11
Access Networks
An access network is a packet-switching network that provides high-speed Internet con- nectivity to homes. It is anticipated that access networks will also provide additional
services, such as voice over IP or ATM, and video on demand. Access networks have different features and requirements than LANs, MANs, and WANs. Currently, there are
two different access networks in place; one is provided over the telephone line and the other over the TV cable. New access networks, such as the ATM passive optical net-
work APON
, and Ethernet-based and wireless-based access networks are also beginning to emerge.
Telephone operators provide high-speed access to the Internet over the telephone line in addition to basic telephone services. Video on demand and voice over IP or ATM will
also be provided in the future. A family of modems known as x-type digital subscriber line xDSL
has been developed to provide high-speed access to the Internet over the telephone line. Of the xDSL modems, the asymmetric DSL ADSL is the most popular one.
Cable operators provide access to the Internet over their cable network in addition to the distribution of TV channels. In addition, voice over IP and video on demand services
over the cable have been introduced recently. The cable-based access network uses the data-over-cable service interface specification DOCSIS
. APON is a cost-effective alternative to the telephone-based and cable-based access
networks. An APON uses an optical distribution network, which consists of optical fibers and passive splitters. It can be used to provide high-speed Internet connection, voice over
IP, voice over ATM, and video on demand services. In this chapter, we describe the ADSL-based access network, the cable-based access
network, and the APON. The ADSL-based access network and the APON have been designed to support ATM and consequently they are connection-oriented networks. The
cable-based access network supports the IP network. Although the cable-based access net- work is not connection-oriented, it has been included in this chapter due to its importance
in the access network market.
11.1 THE ADSL-BASED ACCESS NETWORKS
ADSL is one of the access technologies that can be used to convert the telephone line into a high-speed digital link. It is part of a family of technologies called the x-type digital
subscriber line xDSL , where x stands for one of several letters of the alphabet and it
indicates a different transmission technique. Examples of the xDSL family technologies
Connection-oriented Networks Harry Perros
2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2
262 ACCESS NETWORKS
are: asymmetric DSL ADSL, high data rate DSL HDSL, symmetric DSL SDSL, ISDN DSL IDSL,
and very high data rate DSL VDSL. Some of the xDSL technologies use analog signaling methods to transport analog or digital information over the twisted pair,
while others use true digital signaling to transport digital information. A list of speci- fications for the xDSL family technologies is given in Table 11.1. In access networks,
downstream means from the network to the user, and upstream means from the user to
the network. These specifications are likely to change as the technology evolves. The very high data rate DSL VDSL, as its name implies, achieves very high data rates
over the twisted pair. However, the distance over which such rates can be transported is limited. Currently, it can achieve a downstream data rate of 52 Mbps and an upstream data
rate of 6 Mbps over a distance of up to 1000 feet. For the same distance, it can also provide symmetric rates of 26 Mbps downstream and 26 Mbps upstream. The longest distance it
can be transported is currently 5000 feet, for which it can achieve 13 Mbps downstream and 1.6 Mbps upstream. VDSL can be used to deliver high quality video together with
access to the Internet and regular telephone services. Because of the distance limitation, it is envisioned that it will be used to deliver information from a cabinet in the street
which is connected to an APON.
The asymmetric digital subscriber line ADSL technology utilizes the existing twisted pair from the central office to the home to transport data in addition to the basic telephone
services. It was originally designed to provide video on demand services transported over switched DS1 or E1 links. This type of traffic is referred to in the ADSL standard as the
synchronous transfer mode STM traffic. In its current standard ITU-T G.992.1 full rate
ADSL has been defined to carry either ATM or STM traffic or both. ADSL is primarily used for ATM traffic, and there is a limited number of applications for STM traffic.
As its name implies, ADSL provides asymmetrical data rates with the downstream rate being considerably higher than the upstream rate. The data rate depends on the length of
the twisted pair, the wire gauge, presence of bridged taps, and cross-couple interference. Ignoring bridged taps, currently ADSL can deliver a full DS1 or E1 signal downstream
over a single unloaded 24 gauge twisted pair for a maximum distance of 18,000 feet. Up to 6.1 Mbps is possible for a maximum distance of 12,000 feet, and 8.128 Mbps for
a maximum distance of 9000 feet. Upstream data rates currently range between 64 Kbps and 800 Kbps. The ADSL data rates and reach have recently been improved with two
new standards: ADSL2 and ADSL2+ see Section 11.1.5.
The deployment of ADSL over the twisted pair, requires an ADSL transmission unit at either end of the line. The ADSL transmission unit at the customer site is referred to
Table 11.1 xDSL maximum data rates.
xDSL type Maximum data rate
Usage Downstream
Upstream
ADSL 8.128 Mbps
800 Kbps Data
HDSL 1.544 Mbps
2.048 Mbps T1E1 replacement
SDSL 2.3 Mbps
2.3 Mbps Data
ISDL 144 Kbps
144 Kbps Data
VDSL 52 Mbps
6 Mbps Videodata
