11 g mixed mode Secondly, mixed mode now also implies that the MAC will be simultaneously loaded with time-bounded (periodic) and best-effort traffic. This will have a significant impact on throughput and capacity. Taking these MAC overheads into account, Table 3.13 shows typical throughputs for TCP/IP best-effort data and/or UDP throughput. The table is from an Atheros White Paper and includes their proprietary bonded (40 MHz) channel solution giving a max throughput of 108 Mbps. Note that time-bounded services would normally use UDP (unacknowledged datagram protocol) rather than TCP (with transmission retries). SIP (session-initiation protocol) places an additional bandwidth overhead on the UDP throughput of approximately 8 kilobytes every time a voice, video or voice and video session starts or is modified. 3.6.3 Mobility, Handover and Power Control Overheads The above also excludes any measurement and signalling overheads introduced by the need to support mobility. As long as a user stays in one place then these overheads can be avoided. This may be/probably is the case for Skype laptop users but may not/probably will not be case for people expecting to use WiFi voice from their mobile broadband device either at a public access hot spot or on a corporate, SOHO or home wireless LAN. Mobility overheads include the need to do measurement reporting (802.11k), the use of mea- surement reporting to manage handover algorithms (802.11f) and/or the use of measurement reporting to manage per packet power control (802.11h). Most transceivers can now collect RSSI (received signal strength indication) at a per packet level. Given that the channel is reciprocal (same RF channel on the uplink and downlink) it is Table 3.13 MAC overheads when supporting TCP or UDP Max TCP Max UDP 802.11 b 11 Mbps g plus b 54 Mbps g only 54 Mbps 54 Mbps a turbo 108 Mbps Making Telecoms Work Table 3.14 Power control dynamic range in 802.11. Reproduced by permission of California EDU Power in dBm Power in milliwatts/microwatts 100 milliwatts 50