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Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer PART I: Peer-to-Peer Protocols Peer-to-Peer Protocols and Service Models ARQ Protocols and Reliable Data Transfer Flow C t l Fl Control Timing Recovery TCP Reliable Stream Service & Flow Control Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer PART II: Data Link Controls Framing Point-to-Point Protocol High-Level Data Link Control Link Sharing Using Statistical Multiplexing Chapter Overview Peer-to-Peer protocols: many protocols involve the interaction between two peers Service Models are discussed & examples given Detailed discussion of ARQ provides example of development of peer-to-peer protocols Flow control, TCP reliable stream, and timing recovery y Data Link Layer Framing PPP & HDLC protocols Statistical multiplexing for link sharing Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer Peer to Peer Peer-to-Peer Protocols and Service Models Peer-to-Peer Protocols p Peer-to-Peer processes execute layer-n protocol to provide service to p n + 1 peer p process n + 1 peer process y ( layer-(n+1)) Layer-(n+1) peer calls SDU SDU layer n layer-n and passes PDU Service Data Units n peer process n peer process (SDUs) for transfer Layer-n peers exchange Protocol Data Units (PDUs) to effect transfer n – 1 peer process n – 1 peer process Layer-n delivers SDUs to layer-(n+1) destination layer (n+1) peer Service Models The service model specifies the information transfer service layer-n provides to layer-(n+1) p The most important distinction is whether the service is: Connection-oriented Connectionless Service model possible features: A bit i t t Arbitrary message size or structure Sequencing and Reliability Timing Pacing and Flow control Timing, Pacing, Multiplexing y g y Privacy, integrity, and authentication Connection-Oriented Transfer Service Connection Establishment Connection must be established between layer-(n+1) peers Layer-n protocol must: Set initial parameters, e.g. sequence b d Allocate resources, e.g. b ff numbers; and All t buffers Message transfer phase Exchange of SDUs Disconnect phase Example: TCP, PPP p process n + 1 peer p n + 1 peer process send receive SDU y Layer n connection-oriented service SDU Connectionless Transfer Service setup, No Connection setup simply send SDU Each message send independently Must provide all address information per message Simple & quick Example: UDP, IP n + 1 peer process n + 1 peer process send receive SDU Layer n connectionless service Message Size and Structure What message size and structure will a service model accept? ff Different services impose restrictions on size & structure of data it will transfer Single bit? Block of bytes? Byte stream? Ex: Transfer of voice mail = 1 long message Ex: Transfer of voice call = byte stream 1 voice mail= 1 message = entire sequence of speech samples (a) 1 call = sequence of 1-byte messages (b) Segmentation & Blocking size To accommodate arbitrary message size, a layer may have to deal with messages that are too long or too short for its protocol Segmentation & Reassembly: a layer breaks long messages into smaller blocks and reassembles these at the destination Blocking & Unblocking: a layer combines small messages into bigger blocks prior to transfer 1 long message 2 or more short messages 2 or more blocks 1 block Reliability & Sequencing Reliability: Are messages or information stream delivered error-free and without loss duplication? or d li ti ? Sequencing: Are messages or information stream delivered in order? detection, ARQ protocols combine error detection retransmission, and sequence numbering to provide reliability & sequencing Examples: TCP and HDLC Pacing and Flow Control Messages can be lost if receiving system does not have sufficient buffering to store arriving messages If destination layer-(n+1) does not retrieve its enough, layer-n information fast enough destination layer n buffers may overflow Pacing & Flow Control provide backpressure mechanisms that control transfer according to availability of buffers at the destination Examples: TCP and HDLC Timing Applications involving voice and video generate units of information that are related temporally Destination application must reconstruct temporal relation in voice/video units Network transfer introduces delay & jitter Timing Recovery protocols use timestamps & sequence numbering to control the delay & jitter in delivered information Examples: RTP & associated protocols in Voice over IP Multiplexing layer-(n+1) Multiplexing enables multiple layer (n+1) users to share a layer-n service A multiplexing tag is required to identify specific users at the destination Examples: UDP, IP Privacy, Integrity, & Authentication Privacy: ensuring that information transferred cannot be read by others Integrity: ensuring that information is not altered during transfer Authentication: verifying that sender and/or receiver are who they claim to be Security protocols provide these services and are discussed in Chapter 11 Examples: IPSec, SSL vs. End-to-End vs Hop-by-Hop A service feature can be provided by implementing a protocol end to end end-to-end across the network across every hop in the network Example: Perform error control at every hop in the network or only between the source and destination? Perform flow control between every hop in the network or only between source & destination? We next consider the tradeoffs between the two approaches Error control in Data Link Layer Packets Packets ( ) (a) Data Link operates Data link Data link over wire-like, layer Frames layer directly-connected A B systems Physical Physical layer layer Frames can be lost corrupted or lost, but arrive in order (b) p Data link performs 3 error-checking & 12 3 21 12 21 Medium retransmission 2 Ensures error-free E f A B 1 packet transfer 1 Physical layer entity between two systems y 2 Data link layer entity 3 Network layer entity Error Control in Transport Layer Transport layer protocol (e.g. TCP) sends segments across network and performs end-to-end error checking & retransmission Underlying network is assumed to be unreliable Messages Messages Transport Segments Transport layer layer Network Network Network Network layer layer layer layer Data link Data link Data link Data link layer layer layer layer End system End system A Physical Physical Physical Physical layer layer B layer layer Network Segments can experience long delays, can be lost, or arrive out-of-order because packets can follow different paths across network End-to-end error control protocol more difficult C 1 2 3 2 1 End System α End System β 4 3 21 12 3 2 1 1 2 3 2 1 1 2 3 4 Medium 2 A B 1 Network 3 Network layer entity 4 Transport layer entity End-to-End Approach Preferred Hop-by-hop Hop-by-hop cannot ensure t E2E correctness Data Data Data Data 1 2 3 4 5 y Faster recovery ACK/ ACK/ ACK/ ACK/ NAK NAK NAK NAK Simple End-to-end ACK/NAK inside the network More scalable 1 2 3 4 5 l it t if complexity at Data Data Data Data the edge Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer ARQ Protocols and Reliable Data Transfer Automatic Repeat Request (ARQ) Purpose: to ensure a sequence of information packets is delivered in order and without errors or duplications despite transmission errors & losses We will look at: Stop-and-Wait ARQ Go-Back N ARQ Selective Repeat ARQ Basic elements of ARQ: Error-detecting code with high error coverage ACKs (positive acknowledgments NAKs (negative acknowlegments) Timeout mechanism Stop-and-Wait ARQ Transmit a frame, wait for ACK Error-free Packet packet Information frame Transmitter Receiver Timer set after (Process A) (Process B) h frame each f transmission Control frame Header CRC Information Header packet CRC Information frame Control frame: ACKs Need for Sequence Numbers (a) Frame 1 lost Time-out Time A Frame Frame Frame Frame 0 1 1 2 ACK ACK B ( ) (b) ACK lost Time-out Time out Time A Frame Frame Frame Frame 0 1 1 2 ACK ACK ACK B In cases (a) & (b) the transmitting station A acts the same way But in case (b) the receiving station B accepts frame 1 twice Question: How is the receiver to know the second frame is also frame 1? Answer: Add frame sequence number in header Slast is sequence number of most recent transmitted frame Sequence Numbers (c) Premature Time-out Time-out Time A Frame 0 Frame Frame Frame ACK 0 1 2 ACK B The transmitting station A misinterprets duplicate ACKs Incorrectly assumes second ACK acknowledges Frame 1 Question: How is the receiver to know second ACK is for frame 0? Answer: Add frame sequence number in ACK header Rnext is sequence number of next frame expected by the receiver Implicitly acknowledges receipt of all prior frames 1-Bit Sequence Numbering Suffices 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 Slast Rnext Timer Transmitter Slast Receiver A B Rnext Global State: Error-free frame 0 (Slast, Rnext) (0,0) (0,1) arrives at receiver ACK for ACK for frame 0 frame 1 arrives at arrives at transmitter transmitter Error-free frame 1 Error free arrives at receiver (1,0) (1,1) Stop-and-Wait ARQ Transmitter Receiver Ready state Always in Ready State Await request from higher layer for Wait for arrival of new frame packet transfer When frame arrives, check for errors Wh request arrives, t When t i transmitit If no errors detected and sequence frame with updated Slast and CRC number is correct (Slast=Rnext), then Go to Wait State accept frame, Wait state p update Rnext, Wait for ACK or timer to expire; send ACK frame with Rnext, block requests from higher layer deliver packet to higher layer If timeout expires If no errors detected and wrong retransmit frame and reset timer t it f d t ti sequence number discard frame If ACK received: send ACK frame with Rnext If sequence number is incorrect or if errors detected: ignore ACK If errors detected If sequence number is correct (Rnext frame discard f = Slast +1): accept frame, go to Ready state Applications of Stop-and-Wait ARQ IBM Binary Synchronous Communications protocol (Bisync): character-oriented data li k control link t l Xmodem: modem file transfer protocol Trivial File Transfer Protocol (RFC 1350): simple protocol for file transfer over UDP Stop-and-Wait Efficiency Last frame