GSM protocol stack
This article describes basics of GSM protocol stack which covers Layer-1, Layer-2 and Layer-3 modules of MS(Mobile Station),BTS(Base Transceiver Station),BSC(Base Station Controller) and MSC(Mobile Switching Center). To gain in depth knowledge one has to understand all the message formats of all the modules such as radio,LAPD,LAPDm,RRM,MM,CM,BTSM,BSSMAP,SCCP,MTP as described in 3GPP release documents. Following figure describes the GSM protocol stack at the GSM network elements.
GSM Layer 1
As mentioned in the figure, FDMA/TDMA is the air interface(radio), also called Um interface. For more on this interface refer GSM tutorial in tutorials section. At MS, FDMA/TDMA is used which is also followed at BTS, BTS takes this format from MS and convert it to 64kbps digital format for the digital link and interfaces with BSC. BSC communicates with MSC in the same format.
GSM Layer 2
Layer 2 is the data link layer, which does following three main functions.
- Establish,maintain and tear down the link
- Flow control
- Work on the Layer-3 frames
At Layer-2 LAPD and LAPDm is used. LAPD is the ISDN(Integrated Services Digital Network) protocol for D Channel. LAPDm is the modified version of LAPD for mobile station. LAPDm does not have CRC for Error detection. This layer uses any of the following format to carry frames. 1.)Format A for DCCHs(for channels having no information field) 2.)Format B for DCCHs(containing an information field) 3.)Format Bbis for BCCH, PCH, and AGCH. 4.)Format C for random access signals The maximum LAPDm frame length is 23 bytes i.e. 184bits.
Depending on type of frame format LAPDm will have Address field(8 bits), Control field(8 bits), Frame Length(8 bits),signaling data(23 octets) and fill in data. Address field carry two important parameters C/R and SAPI. C/R indicates whether the frame is command or response and also mention whether the direction of frame is BS to MS or from MS to BS. SAPI takes either value of 0,3 or other values. For SAPI of 0 is used for messages from the RRM,MM and CC , and SAPI of value 3 for message from the SMS and Supplementary Services (SS) messages.
Control field of this layer-2 contains sequence numbers and type field to differentiate various frames. There are three type of frames supported here, supervisory,unnumbered information transfer and control function( unacknowledged mode), numbered information transfer(multiframe acknowledged mode). The frame length field contains the length of the layer 3 message within the information field of the LAPDm frame. If the message is less than the length specified in parameter N201 (standard specifies this) of the radio interface, fill-in data octets are used to fill up the remaining gap. Value of fill-in data is specified in GSM TS document.
LAPD at BTS converts potentially unreliable physical link of MS into reliable link.This connects with BSC's MTP part. This is done with the use of CRC and ARQ techniques. ARQ stands for Automatic Repeat Request. ARQ works on the principle of re-transmission of packet when the erroneous packet is received at the receiver.
GSM protocol stack can be explored by studying deep into protocol layers at various network elements viz. MS, BTS, BSC and MSC.
GPRS Protocol stack
Refer GPRS protocol stack for more information.