Praise for Fundamentals of WiMAX

264 Chapter 7 • Networking and Services Aspects of Broadband WirelessTable 7.5 Gains Achievable Through Header CompressionMinimumProtocol Type Header Size (bytes)Compressed HeaderSize (bytes)Bandwidth Savings(%)IPv4/TCP 40 4 90IPv4/UDP 28 1 96.4IPv4/UDP/RTP 40 1 97.5IPv6/TCP 60 4 93.3IPv6/UDP 48 3 93.75IPv6/UDP/RTP 60 3 95Header compression uses the concept of flow context, a collection of information aboutstatic and dynamic fields and change patterns in the packet header. This context is used by thecompressor and the decompressor to achieve maximum compression. The first few packets of aflow are sent without compression and are used to build the context on both sides. The numberof initial uncompressed packets is determined based on link BER and round-trip time. Usingperiodic feedback about link conditions, the amount of compression can also be varied. Once acontext is established, compressed packets are sent with a context identifier prefixed to it.Several header-compression techniques have been developed over the years [15, 19, 31]. Wediscuss only one of them, robust header compression (ROHC) [10], which is supported inWiMAX. ROHC is a more complex technique, but works well under conditions of high BERand long round-trip times and can reduce the header size to a minimum of 1 byte. An extensibleframework for compression, ROHC can be used on a variety of headers, including IP/UDP/RTPfor VoIP and IP/ESP for VPN.At the beginning of a flow, a static update message that contains all the fields not expectedto change such as IP source and destination address, is sent. Dynamic fields are sent uncompressedin the beginning and when there is a failure. Otherwise, dynamic fields are sent compressed,using a window-based least-significant bits encoding. ROHC includes an error-recoveryprocess at the decompressor, as shown in Figure 7.18. A CRC that is valid for the uncompressedheader is sent with each compressed header. If the CRC fails after decompression, the decompressortries to interpolate from the previous headers the missing data and checks again. This istried a few times; and if unsuccessful, a context update is requested. The compressor then sendsenough information to fix the context. This error-recovery mechanism is what makes ROHCcompression scheme robust. ROHC is widely recognized as a critical piece of any wireless IPnetwork, and the IETF has a charter dedicated to continually making additions and enhancementsto ROHC [53].One negative consequence of using header compression over the air link is that the bandwidthrequirements of a particular application become different over the air and in the rest of the