bit ACK First frame bit enters channel arrives enters channel Channel idle while transmitter waits for ACK A t B t First frame bit Receiver arrives at Last frame bit processes frame receiver arrives at and receiver prepares ACK 10000 bit frame @ 1 Mbps takes 10 ms to transmit it for th ffi i 10/11 If wait f ACK = 1 ms, then efficiency = 10/11= 91% If wait for ACK = 20 ms, then efficiency =10/30 = 33% Stop-and-Wait Model t0 = total time to transmit 1 frame A tproc B frame tprop tproc tack tprop tf time t 0 = 2 t prop + 2 t proc + t f + t ack bits/info frame nf na bits/ACK frame = 2 t prop + 2 t proc + + R R channel transmission rate S&W Efficiency on Error-free channel Eff ti transmission rate: Effective t i i t bits for header & CRC number of information bits delivered to destinatio n n f − no R 0 eff ff = = , total time required to deliver the information bits t0 Transmission efficiency: Effect of n f − no no 1− frame overhead Reff t0 nf η0 = = = . R R na 2(t prop + t proc ) R 1+ + nf nf Eff f Effect of Effect of Delay-Bandwidth Product ACK frame Example: Impact of Delay- Bandwidth Product bits, nf=1250 bytes = 10000 bits na=no=25 bytes = 200 bits 2xDelayxBW 1 ms 10 ms 100 ms 1 sec Efficiency Effi i 200 km 2000 km 20000 km 200000 km 1 Mbps 103 104 105 106 88% 49% 9% 1% 1 Gbps 106 107 108 109 1% 0.1% 0.01% 0.001% Stop-and-Wait does not work well for very high speeds or long propagation delays S&W Efficiency in Channel with Errors Let 1 – Pf = probability frame arrives w/o errors Avg. # of transmissions to first correct arrival is then 1/ (1–Pf ) g g g "If 1-in-10 get through without error, then avg. 10 tries to success" Avg. Total Time per frame is then t0/(1 – Pf) n f − no no t0 1− Reff 1 − Pf nf η SW = = = (1 − Pf ) R R na 2(t prop + t proc ) R 1+ + nf nf Effect of frame loss Example: Impact Bit Error Rate bits, nf=1250 bytes = 10000 bits na=no=25 bytes = 200 bits Find efficiency for random bit errors with p=0, 10-6, 10-5, 10-4 −n f p 1 − Pf = (1 − p ) ≈e nf f l d ll for large n f and small p 1 – Pf 0 10-6 6 10-5 5 10-4 4 Efficiency 1 Mbps 1 0.99 0 99 0.905 0 905 0 368 0.368 & 1 ms 88% 86.6% 79.2% 32.2% Bit errors impact performance as nfp approach 1 Go-Back-N Improve Stop-and-Wait by not waiting! Keep channel busy by continuing to send frames Allow a window of up t Ws outstanding frames All i d f to t t di f Use m-bit sequence numbering If ACK for oldest frame arrives before window is exhausted, we can continue transmitting If window is exhausted, pull back and retransmit all outstanding frames Alternative: Use timeout Go-Back-N ARQ Go-Back-4: Go Back 4: 4 frames are outstanding; so go back 4 fr fr fr fr fr fr fr fr fr fr fr fr fr fr Time 0 1 2 3 4 5 6 3 4 5 6 7 8 9 A B A A A out of sequence A A A A A A C C C frames C C C C C C K K K K K K K K K 1 2 3 4 5 6 7 8 9 Rnext 0 1 2 3 3 4 5 6 7 8 9 Frame transmission are pipelined to keep the channel busy F ith d b t t f Frame with errors and subsequent out-of-sequence f i d frames are ignored Transmitter is forced to go back when window of 4 is exhausted Window size long enough to cover round trip time p Time-out expires Stop-and-Wait ARQ fr fr fr Time 0 0 1 A B Receiver is A looking for C K 0 Rnext=0 1 Go-Back-N ARQ Four frames are outstanding; so go back 4 fr fr fr fr fr fr fr fr fr fr fr 0 Ti Time 1 2 3 0 1 2 3 4 5 6 A B A A A A A A Receiver is Out-of- C C C C C C looking for sequence K K K K K K Rnext=0 frames 1 2 3 4 5 6 Go-Back-N with Timeout Go-Back-N Problem with Go Back N as presented: If frame is lost and source does not have frame to d then i d ill t be h t d d send, th window will not b exhausted and recovery will not commence Use timeout with each f U a ti t ith h frame When timeout expires, resend all outstanding frames Go-Back-N Transmitter & Receiver Transmitter Receiver Send Window ... Receive Window Frames transmitted S last Srecent Slast+Ws-1 and ACKed Frames Buffers Rnext i d received oldest un- Timer Slast ACKed frame Timer 1 Slast+1 Receiver will only accept ... a frame that is error-free and most recent that has sequence number Rnext Timer Srecent transmission When such frame arrives Rnext is ... incremented by one, so the Sllastt+Ws-1 1 q max Seq # y receive window slides forward by allowed one Sliding Window Operation Transmitter Send Window m-bit Sequence Numbering ... Frames transmitted S 0 and ACKed last Srecent Slast+Ws-1 2m – 1 1 2 Transmitter waits for error-free ACK frame with sequence number Slast b Sllast send When such ACK frame arrives, Slast is incremented by one and b one, window i the send window slides forward i + Ws – 1 i+1 by one Maximum Allowable Window Size is Ws = 2m-1 M = 22 = 4, Go-Back - 4: Transmitter goes back 4 fr fr fr fr fr fr fr fr Time A 0 1 2 3 0 1 2 3 A A A A B C C C C Receiver has Rnext= 0, but it does not K K K K know whether its ACK for frame 0 was 1 2 3 0 received, so it does not know whether Rnext 0 1 2 3 0 this is the old frame 0 or a new frame 0 M = 22 = 4, Go-Back-3: Transmitter goes back 3 fr fr fr fr fr fr Time A 0 1 2 0 1 2 A A A B C C C K K K Receiver has Rnext= 3 , so it 1 2 3 rejects the old frame 0 Rnext 0 1 2 3 ACK Piggybacking in Bidirectional GBN SArecent RA next Transmitter Receiver Receiver Transmitter T itt SBrecent RB next "A" Receive Window "B" Receive Window B RA next RB next "A" Send Window "B" Send Window ... ... SA last SA last+WA s-1 SB last SB last+WB s-1 Buffers Buffers Timer Ti SA last Timer SB last SA last+1 SBlast+1 Timer Note: Out-of- Timer ... ... sequence error-free Timer SArecent Timer SBrecent ... frames discarded ... A A after Rnext examined Timer S last+W s-1 Timer SB last+WB s-1 Applications of Go-Back-N ARQ (High-Level bit- HDLC (High Level Data Link Control): bit oriented data link control V.42 modem: error control over telephone modem links Required Timeout & Window Size Tout Tproc Tprop Tf Tf Tprop Timeout value should allow for: Two propagation times + 1 processing time: 2 Tprop + Tproc A frame that begins transmission right before our frame arrives Tf Next frame carries the ACK, Tf Ws should be large enough to keep channel busy for Tout Required Window Size for Delay-Bandwidth Product Frame = 1250 bytes =10,000 bits, R = 1 Mbps 2(tprop + tproc) 2 x Delay x BW Window 1 ms 1000 bits 1 10 ms 10 000 bits 10,000 2 100 ms 100,000 100 000 bits 11 1 second , , 1,000,000 bits 101 Efficiency of Go-Back-N GBN is completely efficient, if Ws large enough to keep channel busy, and if channel is error-free Assume Pf frame loss probability, then time to deliver a frame is: tf if first frame transmission succeeds (1 – Pf ) ( Tf + Wstf /(1-Pf) if the first transmission does not succeed Pf Ws t f Ws t f tGBN = t f (1 − Pf ) + Pf {t f + } = t f + Pf d and 1 − Pf 1 − Pf n f − no no 1− tGBN nf ηGBN = = (1 − Pf ) R 1 + (Ws − 1) Pf Delay-bandwidth product determines Ws Example: Impact Bit Error Rate on GBN bits, nf=1250 bytes = 10000 bits na=no=25 bytes = 200 bits Compare S&W with GBN efficiency for random bit errors with p = 0, 10-6, 10-5, 10-4 and R = 1 Mbps & 100 ms 1 Mbps x 100 ms = 100000 bits = 10 frames → Use Ws = 11 Effi i Efficiency 0 10-6 6 10-5 5 10-4 4 S&W 8.9% 8 9% 8 8% 8.8% 8.0% 8 0% 3.3% 3 3% GBN 98% 88.2% 45.4% 4.9% Go-Back-N significant improvement over Stop-and-Wait for large delay-bandwidth product Go-Back-N becomes inefficient as error rate increases Selective Repeat ARQ Go-Back-N ARQ inefficient because multiple frames are resent when errors or losses occur Selective Repeat retransmits only an individual frame Timeout causes individual corresponding frame to be resent NAK causes retransmission of oldest un-acked frame Receiver maintains a receive window of sequence numbers that can be accepted Error-free, but out-of-sequence frames with sequence numbers within the receive window are buffered Arrival of frame with Rnext causes window to slide forward by 1 or more Selective Repeat ARQ fr fr fr fr fr fr fr fr fr fr fr fr fr fr Time 0 1 2 3 4 5 6 2 7 8 9 10 11 12 A B A A N A A A A A A A A A C C A C C C C C C C C C K K K K K K K K K K K K 1 2 2 2 2 2 7 8 9 1 1 1 0 1 2 Selective Repeat ARQ Transmitter Receiver Send Window Receive Window ... Frames Frames transmitted S received Rnext Rnext + Wr-1 last Srecent Slast+ Ws-1 and ACKed Buffers Buffers Timer Slast Rnext+ 1 Timer Slast+ 1 Rnext+ 2 ... Timer Srecent ... Rnext+ Wr- 1 max Seq # ... accepted Slast+ Ws - 1 Send & Receive Windows Transmitter Receiver 0 0 2m-1 1 2m-1 1 2 2 Rnextt Slast send receive j window i window i i + Ws – 1 i+1 j + Wr – 1 Moves k forward when ACK Moves forward by 1 or more arrives with Rnext = Slast + k h frame arrives with when f i ith k = 1, …, Ws-1 Seq. # = Rnext What size Ws and Wr allowed? M 2 4, 3, 3 Example: M=22=4 Ws=3 Wr=3 Frame 0 resent S d Send {0,1,2} {1,2} {2} {.} Window fr0 fr1 fr2 fr0 A Time B ACK1 ACK2 ACK3 Receive {0,1,2} {1,2,3} {0 1 2} {1 2 3} {2 3 0} {2,3,0} {3 0 1} {3,0,1} Window Old frame 0 accepted as a new frame because it falls in the receive window Ws + Wr = 2m is maximum allowed M 2 4, 2, 2 Example: M=22=4 Ws=2 Wr=2 Frame 0 resent S d Send {0,1} {1} {.