 a network density capital and operational cost. The user's data duty cycle (time between battery recharge) will reduce as well. These caveats aside, multiplexing in its many forms does produce an efficiency gain, but this will not always be quite as much as claimed by the vendor community. The efficiency gain can be substantial if we are tolerant to delay and delay variability or can access delivery bandwidth in periods of low demand. For example our business offices are situated in a residential area from which most people commute every day. Day-time contention rates on our ADSL lines are low, evening contention rates are high, which suits us just fine. Standardised Description Frameworks - OSI Seven-Layer Model as a Market and Business Descriptor All of the above suggests a need for a standardised description framework that can be used as the basis for analysing how different functions in a network interact together and the coupled effects that determined delivery cost and delivery value. Such a framework already exists and has been in use since 1983. 6 Known as the Open Systems Interconnection Model, this seven-layer structure remains as relevant today as it has ever been and encapsulates the topics addressed in all subsequent parts and chapters in this book. (See Table 1.1.) There are two immediately obvious ways to approach an analysis using this model, to start at the top and work downwards or to start at the bottom and work upwards. Most industry 6 http://www.tcpipguide.com/free/t_HistoryoftheOSIReferenceModel.htm. Making Telecoms Work Table 1.1 OSI seven-layer model Description Example protocol or layer function Layer 7 Application Layer Windows or Android Layer 6 Presentation Layer HTML or XML Layer 5 Session Layer Reservation protocols Layer 4 Transport Layer TCP prioritisation protocols Layer 3 Network Layer IP address protocols Layer 2 Data Link Layer ATM and Ethernet Medium Access Control Layer 1 Physical layer Fibre, cable, copper, wireless including free-space optical, infrared, ultraviolet and ra