Networking: A Beginner's Guide
Surprisingly, DS1 requires only two twisted-pairs, not fiber-optic cable or anything
exotic. (For details on how much data can be carried over simple telephone wire, see the
preceding section on DSL.)
DS1 connections are commonly used as digital connections between a company's
PBX and a point of presence (POP) for a long-distance telephone carrier. They are also
commonly used to connect LANs to the Internet. A DS1 connection can handle up to 24
voice calls or as many as 24 data connections simultaneously. Or, using a multiplexer
and a DS1, you can form one big 1.544 Mbps connection.
A popular technology called fractional T-1 also exists, where a full DS1 is installed,
but only the number of channels you pay for are turned on and available for use.
Fractional T-1 is great because you can buy just the bandwidth you need, and
increasing the bandwidth (up to the maximum for a DS1) is just a phone call (and some
more money!) away.
DS0, DS1, and DS3 WAN connections use frame-relay signaling technology on the
RBOC's side of the connection. Understanding the ins and outs of frame relay isn't especially
important, although you should be aware that when you install a DSx connection to the Internet for
your LAN, you are really using frame-relay services.
At your end of a DS1 connection are two key pieces of equipment: a CSU/DSU that
converts the DS1 signals into network signals, and a router that directs data between
the DS1 and the LAN.
Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode, commonly called just ATM, is a very high-speed
technology for transmitting data between locations. ATM is a multiplexed, cell-based
networking technology that collects data into entities called cells and then transmits the
cells over the ATM network connection.
ATM networks can carry both voice and data. ATM is very fast, with speeds
ranging from 155 Mbps to 622 Mbps, and in some cases can go as high as 10 Gbps.
Usually, ATM is used only by relatively large companies that need ATM's speed
for their WAN links, or by companies that need to send enormous amounts of data
through a network connection, such as a lot of video data.
X.25 connections have been available for a long time, but they are not typically used for
WAN connections because of the overhead involved. Also, the trade-off between price
and bandwidth is not competitive with other solutions. Some older networks might
still have X.25 connections in place, however, and they were commonly used in Europe.
X.25 is a packet-switched WAN connection, in which data travels through an X.25
cloud, which works similarly to the Internet but uses a private/public X.25 network.
X.25 connections are typically relatively slow (56 Kbps), but might be faster.