} Window fr0 fr1 fr0 A Time B ACK1 ACK2 Receive {0,1} {0 1} {1 2} {1,2} {2 3} {2,3} Window Old frame 0 rejected because it falls outside the receive window Why Ws + Wr = 2m works Transmitter sends frames 0 Receiver window starts at {0, …, Wr} to Ws-1; send window empty Window slides forward to All arrive at receiver {Ws,…,Ws+Wr-1} C s ost All ACKs lost Receiver rejects frame 0 because it Transmitter resends frame 0 is outside receive window 0 0 2m-1 1 2m-1 1 Slast W 1 Ws +Wr-1 2 2 receive Rnext Ws send window window 1 Ws-1 Applications of Selective Repeat ARQ TCP (Transmission Control Protocol): transport layer protocol uses variation of l ti t to id li bl t selective repeat t provide reliable stream service Service Specific Connection Oriented g g Protocol: error control for signaling messages in ATM networks Efficiency of Selective Repeat probability, Assume Pf frame loss probability then number of transmissions required to deliver a frame is: tf / (1-Pf) n f − no t f /(1 − Pf ) no η SR = = (1 − )(1 − Pf ) R nf Example: Impact Bit Error Rate on Selective Repeat bits, nf=1250 bytes = 10000 bits na=no=25 bytes = 200 bits Compare S&W, GBN & SR efficiency for random bit errors p 0, R with p=0, 10-6, 10-5, 10-4 and R= 1 Mbps & 100 ms Efficiency 0 10-6 10-5 10-4 S&W 8.9% 8.8% 8.0% 3.3% GBN 98% 88.2% 45.4% 4.9% SR 98% 97% 89% 36% Selective R t t f d S&W b t S l ti Repeat outperforms GBN and S&W, but efficiency drops as error rate increases Comparison of ARQ Efficiencies Assume na and no are negligible relative to nf, and L = 2(tprop+tproc)R/nf =(Ws-1), then p Selective-Repeat: no η SR = (1 − Pf )(1 − ) ≈ (1 − Pf ) nf Go-Back-N: For Pf≈0, SR & GBN same 1 − Pf 1 − Pf η GBN = = 1 + (WS − 1) Pf 1 + LPf 1, For Pf→1, GBN & SW same Stop-and-Wait: (1 − Pf ) 1 − Pf η SW = ≈ n 2(t prop + t proc ) R 1 + L 1+ a + nf nf ARQ Efficiencies ARQ Efficiency Com parison Selective 15 1.5 Repeat Go Back N 10 ncy 1 Efficien 0.5 Stop and Wait 100 0 Go Back N 100 -9 -8 -7 -6 -5 -4 -3 10 10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 -2-2 -1-1 - LOG(p) p Stop and Wait 10 Delay-Bandwidth product = 10, 100 Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer Flow Control Flow Control buffer fill Information frame Transmitter Receiver Control frame Receiver h li it d b ff i t store arriving R i has limited buffering to t i i frames Several situations cause b ff overflow S l it ti buffer fl Mismatch between sending rate & rate at which user can retrieve data Surges in frame arrivals Flow control prevents buffer overflow by regulating rate at which source is allowed to send information X ON / X OFF threshold Information frame Transmitter Receiver Transmit X OFF Transmit Time A on off on off B Time 2Tprop Threshold must activate OFF signal while 2 Tprop R bits still remain in buffer Window Flow Control Return of permits tcycle A Time B Time Sliding Window ARQ method with Ws equal to buffer available Transmitter can never send more than Ws frames ACKs that slide window forward can be viewed as permits to transmit more Can also pace ACKs as shown above p (ACKs) at end of cycle regulates transmission rate Return permits ( ) y g Problems using sliding window for both error & flow control Choice of window size p y Interplay between transmission rate & retransmissions TCP separates error & flow control Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer Timing Recovery Timing Recovery for Synchronous Services Synchronous source Network output sends periodic not periodic information blocks Network Applications that involve voice, audio, or video can generate a synchronous information stream Information carried by equally-spaced fixed-length packets Network multiplexing & switching introduces random delays Packets experience variable transfer delay Jitter (variation in interpacket arrival times) also introduced Ti i recovery re-establishes th synchronous nature of th stream Timing t bli h the h t f the t Introduce Playout Buffer Packet Arrivals y Packet Playout Playout Buffer Packet Arrivals Sequence numbers help order packets Packet Playout Tmax • Delay first packet by maximum network delay • All other packets arrive with less delay • Playout packet uniformly thereafter Arrival times Time Playout clock must Send be synchronized to times Receiver too transmitter clock Playout slow; times buffer fills and fl overflows Tplayoutt l time Receiver too fast Receiver speed Time buffer starvation Time just right Many late packets Tplayout time Tplayout time Clock Recovery Timestamps inserted in Buffer for information blocks packet payloads indicate when info was Playout produced Error command signal + Smoothingg Adjust Add frequency filter t4 t3 t2 t1 Recovered - Timestamps clock Counter Counter attempts to replicate transmitter clock Frequency of counter is adjusted according to arriving timestamps Jitter introduced by network causes fluctuations in buffer & in local clock Synchronization to a Common Clock Transmitter Receiver M M fs Network fr M=#ticks in local clock In time that net clock Δf=fn-fs=fn-(M/N)fn does N ticks fn fn/fs=N/M N ticks N ticks fr=fn-Δf Network clock Clock recovery simple if a common clock is available to transmitter & receiver E.g. SONET network clock; Global Positioning System (GPS) Transmitter sends Δf of its frequency & network frequency Receiver adjusts network frequency by Δf Packet delay jitter can be removed completely Example: Real-Time Protocol real- RTP (RFC 1889) designed to support real time applications such as voice, audio, video RTP provides means to carry: Type of information source Sequence numbers Timestamps Actual timing recovery must be done by higher layer protocol MPEG2 for video, MP3 for audio Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer TCP Reliable Stream Service & Flow Control TCP Reliable Stream Service TCP transfers byte Application Layer stream in order, without Application Layer reads writes bytes into send ff buffer through socket errors or duplications bytes from receive buffer y through socket Write 45 bytes Write 15 bytes Read 40 bytes Application layer Write 20 bytes Read 40 bytes Transport layer g Segments Transmitter Receiver Send buffer Receive buffer ACKs TCP ARQ Method • TCP uses Selective Repeat ARQ • Transfers byte stream without preserving boundaries p • Operates over best effort service of IP • Packets can arrive with errors or be lost • Packets can arrive out-of-order • Packets can arrive after very long delays • Duplicate segments must be detected & discarded • Must protect against segments from p p g g previous connections • Sequence Numbers • Seq. # is number of first byte in segment payload Very l • V Seq. # (32 bits) to deal with l long S #s delays bit ) t d l ith long d l • Initial sequence numbers negotiated during connection setup ( y p (to deal with very old duplicates)) • Accept segments within a receive window Transmitter Receiver Send Window Receive Window Slast + Wa-1 Rlast Rlast + WR – 1 ... ... ... octets Rnext Rnew S S Slast + Ws – 1 transmitted last recent & ACKed Slast oldest unacknowledged byte Rlast highest-numbered byte not Srecent highest-numbered yet read by the application transmitted byte Rnext next expected byte Slast+Wa-1 highest-numbered byte Rnew highest numbered byte that can be transmitted received correctly Slast+Ws-1 highest-numbered byte Rlast+WR-1 highest-numbered that can be accepted from the p byte that can be accommodated application in receive buffer TCP Connections TCP Connection One connection each way Address, #, Identified uniquely by Send IP Address Send TCP Port # Receive IP Address, Receive TCP Port # Three Way Connection Setup with Three-Way Handshake Three-way exchange to negotiate initial Seq. #'s for connections in each direction Data Transfer Exchange segments carrying data Graceful Close Close each direction separately Three Phases of TCP Connection Host A Host B Three-way Handshake Data Transfer Graceful Close 1st Handshake: Client-Server Connection Request Initial Seq. # from li t to client t server SYN bit set indicates request to establish connection from client to server 2nd Handshake: ACK from Server ACK Seq. # = Init. Seq Init Seq. # + 1 ACK bit set acknowledges connection request; Client- to-Server connection established 2nd Handshake: Server-Client Connection Request Initial Seq. # from to li t server t client SYN bit set indicates request to establish connection from server to client 3rd Handshake: ACK from Client ACK Seq. # = Init. Seq Init Seq. # + 1 ACK bit set acknowledges connection request; Connections in both directions established TCP Data Exchange Application Layers write bytes into buffers TCP sender forms segments When bytes d threshold timer expires Wh b t exceed th h ld or ti i Upon PUSH command from applications Consecutive b t f C buffer i t d in l d ti bytes from b ff inserted i payload Sequence # & ACK # inserted in header Checksum calculated and i l d d i h d Ch k l l t d d included in header TCP receiver Performs selective repeat ARQ functions Writes error-free, in-sequence bytes to receive buffer Data Transfer: Server-to-Client Segment 12 bytes of payload Push set 12 bytes of payload carries telnet option negotiation Graceful Close: Client-to-Server Connection Client initiates closing of its connection to server Graceful Close: Client-to-Server Connection ACK Seq. # = Seq. Previous Seq # + 1 Server ACKs request; client- to-server to server connection closed Flow Control TCP receiver controls rate at which sender transmits to prevent buffer fl b ff overflow TCP receiver advertises a window size specifying number of bytes that can be accommodated by receiver WA = WR – (Rnew – Rlast) TCP sender obliged to keep # outstanding bytes below WA (Srecent - Slast) ≤ WA Send Window Receive Window Slast + WA-1 WA ... ... ... Rlast Rnew Rlast + WR – 1 Slast Srecent Slast + Ws – 1 TCP window flow control Host A Host B t0 t1 t2 t3 t4 TCP Retransmission Timeout g TCP retransmits a segment after timeout p period Timeout too short: excessive number of retransmissions Timeout too long: recovery too slow Timeout depends on RTT: time from when segment is sent to when ACK is received Round trip time (RTT) in Internet is highly variable Routes vary and can change in mid-connection Traffic fluctuates TCP uses adaptive estimation of RTT Measure RTT each time ACK received: τn tRTT(new) = α tRTT(old) + (1 – α) τn α = 7/8 typical RTT Variability Estimate variance σ2 of RTT variation Estimate for timeout: tout = tRTT + k σRTT If RTT highly variable, timeout increase accordingly If RTT nearly constant, timeout close to RTT estimate Approximate estimation of deviation dRTT(new) = β dRTT(old) + (1-β) | τn - tRTT | tout = tRTT + 4 dRTT Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer PART II: Data Link Controls Framing Point-to-Point Protocol High-Level Data Link Control Link Sharing Using Statistical Multiplexing Data Link Protocols Packets Packets Data Links Services Framing Data link Data link Error control layer layer A Frames Fl t l Flow control Physical Physical B layer Multiplexing layer Link Maintenance Security: Authentication & Encryption Examples Directly connected, wire-like PPP errors, out of Losses & errors but no out-of- sequence frames HDLC Applications: Direct Links; Ethernet LAN LANs; Connections across IEEE 802.11 (Wi Fi) LAN WANs Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer Framing Framing transmitted received Mapping stream of frames frames physical