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• Rate adaptation: Telephone wires are bundled together in multi-pair binders, containing 25 or more twisted wire pairs. As a result, electrical signals from one pair might electro-magnetically couple onto adjacent pairs in the binder. This phenomenon, called crosstalk, can impede the data rate of ADSL. ADSL2 addresses this problem by seamlessly adapting the data rate. Specifically, the receiver monitors the link’s signal-to-noise ratio (SNR), and if it determines that a data rate change is necessary, then it sends a message to the transmitter. The transmitter then sends a Sync Flag signal, which is used as a marker to designate the exact time at which the new data rate is to be used. The Sync Flag signal is detected by the receiver, and the transmitter and receiver both change to the new data rate.
• Bonding for higher data rates: Data rates can be significantly increased by bonding multiple phone links together. ADSL2 supports the ATM Forum’s inverse ATM multiplexer standard, which was developed for transporting ATM traffic over multiple low-speed links. This standard permits ADSL2 to bind two or more twisted pairs to a single ADSL connection, which results in higher downstream data rates.
In addition to the above features, ADSL2 has a channelization capability, whereby it can split the bandwidth into different channels for different applications. ADSL channelization capability provides support for Channelized Voice over DSL (CVoDSL). Specifically, ADSL2 reserves a number of subchannels (tones) in the upstream and downstream spectrum for the transport of PCM 64-Kbps voice data (see Figure 11.8).
With the current ADSL standard, voice can be transmitted over IP (VoIP) by encapsulating the IP packets in PPP and subsequently transmitting the PPP frames over AAL5/ATM/ ADSL. Voice can also be carried over ATM (VoATM) by transmitting the voice packets over AAL2/ATM/ADSL. The protocol stacks for VoATM and VoIP are shown in Figure 11.9. VoIP and VoATM is in contrast with CVoDSL which is transmitted directly over the ADSL physical layer, without having to use any higher-level protocols. Recall that POTS is transmitted directly over the twisted pair using subchannels 1 to 6.
THE ADSL-BASED ACCESS NETWORKS
64 Kbps DS0
Figure 11.8 Channelized voice over DSL (CVoDSL).
Figure 11.9 CVoDSL, VoATM, VoIP.
Figure 11.10 The ADSL2+ downstream bandwidth.
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
1.1MHz, 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.
11.2 THE CABLE-BASED ACCESS NETWORK
A cable network architecture consists of the headend, multiple optical fiber trunks extending 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 MAN/WAN 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 transmission. 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.