Bearer Channels THE ADSL-BASED ACCESS NETWORKS

THE ADSL-BASED ACCESS NETWORKS 271 KHz . . . . . . POTS Upstream Downstream . . . 64 Kbps DS0 Figure 11.8 Channelized voice over DSL CVoDSL. IP PPP ATM AAL2 IP PPP ATM Customer Premises Network Baseband POTS ADSL Physical layer VoIP VoATM CVoDSL POTS AAL5 AAL2 AAL5 Figure 11.9 CVoDSL, VoATM, VoIP. 1.1 MHz . . . . . . POTS Upstream Downstream . . . . . . ADSL2+ ADSL2 2.2 MHz KHz 0.14 MHz Figure 11.10 The ADSL2+ downstream bandwidth. ADSL2+ ADSL2+ doubles the downstream bandwidth thereby increasing the downstream data rate on telephone lines shorter than 5000 feet. ADSL and ADSL2 use a bandwidth of 1.1 MHz, whereas ADSL2+ specifies a bandwidth of 2.2 MHz see Figure 11.10. This results in a significant increase in the downstream speed on short lines. 272 ACCESS NETWORKS

11.2 THE CABLE-BASED ACCESS NETWORK

A cable network architecture consists of the headend, multiple optical fiber trunks extend- ing from the headend, and coaxial cables. The headend transmits the TV channels which are distributed to the homes over the cable network. Each fiber trunk extending from the headend terminates at an optical network unit ONU. From the ONU, a number of coaxial cables fan out into the neighbourhood, each serving a number of homes see Figure 11.11. Typically, about 500 homes are served by the same optical fiber. Due to the combination of fiber optics and coaxial cables, this architecture is known as the hybrid fiber coaxial HFC architecture. High-speed access to the home is provided over an HFC plant using the data-over- cable service interface specification DOCSIS. This specification was developed by Cable Television Laboratories CableLabs for the cable industry in North America, Europe, and other regions. It is also applicable to older all-coax cable TV plants. DOCSIS permits a transparent bidirectional transfer of IP traffic between the cable system’s headend and the homes. This is realized using a cable modem termination system CMTS at the headend, and a cable modem CM at each home see Figure 11.12. The CMTS is a packet switch that is equipped with network interfaces and interfaces to the data-over-cable system. The network interfaces are used to communicate with one or more MANWAN networks to which it is connected, and the interfaces to the data-over-cable system are used to transmit and receive data from the CMs over the HFC cable network. The maximum distance between the CMTS and a CM is 100 miles, but it is typically limited to 10 to 15 miles. The cable network is a shared-medium tree-like network with analog two-way trans- mission. In the downstream direction, the cable network operates in the range of 50 MHz to 864 MHz. Within this range multiple analog television signals are transmitted in 6-MHz channels, as well as other narrowband and wideband digital signals. In the upstream direction, the cable network operates between 5 MHz and 42 MHz. Within this passband, analog television signals in 6-MHz channels as well as other signals might be present. In the reference architecture shown in Figure 11.12, we show a single interface to the WANMAN network, and a single interface to the data-over-cable access network. Data that is transmitted to the CMs is modulated onto a carrier; it is then multiplexed with all of the television signals and the other signals in the downstream direction. The resulting signal is transmitted out on the optical fiber which terminates at the ONU, and from there it is distributed to all of the homes attached to the coax cables that fan out from the ONU. The data stream transmitted by the CMTS is extracted by each CM, from where it extracts the IP packets destined to it. On the upstream side, each CM transmits IP packets towards the CMTS. These packets are modulated on a Cable TV headend Optical fiber ONU Coaxial cable Figure 11.11 The hybrid fiber coaxial HFC architecture. THE CABLE-BASED ACCESS NETWORK 273 CMTS … Data Video Video Tx Rx … ONU Fiber Coax CM CM CM MANWAN network Data Figure 11.12 The DOCSIS reference architecture. CMTS DS TC DOCSIS MAC Link security 802.2 LLC Bridge 802.3 MAC 802.2 LLC IP CM 802.3 10Base TC Cable PMD DOCSIS MAC Link security 802.2 LLC Bridge Data link layer IP PHY layer Cable PMD Cable PMD US cable PMD Cable network Figure 11.13 The protocol stacks of CMTS and CM. carrier ranging from 5 MHz to 42 MHz. Multiple carriers can also be used. A specially designed MAC, referred to as the DOCSIS MAC, assures that there are no collisions in the upstream direction. Both CM and CMTS support the IP protocol and other IP-related protocols. The pro- tocol stacks of the CMTS and CM are shown in Figure 11.13. The protocol stack of the CM at the data-over-cable interface, i.e., at the side of the cable network, consists of the physical layer, the DOCSIS MAC protocol, the link security layer, and the IEEE 802.2 LLC. The physical layer consists of the transmission convergence TC sublayer and the cable physical medium dependent PMD sublayer. The TC sublayer is only present in the downstream direction DS – that is, from the CMTS to the CM. It is not present in the upstream direction US – that is, from the CM to CMTS. The DOCSIS MAC protocol controls the upstream transmission of the CMs, and provides QoS and other fea- tures. The CM communicates with the customer premises equipment CPE via Ethernet. The CM could also be part of a router that forwards packets to CPEs using MPLS label switching or classical IP-based forwarding. The protocol stack of the CMTS at the data-over-cable interface i.e., at the side of the cable network is similar to that of the CM’s protocol at the side of the cable network. The stack of the CMTS on the interface to the MANWAN network i.e., at the network