layer bits into frames Mapping frames into Framing bit stream Frame boundaries can be determined using: Character Counts 0110 0101 Control Characters 011011 111110 Flags CRC Checks 01 11 Character-Oriented Framing Data to be sent A DLE B ETX DLE STX E After stuffing and framing DLE STX A DLE DLE B ETX DLE DLE STX E DLE ETX Frames consist of integer number of bytes Asynchronous transmission systems using ASCII to transmit printable characters Octets with HEX value <20 are nonprintable O ih l 20 i bl Special 8-bit patterns used as control characters STX (start of text) = 0x02; ETX (end of text) = 0x03; Byte dt i t bl h t in frame B t used to carry non-printable characters i f DLE (data link escape) = 0x10 DLE STX (DLE ETX) used to indicate beginning (end) of frame Insert extra DLE in front of occurrence of DLE STX (DLE ETX) in frame All DLEs occur in pairs except at frame boundaries Framing & Bit Stuffing HDLC frame Flag Address Control Information FCS Flag a y u be of bits any number o b ts Frame delineated by flag character t ffi to t f fl HDLC uses bit stuffing t prevent occurrence of flag 01111110 inside the frame Transmitter inserts extra 0 after each consecutive five 1s inside the frame Receiver checks for five consecutive 1s if next bit = 0, it is removed , g if next two bits are 10, then flag is detected If next two bits are 11, then frame has errors Example: Bit stuffing & de- stuffing (a) Data to be sent 0110111111111100 After stuffing and framing 0111111001101111101111100001111110 (b) Data received 01111110000111011111011111011001111110 After destuffing and deframing *000111011111-11111-110* PPP Frame Flag Address Control Protocol g Flag Information CRC 01111110 1111111 00000011 01111110 integer # of bytes All stations are to Unnumbered Specifies what kind of packet is contained in the accept the frame frame payload, e.g., LCP, NCP, IP, OSI CLNP, IPX PPP uses similar frame structure as HDLC, except Protocol type field Payload contains an integer number of bytes PPP uses the same flag, but uses byte stuffing Problems with PPP byte stuffing Size of frame varies unpredictably due to byte insertion Malicious users can inflate bandwidth by inserting 7D & 7E Byte-Stuffing in PPP character oriented PPP is character-oriented version of HDLC Flag is 0x7E (01111110) Control escape 0x7D (01111101) Any occurrence of flag or control escape inside of frame is replaced with 0x7D followed by original octet XORed with 0x20 (00100000) Data to be sent 41 7D 42 7E 50 70 46 7E 41 7D 5D 42 7D 5E 50 70 46 7E After stuffing and framing Generic Framing Procedure GFP payload area 2 2 2 2 0-60 PLI cHEC Type yp tHEC GEH GFP payload p y Payload Core Payload Type GFP GFP length header type header extension payload indicator error error headers checking checking GFP combines frame length indication with CRC PLI indicated length of frame, then simply count characters cHEC (CRC-16) protects against errors in count field (single-bit error correction + error detection) GFP designed to operate over octet-synchronous physical layers (e.g. SONET) Frame-mapped mode for variable-length payloads: Ethernet Transparent mode carries fixed-length payload: storage devices GFP Synchronization & Scrambling three states Synchronization in three-states Hunt state: examine 4-bytes to see if CRC ok If no, move forward by one-byte yes pre-sync If yes, move to pre sync state Pre-sync state: tentative PLI indicates next frame If N successful frame detections, move to sync state If no match, go to hunt state Sync state: normal state p y go Validate PLI/cHEC, extract payload, g to next frame Use single-error correction Go to hunt state if non-correctable error Scrambling Payload is scrambled to prevent malicious users from inserting long strings of 0s which cause SONET equipment to lose bit clock synchronization (as discussed in line code section) Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer Point to Point Point-to-Point Protocol PPP: Point-to-Point Protocol Data link protocol for point-to-point lines in Internet Router-router; dial-up to router 1 Provides Framing and Error Detection 1. Character-oriented HDLC-like frame structure 2. 2 Link Control Protocol Bringing up, testing, bringing down lines; negotiating options Authentication: key capability in ISP access 3. 3 A family of Network Control Protocols specific to different network layer protocols , y , ( ), pp IP, OSI network layer, IPX (Novell), Appletalk PPP Applications PPP used in many point-to-point applications Telephone Modem Links 30 kbps Packet Mb to P k t over SONET 600 Mbps t 10 Gb Gbps IP→PPP→SONET PPP is also used over shared links such as Ethernet to provide LCP, NCP, and authentication features PPP over Ethernet (RFC 2516) Used over DSL PPP Frame Format 1 or 2 variable 2 or 4 Flag Address Control Protocol Information Flag FCS 01111110 1111111 00000011 01111110 CRC 16 or t ti to All stations are t CRC 32 accept the frame HDLC Unnumbered frame • PPP can support multiple network protocols simultaneously Specifies what ki d of packet i contained i th payload • S ifi h t kind f k t is t i d in the l d •e.g. LCP, NCP, IP, OSI CLNP, IPX... PPP Example PPP Phases 1 Carrier 1. Home PC to Internet Service Dead detected Provider 7. Carrier 1. PC calls router via modem dropped 2. PC and router exchange LCP packets to negotiate PPP Failed parameters Terminate Establish 3. Check on identities p 2. Options p g 4. NCP packets exchanged to 6. Done negotiated configure the network layer, e.g. TCP/IP ( requires IP address Failed assignment) 5. 5 Open Authenticate 5 Data transport, e g send/receive 5. transport e.g. IP packets 6. NCP used to tear down the 3. Authentication network layer connection (free up 4. NCP dd ) d to h t IP address); LCP used t shut completed down data link layer connection configuration Network 7. Modem hangs up PPP Authentication Password Authentication Protocol Initiator must send ID & password Authenticator replies with authentication success/fail Aft several attempts, LCP closes li k After l tt t l link Transmitted unencrypted, susceptible to eavesdropping Challenge-Handshake Authentication Protocol Challenge Handshake (CHAP) Initiator & authenticator share a secret key Authenticator d h ll A th ti t sends a challenge ( d (random # & ID) Initiator computes cryptographic checksum of random # & ID using the shared secret key Authenticator also calculates cryptocgraphic checksum & compares to response Authenticator can reissue challenge during session Example: PPP connection setup in dialup modem to ISP LCP Setup p PAP IP NCP setup Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer High Level High-Level Data Link Control High-Level Data Link Control (HDLC) Bit-oriented Bit oriented data link control Derived from IBM Synchronous Data Link Control (SDLC) Related to Link Access Procedure Balanced (LAPB) LAPD in ISDN LAPM in cellular telephone signaling NLPDU Network Network layer layer "Packet" DLSDU DLSAP DLSAP DLSDU DLPDU Data link Data link layer layer "Frame" Physical Physical layer layer HDLC Data Transfer Modes Normal Response Mode Used in polling multidrop lines Commands Primary Responses Secondary S d Secondary S d Secondary Asynchronous Balanced Mode Used in full-duplex point-to-point links Primary Commands Responses Secondary Secondary Responses Commands Primary Mode is selected during connection establishment HDLC Frame Format Flag Address Control Information FCS Flag Control field gives HDLC its functionality Codes in fields have specific meanings and uses Flag: delineate frame boundaries Address: identify secondary station (1 or more octets) mode In ABM mode, a station can act as primary or secondary so address changes accordingly Control: purpose & functions of frame (1 or 2 octets) standardized, Information: contains user data; length not standardized but implementations impose maximum Frame Check Sequence: 16- or 32-bit CRC Control Field Format Information Frame 1 2-4 5 6-8 0 N(S) P/F N(R) Supervisory Frame 1 0 S S P/F N(R) Unnumbered Frame 1 1 M M P/F M M M S: Supervisory Function Bits M: Unnumbered Function Bits N(R): Receive Sequence Number P/F: Poll/final bit used in interaction N(S): Send Sequence Number between primary and secondary Information frames Each I-frame contains sequence number N(S) Positive ACK piggybacked N(R)=Sequence number of next frame expected acknowledges all frames up to and including N(R)-1 3 or 7 bit sequence numbering Maximum window sizes 7 or 127 Poll/Final Bit NRM: Primary polls station by setting P=1; Secondary sets F=1 in last I-frame in response p Primaries and secondaries always interact via paired P/F bits Error Detection & Loss Recovery Frames lost due to loss-of-synch or receiver buffer overflow Frames may undergo errors in transmission CRCs detect errors and such frames are treated as lost Recovery through ACKs, timeouts & retransmission S b i to identify t f Sequence numbering t id tif out-of-sequence & duplicate frames C for HDLC provides f options that implement several ARQ methods Supervisory frames (ACK, (Don t Used for error (ACK NAK) and flow control (Don't Send): Receive Ready (RR), SS=00 p ( ) p ggy ACKs frames up to N(R)-1 when piggyback not available REJECT (REJ), SS=01 Negative ACK indicating N(R) is first frame not received tl T itt t d d later frames correctly. Transmitter must resend N(R) and l t f Receive Not Ready (RNR), SS=10 N(R) 1 I-frames ACKs frame N(R)-1 & requests that no more I frames be sent Selective REJECT (SREJ), SS=11 Negative ACK for N(R) requesting that N(R) be selectively retransmitted Unnumbered Frames Setting of Modes: SABM: Set Asynchronous Balanced Mode UA: acknowledges acceptance of mode setting commands DISC: terminates logical link connectio Information Transfer between stations UI: Unnumbered information Recovery used when normal error/flow control fails FRMR: frame with correct FCS but impossible semantics RSET: indicates sending station is resetting sequence numbers g XID: exchange station id and characteristics Connection Establishment & Release Supervisory frames used to establish and release data link connection In HDLC Set Asynchronous Balanced Mode (SABM) Disconnect (DISC) Unnumbered Acknowledgment (UA) SABM UA Data UA transfer DISC Example: HDLC