rection. The downstream transmission originates at the headend node and is transmitted to all end-users located at the tips of the branches in the tree and branch network. An upstream transmission originates from an end-user terminal and reaches the headend node through a multipoint-to-point access network where the access medium is shared by all end-users that are communicating with the same headend. This is the network shown in Figure 17.18. Here we see a multiple of 5- to 42-MHz channels that are frequency-division multiplexed in the fiber node (FN) and that are 17.7 TWO-WAY VOICE AND DATA OVER CATV SYSTEMS BASED ON THE DOCSIS 2.0 SPECIFICATION Other WANs Master Headend Distribution ring Headend Fiber trunk Fiber node Coax trunk Trunk splitter Coax feeder Fiber serving area Bidirectional Home drop amplifier End-user Figure 17.18 A pictorial overview of the DOCSIS networks and their relationship with the outside world. then transmitted by a single fiber trunk to the CATV headend. This operation is called frequency stacking. 17.7.3 Layer 2 - Data-Link Layer The data-link layer consists of two sublayers: the media access control (MAC) and the IEEE 802.2 Logical Link Control (LLC). The MAC sublayer, besides conventional MAC functions, provides the additional mechanisms that are necessary for sharing the cable. These include: COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) Ranging to compensate for different cable losses SO that the CMTS receives all CM signals at the same level Ranging to compensate for different cable delays Assignment of frequencies to the CM Allocation of time slots (TDMA) for upstream data flow 17.7.4 Layer 3 and Above Standard TCP/IP protocols are used for the network layer and above. This ensures Internet compatibility over the cable network. 17.8 SUBSPLIT/EXTENDED SUBSPLIT FREQUENCY PLAN The majority of CATV systems, particularly in North America, are upgraded to sub- split8 (5-30 MHz) or extended subsplit (5-42 MHz) operation. This scenario represents the worst-case design in terms of ingr