Access Network Categories

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Today, I would like to share with you an article about Access Network Categories.

Figure 1-1: Access networks

The above figure shows the access networks' links highlighted in red.

Access networks can be loosely divided into three categories:

· Residential access networks, connecting a home end system into the network;

· Institutional access networks, connecting an end system in a business or educational institution into the network;

· Mobile access networks, connecting a mobile end system into the network

These categories are not hard and fast; some corporate end systems may well use the access network technology that we ascribe to residential access networks, and vice versa. Our descriptions below are meant to hold for the common (if not every) case.

Residential Access Networks

A residential access network connects a home end system (typically a PC, but perhaps a Web TV or other residential system) to an edge router. Probably the most common form of home access is using a modem over a POTS (plain old telephone system) dialup line to an Internet service provider (ISP). The home modem converts the digital output of the PC into analog format for transmission over the analog phone line. A modem in the ISP converts the analog signal back into digital form for input to the ISP router. In this case, the "access network" is simply a point-to-point dialup link into an edge router. The point-to-point link is your ordinary twisted-pair phone line. Today's modem speeds allow dialup access at rates up to 56 Kbps. However, due to the poor quality of twisted-pair line between many homes and ISPs, many users get an effective rate significantly less than 56 Kbps.

While dialup modems require conversion of the end system's digital data into analog form for transmission, so-called narrowband ISDN technology (Integrated Services Digital Network) allows for all-digital transmission of data from a home end system over ISDN "telephone" lines to a phone company central office. Although ISDN was originally conceived as a way to carry digital data from one end of the phone system to another, it is also an important network access technology that provides higher speed access (e.g., 128 Kbps) from the home into a data network such as the Internet. In this case, ISDN can be thought of simply as a "better modem".

Dialup modems and narrowband ISDN are already widely deployed technologies. Two new technologies, Asymmetric Digital Subscriber Line (ADSL) and hybrid fiber coaxial cable (HFC) are currently being deployed. ADSL is conceptually similar to dialup modems: it is a new modem technology again running over existing twisted pair telephone lines, but can transmit at rates of up to about 8 Mbps from the ISP router to a home end system. The data rate in the reverse direction, from the home end system to the central office router, is less than 1 Mbps. The asymmetry in the access speeds gives rise to the term "Asymmetric" in ADSL. The asymmetry in the data rates reflects the belief that home users are more likely to be a consumer of information (bringing data into their homes) than a producer of information.

ADSL uses frequency division multiplexing, as described in the previous section. In particular, ADSL divides the communication link between the home the ISP into three non-overlapping frequency bands:

o a high-speed downstream channel, in the 50 KHz to 1 MHz band;

o a medium-speed upstream channel, in the 4 KHz to 50 KHz band;

o and an ordinary POTs two-way telephone channel, in the 0 to 4 KHz band.

One of the features of ADSL is that the service allows the user to make an ordinary telephone call, using the POTs channel, while simultaneously surfing the Web. This feature is not available with standard dailup modems. The actually amount of downstream and upstream bandwidth available to the user is a function of the distance between the home modem and the ISP modem, the gauge of the twisted pair line, and the degree of electrical interference. For a high-quality line with negligible electrical interference, an 8 Mbps downstream transmission rate is possible if the distance between the home and the ISP is less than 3,000 meters; the downstream transmission rate drops to about 2 Mbps for a distance of 6,000 meters. The upstream rate ranges from 16 Kbps to 1 Mbps.

While ADSL, ISDN and dailup modems all use ordinary phone lines, HFC access networks are extensions of the current cable network used for broadcasting cable television. In a traditional cable system, a cable head end station broadcasts through a distribution of coaxial cable and amplifiers to residences. As illustrated in Figure 1-2, fiber optics connect the cable head end to neighborhood-level junctions, from which traditional coaxial cable is then used to reach individual houses and apartments. Each neighborhood juncture typically supports 500 to 5000 homes.

Figure 1-2: A hybrid fiber-coax access network

As with ADSL, HFC requires special modems, called cable modems. Companies that provide cable Internet access require their customers to either purchase or lease a modem. Typically, the cable modem is an external device and connects to the home PC through a 10-BaseT Ethernet port. Cable modems divide the HFC network into two channels, a downstream and an upstream channel. As with ADSL, the downstream channel is typically allocated more bandwidth and hence a larger transmission rate. For example, the downstream rate of the CyberCable system is 10 Mbps and the upstream rate is 768 Kbps. However, with HFC (and not with ADSL), these rates are shared among the homes, as we discuss below.

One important characteristic of the HFC is that it is a shared broadcast medium. In particular, every packet sent by the headend travels downstream on every link to every home; and every packet sent by a home travels on the upstream channel to the headend. For this reason, if several users are receiving different Internet videos on the downstream channel, actual rate at which each user receives its video will be significantly less than downstream rate. On the other hand, if all the active users are Web surfing, then each of the users may actually receive Web pages at the full downstream rate, as a small collection of users will rarely receive a Web page at exactly the same time. Because the upstream channel is also shared, packets sent by two different homes at the same time will collide, which further decreases the effective upstream bandwidth. Advocates of ADSL are quick to point out that ADSL is a point-to-point connection between the home and ISP, and therefore all the ADSL bandwidth is dedicated rather than shared. Cable advocates, however, argue that a reasonably dimensioned HFC network provides higher bandwidths than ADSL. The battle between ADSL and HFC for high speed residential access has clearly begun.

Enterprise Access Networks

In enterprise access networks, a local area network (LAN) is used to connect an end system to an edge router. There are many different types of LAN technology. However, Ethernet technology is currently by far the most prevalent access technology in enterprise networks. Ethernet operates 10 Mbps or 100 Mbps (and now even at 1 Gbps). It uses either twisted-pair copper wire are coaxial cable to connect a number of end systems with each other and with an edge router. The edge router is responsible for routing packets that have destinations outside of that LAN. Like HFC, Ethernet uses a shared medium, so that end users share the the transmission rate of the LAN. More recently, shared Ethernet technology has been migrating towards switched Ethernet technology. Switched Ethernet uses multiple coaxial cable or twisted pair Ethernet segments connected at a "switch" to allow the full bandwidth an Ethernet to be delivered to different users on the same LAN simultaneously.

Mobile Access Networks

Mobile access networks use the radio spectrum to connect a mobile end system (e.g., a laptop PC or a PDA with a wireless modem) to a base station, as shown in Figure 1-1. This base station, in turn, is connected to an edge router of a data network.

An emerging standard for wireless data networking is Cellular Digital Packet Data (CDPD). As the name suggests, a CDPD network operates as an overlay network (i.e., as a separate, smaller "virtual" network, as a piece of the larger network) within the cellular telephone network. A CDPD network thus uses the same radio spectrum as the cellular phone system, and operates at speeds in the 10's of Kbits per second. As with cable-based access networks and shared Ethernet, CDPD end systems must share the transmission media with other CDPD end systems within the cell covered by a base station. A media access control (MAC) protocol is used to arbitrate channel sharing among the CDPD end systems.

The CDPD system supports the IP protocol, and thus allows an IP end system to exchange IP packets over the wireless channel with an IP base station. A CDPD network can actually support multiple network layer protocols; in addition to IP, the ISO CNLP protocol is also supported. CDPD does not provide for any protocols above the network layer. From an Internet perspective, CDPD can be viewed as extending the Internet dialtone (i.e., the ability to transfer IP packets) across a wireless link between a mobile end system and an Internet router.