using NRM (polling)Address of secondary Primary A Secondaries B, C ll A polls B B, RR 0, B RR, 0 P N(S) N(R) B, I, 0, 0 N(R) B sends 3 info X , , , B, I, 1, 0 frames f B, I, 2, 0,F A rejects fr1 B, SREJ, 1 A polls C C RR, 0, P C, C, RR, 0, F C nothing to send A polls B, B, SREJ, 1,P requests selective B, I, 1, 0 B resends fr1 retrans. fr1 B, I, 3, 0 Then fr 3 & 4 B, I, 4, 0, F A send info fr0 B I, 0 5 B, I 0, to B, ACKs up to 4 Time Frame Exchange using Asynchronous Balanced Mode Combined Station A Combined Station B B, I 0, B I, 0 0 A, I, 0, 0 B, I, 1, 0 A, I, 1, 1 A ACKs fr0 X B sends 5 frames B, I, 2, 1 A, I, 2, 1 B, I, 3, 2 A rejects B, REJ, 1 fr1 B, I, 4, 3 , , , A, I, 3, 1 B goes B, I, 1, 3 back to 1 B, I, 2, 4 B, RR, 2 A ACKs fr1 B, I, 3, 4 B, RR, 3 A ACKs fr2 Flow Control Flow control is required to prevent transmitter from overrunning receiver buffers y y g Receiver can control flow by delaying acknowledgement messages y Receiver can also use supervisory frames to explicitly control transmitter Receive Not Ready (RNR) & Receive Ready (RR) I3 I4 I5 RNR5 RR6 I6 Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer Link Sharing Using Statistical Multiplexing Statistical Multiplexing Multiplexing t t bursty traffic t h d li M lti l i concentrates b t t ffi onto a shared line Greater efficiency and lower cost Header Data payload A B Buffer Output line C Input lines Tradeoff Delay for Efficiency (a) Dedicated lines D di t d li A1 A2 B1 B2 C1 C2 (b) Shared lines A1 C1 B1 A2 B2 C2 Dedicated lines involve not waiting for other users, but lines are used inefficiently when user traffic is bursty Shared lines concentrate packets into shared line; packets buffered (delayed) when line is not immediately available Multiplexers inherent in Packet Switches 1 1 2 2 N N Packets/frames forwarded to buffer prior to transmission from switch Multiplexing occurs in these buffers Multiplexer Modeling Input lines A Output line B Buffer C Arrivals: What is the packet interarrival pattern? Service Time: How long are the packets? Service Discipline: What is order of transmission? Buffer Di i li b ff i f ll hi h k t is dropped? B ff Discipline: If buffer is full, which packet i d d? Performance Measures: Delay Distribution; Packet Loss Probability; Line Utilization Delay = Waiting + Service Times P1 P2 P3 P4 P5 Packet completes transmission Service Packet begins time transmission Waiting Packet arrives time at queue P1 P2 P3 P4 P5 Packets arrive and wait for service Waiting Time: from arrival instant to beginning of service Service Time: time to transmit packet Delay: total time in system = waiting time + service time Fluctuations in Packets in the System (a) Dedicated lines A1 A2 B1 B2 C1 C2 (b) Shared line A1 C1 B1 A2 B2 C2 (c) N(t) Number of packets in the system Packet Lengths & Service Times R bits per second transmission rate L = # bits in a packet X = L/R = time to transmit ("service") a packet Packet lengths are usually variable Distribution of lengths → Dist. of service times C Common models:d l Constant packet length (all the same) Exponential di t ib ti E ti l distribution Internet Measured Distributions fairly constant See next chart Measure Internet Packet Distribution Dominated by TCP traffic (85%) ~40% packets are minimum-sized 40 byte packets for TCP ACKs ~15% packets are maximum-sized Ethernet 1500 frames ~15% packets are 552 & 576 byte packets for TCP implementations that do not use path MTU discovery Mean=413 Mean 413 bytes Stand Dev=509 bytes Source: caida.org M/M/1/K Queueing Model Exponential ser ice E ponential service K – 1 buffer Poisson Arrivals time with rate μ rate λ At most K customers allowed in system 1 customer served at a time; up to K – 1 can wait in queue Mean service time E[X] = 1/μ Key parameter Load: ρ = λ/μ (ρ≈0), Wh λ << μ ( 0) customers arrive i f When t tl d ll i infrequently and usually find system empty, so delay is low and loss is unlikely As λ approaches μ (ρ→1) , customers start bunching up and delays i d l d losses occur more f increase and l tl frequently When λ > μ (ρ>0) , customers arrive faster than they can be processed, so most customers find system full and those that do t h to it b t i times d enter have t wait about K – 1 service ti Poisson Arrivals Average Arrival Rate: λ packets per second Arrivals are equally-likely to occur at any point in time e between co secut e arrivals s an exponential a do Time bet ee consecutive a a s is a e po e t a random variable with mean 1/ λ Number of arrivals in interval of time t is a Poisson random variable with mean λt ( λ t ) k − λt P[ k arrivals i t seconds] = i l in d e k! Exponential Distribution - t/E [X] - λt P[ X > t ] = e =e for t > 0 . 1-e-λt λe-λt t t 0 0 M/M/1/K Performance Results (From Appendix A) Probability of Overflow: (1 − ρ ) ρ K Ploss = 1− ρ K +1 Average T t l Packet Delay: A Total P k t D l ρ ( K + 1) ρ K +1 E[ N ] = − 1− ρ 1 − ρ K +1 E[ N ] E[T ] = λ (1 − PK ) M/M/1/10 10 9 elay X] Maximum 10 E[T]/E[X 8 alized avg de 7 5 6 packets allowed in 4 system norma 3 2 1 Minimum delay is 1 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 service time 1 0.9 load Maximum delay is lity 0.8 10 service times oss b b y lo ro a ility lo s pprobabil 0.7 0.6 0.5 0.4 At 70% load delay & loss begin s 0.3 0 3 o 0.2 0.1 0 increasing 0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 load What if we add more buffers? M/M/1 Queue Exponential service Infinite buffer Poisson Arrivals time with rate μ rate λ Unlimited number of customers allowed in system 0 Pb=0 since customers are never blocked Average Time in system E[T] = E[W] + E[X] μ, When λ << μ customers arrive infrequently and delays are low As λ approaches μ customers start bunching up and average d l increase delays i When λ > μ customers arrive faster than they can be processed and queue grows without bound (unstable) Avg. Avg Delay in M/M/1 & M/D/1 10 9 8 nlized averag . delay 7 normalormalized avgedelay 6 5 4 M/M/1 3 constant 2 service time 1 M/D/1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.99 load 1⎡ ρ ⎤ ⎡ 1 ⎤1 ⎡ ρ ⎤1 1 E[TM ] = ⎢ ⎥ =⎢ ⎥ = ⎢ ⎥ + for M/M/1 model. λ ⎣1 − ρ ⎦ ⎣1 − ρ ⎦ μ ⎣1 − ρ ⎦ μ μ ⎡ ρ ⎤1 ⎡ ρ ⎤1 1 E[T D ] = ⎢1 + ⎥ =⎢ ⎥ + for M/D/1 system. ⎣ 2(1 − ρ ) ⎦ μ ⎣ 2(1 − ρ ) ⎦ μ μ Effect of Scale 100,000 C = 100 000 bps 10,000,000 C = 10 000 000 bps Exp. Dist. with Avg. Packet Exp. Dist. with Avg. Packet Length: 10,000 bits Length: 10,000 bits Service Time: X=0.1 second Service Time: X=0.001 second Arrival Rate: 7.5 pkts/sec Arrival Rate: 750 pkts/sec Load: ρ=0.75 Load: ρ=0.75 Mean Delay: M D l Mean Delay: E[T] = 0.1/(1-.75) = 0.4 sec E[T] = 0.001/(1-.75) = 0 004 sec 0.004 Reduction by factor of 100 Aggregation of flows can improve Delay & Loss Performance Example: Header overhead & Goodput R 64 Let R=64 kbps Assume IP+TCP header = 40 bytes Assume constant packets of total length L= 200, 400, 800, 1200 bytes Find avg. delay vs. goodput (information transmitted excluding header overhead) Service rate μ = 64000/8L packets/second Total load ρ = λ 64000/8L Goodput = λ packets/sec x 8(L-40) bits/packet p Max Goodput = ( ) p (1-40/L)64000 bps Header overhead limits maximum goodput 0.6 06 nds) 0.5 L=1200 Average Delay (secon 0.4 L=800 0.3 L=400 0.2 02 0.1 01 L=200 0 0 8000 16000 24000 32000 40000 48000 56000 64000 Goodput (bits/second) Burst Multiplexing / Speech Interpolation Many Fewer Voice Trunks Calls Part of this burst is lost Voice active < 40% time No buffering, on-the-fly switch bursts to available trunks Can handle 2 to 3 times as many calls Tradeoff: Trunk Utilization vs. Speech Loss Fractional S F i h Loss: f l Speech L fraction of active speech l i f i h lost Demand Characteristics Talkspurt and Silence D ti St ti ti T lk t d Sil Duration Statistics Proportion of time speaker active/idle vs. Speech Loss vs Trunks trunks 10 12 14 16 18 20 22 24 1 ss speech los 0.1 0.01 Typical 48 requirement 0.001 24 32 40 # connections ⎛n⎞ k n ∑ k = m +1 (k − m)⎜ ⎟ p (1 − p ) n − k ⎝k⎠ ⎛n⎞ n! speech loss = where ⎜ ⎟ = . np ⎝ k⎠ k! (n − k )! Effect of Scale Larger flows lead to better performance Multiplexing Gain = # speakers / # trunks Trunks required for 1% speech loss Multiplexing p Speakers Trunks Utilization G i Gain 24 13 1.85 0.74 32 16 2.00 2 00 0.80 0 80 40 20 2.00 0.80 48 23 2.09 0.83 Packet Speech Multiplexing A2 A1 Many voice A3 y terminals generating B3 B2 B1 voice packets Buffer B3 C3 A2 D2 C2 B1 C1 D1 A1 C3 C2 C1 D3 D2 D1 Buffer overflow B2 Digital speech carried by fixed-length packets No packets when speaker silent Synchronous packets when speaker active Buffer packets & transmit over shared high-speed line Tradeoffs: Utilization vs. Delay/Jitter & Loss Packet Switching of Voice 1 2 3 Sent t 1 2 3 Received t Packetization delay: time for speech samples to fill a packet Jitt i bl inter-packet arrivals at destination Jitter: variable i t k t i l t d ti ti Playback strategies required to compensate for jitter/loss y produce fixed end-to-end delay Flexible delay inserted to p y Need buffer overflow/underflow countermeasures Need clock recovery algorithm Chapter 5 Peer-to-Peer Peer to Peer Protocols and Data Link Layer ARQ Efficiency Calculations Stop & Wait Performance 1 successful transmission i – 1 unsuccessful transmissions ∞ E [t t o t a l ] = t 0 + ∑ (i − 1) o ut P[nt = i ] t i =1 ∞ =t 0 + ∑ (i − 1)t o ut (1 − Pf )i −1 Pf i =1 t o u t Pf 1 =t 0 + =t 0 . 1 − Pf 1 − Pf Efficiency: n f − no no 1− E[ttotal ] nf η SW = = (1 − Pf ) = (1 − Pf )η 0 . R na 2(t prop + t proc ) R 1+ + nf nf Go-Back-N Performance f 1 successful transmission f l transmissions i – 1 unsuccessful t i i ∞ E[ttotal ] = t f + ∑ (i − 1)Ws t f P[nt = i ] i =1 ∞ = t f + Ws t f ∑ (i − 1)(1 − Pf ) i −1 Pf i =1 Ws t f Pf 1 + (Ws − 1) Pf = tf + = tf . 1 − Pf 1 − Pf Efficiency: n f − no no 1− E[ttotal ] nf η GBN = = (1 − Pf ) . R 1 + (Ws − 1) Pf
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The quaint New England city of Torrington, Conn., located just 48 minutes west of Hartford, provides residents with a historic, colonial lifestyle. With a population of approximately 36,000, according to the U.S. Census Bureau, there is always something new to explore and discover. Torrington apartments for rent won't leave you disappointed when looking for your new home.
Torrington's history encapsulates centuries prior to its incorporation in 1740. Several Native American artifacts have been found in the area, demonstrating that Torrington has been a great place to live for possibly millennia. Residents today continue to pay homage to their history, preserving many buildings in the town's historic downtown.
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Home of Warner Theatre and The Nutmeg Conservatory for the Arts, Torrington is perfect for those who enjoy the arts because both facilities produce performances year-round. The history behind the city can always be enjoyed at The Torrington Historical Society. Torrington Trails and the Sue Grossman Still River Greenway Trail, located in the city, provides nature lovers with the perfect fill of the outdoors. For hot summer days when the beach sounds like the perfect place to cool down, residents need only head an hour south to enjoy the waves of the Long Island Sound. If you're feeling a need for the city, head two hours west on Interstate 84 for the lights and sights of New York City.
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- Material: Plastic
- Type: Interior
- Color/Finish: Blue
- Product Fit: Direct Fit
- Quantity Sold: Sold Individually
- Door Lock/Key Hole Provision: With door lock button
- Recommended Use: OE Replacement
- Anticipated Ship Out Time: Same day - 1 business day
- Replaces OE Number: 15708045
- Replaces Partslink Number: GM1352102
- Warranty: 1-year, unlimited-mileage warranty
Product reviews
Got it right the first time! by Promotor, February 24, 2014
|Ease of Use/Installation:
|Overall Rating:
I could not believe they got it correct the 1st time with the limited information I was able to give them on this used farm truck..?I had more trouble with giving them a recomendation on their web site....than getting the right part....
Good by Christo, November 8, 2013
|Ease of Use/Installation:
|Overall Rating:
Fit my 99 tahoe good works like its supposed to and seems to be good quality.
Worked Great by Scotty, September 26, 2013
|Ease of Use/Installation:
|Overall Rating:
Great replacement part. Perfect replacement and saved me 80%.
great part by speck, December 2, 2012
|Ease of Use/Installation:
|Overall Rating:
great part and easy to install and looks great to on car
The price was right by smitty, February 15, 2012
|Ease of Use/Installation:
|Overall Rating:
I had called around and the price I was quoted was over 3 times what I paid. I had my mechanic put it in and it was still less money than I would have paid to buy the part from the dealer. I am very satisified.
Not as good as OEM part by , January 7, 2012
|Ease of Use/Installation:
|Overall Rating:
The handle broke within 2 weeks after installation.
easy ordering and fast delivery by Mimi, October 25, 2011
|Ease of Use/Installation:
|Overall Rating:
ordering process and delivery time were a cinch but no where in product description is there an option for colors. interior in my truck is gray and part that I ordered (black) did not offer a choice for color. product works fine but I am concerned with non matching door interior. hopefully I am able to keep vehicle for long time and it will not be an issue.
by mollie, March 10, 2010
|Ease of Use/Installation:
|Overall Rating:
My car failed inspection because of the handle .I took maybe 15 minutes to install.My car is good for 2 years . You are the best


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Pell Grants matter (a lot). But so does what we do with our students in the classroom, writes Wick Sloane, who describes resources he uses to inspire his.
Sometimes just revising a course isn't enough, writes Rob Weir.
Exploring uncharted areas can be mind-stretching and even painful -- but isn't that how we grow? asks Maria Shine Stewart.
In the debut of her column, A Kinder Campus, Maria Shine Stewart remembers two very different campus presences who shaped her career -- and how she channels them in her dealings with others.
With her spiritual life an adjunct to her adjunct life, Maria Shine Stewart offers her top ten (whimsical) tips of survival as a part-time professor.
Finding extra hours in the day to consult or start a business may seem impossible, but Jessica Quillin offers tips for doing so -- including starting slowly and saying No.
Despite warnings from friends, Maria Shine Stewart added a for-profit college to her adjunct teaching schedule. She shares her experiences.
If you are one of the fortunate few with more than one job offer, Eliza Woolf has ideas on how to decide.
Promoting your skills in the business world is a matter of self-marketing, not self-denial, writes Jessica Quillin.
An employee thinks he did the right thing to protect campus safety, but his superiors disagree. C.K. Gunsalus weighs the arguments.
Inside Higher Ed
1015 18th Street NW
Suite 1100
Washington, DC 20036
Ph: 1-202-659-9208
Fax: 1-202-659-9381
1015 18th Street NW, Suite 1100 | PH: 1-202-659-9208
Copyright © 2015


text:
Mequon — Former mayor Connie Pukaite has upset former alderman Bob Ashmore with a successful write-in campaign and claimed the District 2 seat on the Mequon Common Council.
Pukaite received 58 percent of the vote while Ashmore received 40 percent.
The vote total remains unofficial until certified by the board of canvassers:
Connie Pukaite 362
Bob Ashmore 252
According to City Clerk Diane Kowalchuk, the write-in votes were counted four times to ensure accuracy.
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- Sarah Chudnow Community offers a variety of programming for April
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The Hunter Derby Bridle has a classic design with an elegant look. Made with the finest quality English leather and inspired by the classic show hunter bridles of the past, the Hunter Derby Bridle with reins combines traditional craftsmanship with modern enhancements.
Our Wellington Saratoga Premier Bridle delivers excellent quality and outstanding comfort for your horse. Made in England of premium Sedgwick's leather, this fancy-stitched bridle features an integrated single crown with subtle, soft padding over the poll and contouring around the ears. Our subtle signature stitch pattern appears on cheeks and rein fronts for a finished, show ring look.
Made in England, the Wellington Jumper Breastplate is designed with strong elastic for complete freedom of motion.
Wellington reins are the perfect partner to Wellington bridles. These beautiful, high-quality reins are perfect for schooling or show.


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(866) 544-7844
Close-out sale on all inventory from Wednesday February 18th through March 15th!
ATTENTION: Please call us before placing your online order to ensure that we currently have inventory of your desired products. We are no longer accepting customized orders. All liquidation sale products are final and cannot be returned for an exchange or refund. These products are from our manufacturer closeout line. All cabinet doors on sale are brand new, but are sold "as is". There is no guarantee on color match or product warranty. Items that are damaged during shipping will be replaced upon availability.
Please note, our office and retail hours have changed. We are open Monday – Friday from 10:00am to 1:00pm. Please schedule an appointment to meet with us. We no longer accept walk-ins.


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Social Celebs: Week's Best Pics
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Watch the latest Access Hollywood Live
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May 26, 2009 07:55 PM EDT
In celebration of its first anniversary and upcoming grants announcement, Stand Up To Cancer is inviting their @SU2C Twitter followers and its celebrity Twitterers to join in a national SU2C Twitterhunt, starting Tuesday, May 26th and running until all the hidden SU2C gifts are found.
Autographed SU2C T-shirts will be hidden in more than 20 cities across the US. @SU2C on Twitter will tweet the clues, and invite celebrity friends on Twitter to join in tweeting and re-tweeting clues. The SU2C gifts will be hidden in iconic spots in cities including New York, Chicago, Miami, Los Angeles, Las Vegas, San Francisco, Seattle and New Orleans.
SU2C T-shirts signed by celebrities including Kirsten Dunst, "Good Morning America's" Robin Roberts, the cast of "30 Rock," Jennifer Aniston, Charles Barkley, Jack Black, Mekhi Phifer, Josh Groban, Tony Hawk, Chris Tucker, Josh Bell, Dana Delany, Elizabeth Berkeley, Shemar Moore, Zach Levy, Jon Favreau, Phil Ramone, Melina Kanakardes, Scarlett Johansson, Monica Mancini, Kate Bosworth, Melissa Etheridge, and NY indie-pop artist Ingrid Michaelson (whose song "Be OK" benefits SU2C).
These stars, among many others, participated in an historic simulcast September 5, 2008 on ABC, NBC, and CBS, which helped to raise over $100 million for cancer research.
On Wednesday, May 27, one year to the day since its launch, SU2C will award its first grants to multi-disciplinary, multi-institutional research teams. The formation of the SU2C Dream Teams will facilitate unprecedented levels of collaboration across the top cancer centers in the U.S. and worldwide and bring better treatments to patients, faster.
Stand Up To Cancer invites everyone in the Twitterverse to participate in the hunt by following the Twitterhunt @SU2C and re-tweeting the clues. SU2C asks its followers to include #SU2C for trending. This is where the end of cancer begins.
Copyright 2015 by NBC Universal, Inc. All rights reserved.This material may not be published, broadcast, rewritten or redistributed.


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StartUpHire is the place to find Sr. HIS Specialist jobs. Review the list of Sr. HIS Specialist career opportunities to the left. These Sr. HIS Specialist jobs represent some of the most rewarding careers available.
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Location:Remote, TN Exempt/Non-Exempt:Exempt Benefits:Full Comprehensive Benefits Package Employment Type:Full Time Department:Passport Division (Patient Access) Description:Experian Health, formerly Passport Health Communications, is a nationally recognized pioneer developing2/23/2015 Franklin, TN Save This Job
Description LumaSense Technologies Inc, is seeking an energetic, self-starter with a positive ?can do? attitude. This person will be supporting the CEO and his executive management team, as well as directing and coordinating office services and related activities. Due to the2/16/2015 Santa Clara, CA Save This Job


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Blowing Smoke
House Budget Committee chairman Rep. Paul Ryan avoided his failed bid for vice president and stuck to what he likes best — the federal budget. He defended his economic blueprint in remarks delivered at the conservative summit and accused Democrats of "blowing smoke" on the budget and creating hardship for Americans. "By living beyond our means, the government is sending a message. It's saying, 'If you plan ahead - if you make sacrifices for your kids, if you save, you're a sucker.' It is brazenly stealing from our children and from young adults. And it has to stop."
(Photo: Alex Wong/Getty Images)
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text:
I'm a easy going guy, loves to have fun. I like to joke around and just have a good time. I've lived in Nashville for a LONG time, to long if you ask me. I lived outside of this tiny town for a little bit when I moved and served in the Army. I am pretty quiet and shy though. If you wanna talk just shoot me a message
Join OkCupid
Find better matches with our advanced
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—% Match
—% Enemy
27 • Nashville, IL • Man
I'm looking for
- Women
- Ages 19–30
- Near me
- Who are single
- For new friends, long-term dating
My details
- Last online
- Yesterday – 8:03pm
- Orientation
- Straight
- Ethnicity
- White
- Height
- 6′ 0″ (1.83m)
- Body type
- A little extra
- Diet
- Mostly anything
- Smokes
- No
- Drinks
- Rarely
- Drugs
- Never
- Religion
- Other
- Sign
- —
- Education
- Working on university
- Job
- Other
- Income
- Rather not say
- Status
- Single
- Type
- —
- Offspring
- Doesn't have kids, but might want them
- Pets
- Likes dogs and likes cats
- Speaks
- English
Similar Users
My self-summary
Write a little about yourself. Just a paragraph will do.
What I'm doing with my life
Don't overthink this one; tell us what you're doing day-to-day.
Just working now, saving up a little money. Plan to finish up my degree, which is for computer hardware engineering.
I'm really good at
Go on, brag a little (or a lot). We won't judge.
Gaming, fixing computers, making people laugh, having fun.
The first things people usually notice about me
I'm an empty essay… fill me out!
How easy going I can be.
Favorite books, movies, shows, music, and food
Help your potential matches find common interests.
My favorite books are probably any excluding sappy romantic books lol. I love pretty much any move, my favorite is Monty Python and the Holy Grail. I listen to pretty much all music, I like classical just to hear the sweet sounds of a violin.
The six things I could never do without
Think outside the box. Sometimes the little things can say a lot.
Cheese pizza, friends, my car, my computer, any place but Nashville.
I spend a lot of time thinking about
Global warming, lunch, or your next vacation… it's all fair game.
Everything I can lol.
On a typical Friday night I am
Netflix and takeout, or getting your party on — how do you let loose?
Hanging out with friends or just relaxing after work.
The most private thing I'm willing to admit
I'm an empty essay… fill me out!
Lets see, I hate spiders........ a lot. Just creepy man.
You should message me if
Offer a few tips to help matches win you over.
You want to try a relationship, I don't do the whole just dating thing or playing around type stuff, or just want to talk.
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text:
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06Water Slides, Plane Wings and Danny MacAskill: This is Road Cycling Like Never Before...
0724 Reasons Why Snowboarding Was Better In The Early Noughties
08Could Scientists Be About to Build a Terminator? They're Closer Than You Think…
09Win a Holiday for 2 in Chamonix with Three #holidayspam
1017 Holiday Beer Photos That Every Skier Takes to Make Their Friends Jealous
MINI Snowpark Feldberg: Pre Contest Snowboard Parkcheckout - 25.01.2013
A Snowboarding video by qparks - 851 views
One day before the QParks Tourstop, a highly motivated crew of locals followed our call for a first serious parkshooting in the best MINI Snowpark Feldberg ever. Kristof says a few words to this years set up and the local boys shred it hard. Stay tuned for another shooting coming up soon! www.snowpark-feldberg.de
What Do You Think?
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Label: Gohard Entertainment
Bio: Yung Tae i am a young rap artist just on the rise i started rapping at the age of 10 i have always had a passion for music and my mom always played music R N B an HipHop and i use to write rhymes all day in my room just freestyling all the time everyday so one day i got into the studio at the ag...See Full Bio
New hit single "Im Ballin" check it out -Underground Records-


text:
OC3D Reviews ›› Systems
Aria.co.uk Gladiator i5 4.4GHz Z68 Bundle
Systems | 31st May 2011
Following the excellent P67-based i5 Gladiator bundle from Aria, we take a look at one based on the Z68 board. Is it just as good?
Aria.co.uk Gladiator 1155 i5 4.4GHz Bundle
Systems | 9th May 2011
Today we take a look at a pre-overclocked Core i5 bundle from Aria. Which just so happens to be the cheapest overclocked bundle in the UK at the time of going to press.
OC3D & Aria at Gadget Show Live
Systems | 12th April 2011
If you're in the Birmingham area and pass the NEC, you might spot something a little familiar.
OC3D Articles ›› Systems
Asus build 2kw Tri-Sli beast
Marc Clements | 18th June 2008
Asus have released details of the Ares, their highest powered pre-built gaming monster
MSI Release the Wind PC
Matthew Fogg | 4th June 2008
Save Energy and Reduce Noise
VIA Unveils VIA OpenBook Mini-Note Reference Design
Nic Crane | 31st May 2008
VIA today introduced the new VIA OpenBook mini-note reference design targeted at the rapidly growing global market for ultra-portable notebooks.
ASUS To Launch EBOX At Computex
Matthew Fogg | 23rd May 2008
ASUS' desktop version of the EeePC
MSI GX660R Laptop Review
Systems | 5th March 2011
Today we're taking a look at the latest high end gaming laptop from MSI. Does it go as well as it looks? I think we all hope so!
Aria Dominator 560Ti System Review
Systems | 9th February 2011
If the recent hardware releases have you salivating but you want a one-stop upgrade, Aria might have just the thing. We take a look at their Dominator 560Ti Gaming PC.
How to build a £600 Gaming PC
Systems | 13th December 2010
Can you build a modern gaming rig for around £600? That's the question we posed, and you'll be surprised at the answer.
OCUK Ultima Raptor Review
Systems | 27th April 2010
With todays release of the AMD Phenom II X6 1090T, OCUK have provided the first pre-built system. How does it fair?
ASUS K50IN Series Notebook
Systems | 15th February 2010
One of ASUS's greatest success stories has to be the EEE PC netbook, but has this success rubbed off on their full fat Notebook range?
Dell Joins The ULCPC Fray
Derrick Huey | 10th April 2008
Dell will be entering the low-cost, small-size laptop market as early as June
HP Releases Educational Mini-Notebook
Derrick Huey | 10th April 2008
HP announces a new mini-notebook geared towards use by students
Alienware now has QX9770
Nic Crane | 30th March 2008
The Alien asks for more money, but has some nice hardware
Dell has announced the first sub $1000 Blu-Ray compatible notebook.
Nic Crane | 30th March 2008
Dell is the first for cheap(ish) Blu-Ray laptop.
Eee PC Gets Some New Features
Derrick Huey | 26th March 2008
The second-generation Eee PC is set to get some handy new features
"Gecko" Mini Laptop to Retail for Under $300
Dave Stevens | 19th March 2008
Another entry to the cheap laptop market, this time at an incredibly low price.


text:
Angel Makeup
Gilded Angel Makeup Kit
The Gilded Angel Makeup Kit includes everything you need to create the illusion of an angelic beauty - glitter eyeshadow, an angel wing-like stencil, applicator and eyeliner. Let your imagination take flight!
Pearl Glitter Gel
Glam up the party, along with your Angel, Fairy, or Princess costume, in this razzle-dazzle Pearl Glitter Gel. Add shimmer to hair, skin, lips, or clothes with this safe, non-toxic and easily removable gel. Eat your heart out, Ziggy Stardust! Some glitter gel tips: For skin: Add only a small amount.…
White Metallic Hairspray
Transform your look in seconds! Spray in and wash out. This temporary, white, metallic haircolor makes a great Santa Claus or snow princess costume complete. You can also match your locks to your angel costume or dust your tresses for an old person costume! Don't forget the Halloween makeup!
White Cream Makeup
Make Up Tips The base or canvas to any serious costume makeup, this Spirit Exclusive1-ounce tube of professional White cream is a must-have. From Clown to Vampire this product's range of applications is as limitless as the imagination. Face Painting 101: Begin with a clean face and pin a towel…
Devil Character Face Kit
Make Up Tips Angel or devil, you'll do it in minutes with the Devil Character Face Kit. The Kit includes everything you need to be bad, even two devil horns! Create an amazingly evil costume with this kit and be bad on Halloween, then quickly wash off the evidence with soap and water.
Eyebrow Whitener
Make Up Tips Stop those bushy dark eyebrows from ruining a perfect disguise with this Eyebrow Whitener. Just brush the applicator stick over your eyebrows, and watch them age instantaneously! In combination with a white wig (sold separately), this will transform you into the perfect Santa or Jack…


text: http://www.linuxquestions.org – Is anyone using UUCP for mobile networking with devices and workstation that are often not connected to a network? I know it is getting easier to stay connected, but off-the-net still happens and you still want to interact with online resources. When I'm offline, I want to:request file and print services with my favorite network resources queue those requests when I get connected, run the queue of requests notify which were completed use the notification to process the results locally ... or something like that. {history lesson} In the really olde days, data communications was expensive for (HowTos)


text:
This article is part of the Seeking Alpha PRO subscriber-only research library, which covers more than 3,800 companies. Click here for more on subscribing to Seeking Alpha PRO.
Endeavour Silver: A Growing Mexican Silver Producer With Big Upside Potential
Aug. 7, 2014 1:31 PM ET | About: Endeavour Silver Corp. (EXK), Includes: GPL, SIL, SLV This article is now exclusive for PRO subscribers.


text:
Our own Jack Baruth is full of praise for Honda's CVT transmission, and it looks like the 2015 CR-V could be the next vehicle from the Big H to adopt it, replacing the 5-speed automatic gearbox.
Tag: cuv
Josh writes:
What is the deal with minivans? I was thinking the other day that as an outdoor person, minivan's are perfect. They have lots of room for people and gear, AWD (in some cases), lots of roof space, and better MPG's than an SUV. But apparently I can't own one because they're not cool. I could get a wagon though. Isn't a minivan just a super-sized wagon?
Will minivans ever be cool to own?
Here's an interesting study in how two markets, with a similar affinity for small crossovers, will get wildly different products.
With pricing for the Audi Q3 and Mercedes-Benz GLA announced, the fight for the luxury compact crossover sales crown is officially on. It's going to be the most important battle of the year for the luxury car market.
The first photos of the Buick Envision have leaked, with prices said to be ranging from $26,000-$32,000 USD.
What's obvious to us isn't always clear to the rest of the automotive world. To anyone who frequents TTAC, the upcoming Chrysler three-row CUV was destined to be built off of the minivan platform, but some other corners of the auto world didn't seem to get the memo. Chrysler brand boss Al Gardner took the time to clear that up.
This chart, courtesy of IHS Automotive, shows that for the first time in America, crossovers have edged out sedans as the most popular body style.
Buick will launch a new mid-size crossover, dubbed "Envision" in China first, then presumably in other markets. All we can say is "hurry up".
Over the past week, two separate readers have emailed regarding the Lincoln MKC. "How can the most important Lincoln in years be hitting dealer lots," asks one reader "and yet there's zero marketing behind it?"
The Jeep Grand Cherokee received a diesel option earlier this year, but don't look for the Cherokee to get one any time soon – at least not in North America.


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Schools in Pennsylvania are dealing with absentee levels of up to 16% as a result of a Mexican swine flu breakout. The Scranton School District has about 15 confirmed student H1N1 cases, Superintendent William King…
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Things to see and do - New Plymouth
Leaving for New Zealand
New Plymouth :
Unmissable tourist sites
Where to sleep?
New Plymouth Hotel tips
Brougham Heights Motel from166 NZDBook
Bks Egmont Motor Lodge from140 NZDBook
ASURE Saddle and Sulky Motor Lodge from130 NZDBook
New Plymouth Travel guide
New Plymouth : Michelin's recommendations
New Plymouth, near Egmont National Park, is the largest port in Taranaki, boasting delightful flower gardens. Take the Coastal Walkway that runs along it to see the subtle and emblematic Wind Wand, a sculpture by Len Lye. Interesting 19C buildings.
Tourist attractions New Plymouth : Things to see and do
Egmont National Park
NZL - New Plymouth
The snow-capped cone of Mount Egmont overlooks the national park of the same name. It attracts skiers, experienced hikers and mountain-climbers, and "Sunday athletes" in its foothills, making it very popular with the inhabitants of New Plym..
Govett-Brewster Art Gallery
Angle de Queen et King St. NZL - New Plymouth
Govett-Brewster Art Gallery is one of the country's main contemporary art museums. In addition to hosting about twenty exhibitions per year, its also houses the collection of Len Lye, a leading New Zealand artist, sculptor and experimental filmmaker.
Hotels New Plymouth
Carrington Motel
61 Carrington Street NZL - New Plymouth
Prix Prices from : 98 NZD
Carrington Motel offers spacious self-contained studios and apartments, each with a kitchenette and satellite TV. Guests receive 200MB of free Wi-Fi per stay. Free tea, coffee and hot chocolate are included. Carrington Motel New Plymouth is a 7-minu..
Coronation Court Motel
226 Coronation Ave NZL - New Plymouth
Prix Prices from : 102 NZD
Coronation Court Motel is just 5 minutes' drive from New Plymouth city centre. All self-contained accommodation features satellite TV with over 50 channels. Guests enjoy 1 GB of free WiFi access and free on-site parking. Each studio and apartment at..
Pukekura Motor Lodge
141 Coronation Avenue NZL - New Plymouth
Prix Prices from : 149 NZD
Featuring self-contained studios with a flat-screen TV, the 5-star Pukekura Motor Lodge is situated just 2 km from New Plymouth's city centre. Guests enjoy free parking and free use of the on-site fitness centre. Featuring modern interiors, studios ..


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Technical Papers and Presentations
Modelling and Analysis of Acoustic Emissions and Structural Vibration in a Wind Turbine
B. Marmo, and B. Carruthers
Reactec Ltd., Edinburgh, United Kingdom
The onshore wind turbine industry must overcome many technical, commercial, and environmental difficulties. A significant element for planning consent is operational noise. Acoustic limits are strictly enforced and can lead to near-neighbour complaints as well as contractual disputes. Reactec have diagnosed, analysed and solved a problematic tonal resonance for a major wind turbine generator ...
Dual Phononic And Photonic Band Gaps In A Periodic Array Of Pillars Deposited On A Membrane
Y. Pennec, Y. El Hassouani, C. Li, B.D. Rouhani, E.H. El Boudouti, H. Larabi, and A. Akjouj
Institut d'Electronique, de Microélectronique et de Nanotechnologie, Université de Lille, Villeneuve d'Ascq, France
Phononic crystals are a class of materials that exhibit periodic variations in their density and elastic properties. Such crystals modify the propagation of acoustic waves and prohibit the propagation of sound for frequencies within the band gap. We discuss the simultaneous existence of phononic and photonic band gaps in a periodic array of silicon pillars deposited on a homogeneous SiO2 ...
Ultrasound Piezo-Disk Transducer Model For Material Parameter Optimization
L. Spicci, and M. Cati
Esaote SpA, Florence, Italy
The technology involved in high performance ultrasound imaging probes needs a reliable model to help in new projects development and performance simulations. To achieve a useful model, it is necessary to use correct values for all material parameters involved in the electro-acoustical performances of the piezoelectric material, but unfortunately some of these parameters are known only with high ...
Using COMSOL to Solve for Currents along a Thin-Wire Antenna Excited by a Lumped Source
A. Shoory, A. Smorgonskiy, and F. Vega
Electromagnetic Compatibility (EMC) group, Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland
In this paper, we will present simulation results obtained using COMSOL RF module for the current along a wire antenna in both frequency and time domains. The structure is excited through a lumped (voltage or current) source at its center. For the frequency domain simulations we compare the results with the Numerical Electromagnetics Code (NEC-4) and for the time domain results we compare them ...
Modelling Flow through Fractures in Porous Media
E. Holzbecher[1], W.L. Wah[1], and M-S. Litz[2]
[1]Georg-August Universität Göttingen, Germany
[2]Freie Universität Berlin, Germany
There are various alternative options concerning modeling fluid flow within fractures in porous media. We give a general overview, with remarks concerning the modeling using COMSOL Multiphysics. Moreover we define and study two test cases for intercomparison. Finally for one of the testcases some results of an extensive sensitivity study is presented.
Deformation Behavior Of A Liquid Droplet Impacting A Solid Surface
S. Oukach[1], M. Elganaoui[1], B. Pateyron[1], and H. Hamdi[2]
[1]Laboratoire des Sciences des Procèdes Céramiques et de Traitements de Surface SPCTS, Limoges, France
[2]Laboratoire de Mécanique des Fluides et Energétique LMFE, Marrakech, Morocco
The quality of coatings obtained by means of thermal spraying depends strongly on the mechanism of the interaction between the molten droplets and the surface to be covered. The aim of the present study is to simulate the impact of a droplet onto a substrate, in order to have a good understanding of the dynamics of droplets impact. In this study, the process of droplet spreading is described; the ...
From CT Scan to Plantar Pressure Map Distribution of a 3D Anatomic Human Foot
S. Gerbino, and P. Franciosa
University of Molise, School of Engineering, Via Duca degli Abruzzi, Termoli, Italy
Understanding the stress-strain behavior of human foot tissues and pressure map distributions at the plantar interface is of interest into biomechanical investigations. In particular, monitoring plantar pressure maps is crucial to establishing the perceived human comfort of shoe insoles. A 3D anatomical detailed FE human foot model was created, starting from CT (Computer Tomography) scans of a ...
Modelling Of Magnetoelectric Effects In Magnetostrictive/Piezoelectric Multi-Layers Using A Multiphysics Simulator
F. Rasoanoavy[1], V. Laur[1], R. Smaali[2], and P. Queffelec[1]
[1]University of Brest Lab-STICC, Brest, France
[2]Clermont Université, Université Blaise Pascal, Cleremeont-Ferrand, France
In this paper, we use COMSOL, finite element software, to compute the magnetodielectric effect in a multiferroic structure based on a {ferromagnetic /piezoelectric} multi-layer. We also investigate the potential tunable applications of such structures at microwave frequencies. In a first part, we present the piezoelectric and magnetostrictive equations which were then used to develop the global ...
Numerical Modelling Of Moisture Related Mechanical Stress In Wooden Cylindrical Objects Using COMSOL: A Comparative Benchmark
H. Schellen, and J. Van Schijndel
Eindhoven University of Technology, Eindhoven, The Netherlands
For preservation of artefacts in a museum the indoor climate is often restricted to a very narrow interval for temperature, but most of all for relative humidity. In old buildings the museum conditions of artefacts, e.g. near cold walls, mostly are not in line with museum recommendations.To have an impression of indoor museum climates in old buildings, a large number of case studies were carried ...
COMSOL Assistance For The Determination Of Pressure Drops In Complex Microfluidic Channels
J. Berthier, R. Renaudot, P. Dalle, G. Blanco-Gomez, F. Rivera, P. Caillat, and V. Agache
CEA-LETI, Grenoble, France
With the current trend towards always more complexity associated to more functionalities in biotechnological systems, it is required to know with accuracy the pressure drop in the circuitry of microfluidic systems. In general, a full three-dimensional calculation is not tractable due to the limited capacity of the computers. However, computational models can help to produce pressure drop ...


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Artist: Monty Python
Album: Monty Python Sings
Genre: Comedy
Never be rude to an Arab
An Israeli or Saudi or Jew
Never be rude to an Irishman
No matter what you do
Never poke fun at a nigger
A Spik or a Wop or a Kraut
And never put
Writer(s): Terry Jones
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