What's Cooking With T1 Bandwidth?
By Chris Lewis
T1 technology has become a staple in the diet of network managers deploying WAN technologies. But its ubiquitousness doesn't mean it's bland: T1 comes in several flavors to suit different diets. For example, you can order a T1 between two locations to deliver a single channel with 1.536-Mbps throughput; a channelized T1 to connect a central site
to 24 remote locations, with each channel providing 56- or 64-Kbps throughput; a T1 to deliver an ISDN Primary Rate Interface (PRI); or a fracti
onal T1 service to deliver bandwidth in 64-Kbps steps from 128 Kbps and up between two locations.
To further confuse the issue, ordering T1 isn't as simple as just asking for T1. For instance, sometimes a T1 line will be listed as having 1.544-Mbps bandwidth; other times, 1.536 Mbps. Also, the size of a T1 channel is 64 Kbps,
but often it's delivered only as 56 Kbps. Finally, sometimes T1 is referred to DS-1.
The Magic Numbers
In the beginning, there was digital transmission of voice communications. Then data networking came along and piggybacked the existing technology in the voice network infrastructure. This is the origin of the 64-Kbps channel as the base building block for data WAN technology.
To transport a voice signal that is analog by nature over a digital medium, an analog-to-digital (A/D) conversion must be performed. Two variables need to be defined for an A/D conversion--the sampling rate and the number of bits used to represent signal amplitude. The highest frequency trans
mitted for voice communications is 4 KHz. However, a law in A/D conversion states that in order to recreate an analog wave from a digital stream of data, you need to sample the analog wave at twice the rate of the highest frequency you want to recreate. Twice 4 KHz is 8 KHz, which gives us a sampling rate of 8,000 times a second.
To accurately represent the amplitude of an analog wave, assign it a value that can be represented by 8 bits of data (see "Representing Analog Waveforms With Digital Data,"). To represent 4 KHz in digital, we generate 8 bits 8,000 times per second--which equals the magic 64,000 bits per second for a voice channel.
In digital voice networking, this basic 64-Kbps channel is termed a DS-0. The next step up is a DS-1, which is a collection of 24 DS-0 channels. A DS-1 delivered on a copper wire is termed a T1. This nomenclature has become so popular that people now refer to any 1.536-Mbps link as a T1. Although not str
ictly correct, the term T1 is accepted for any kind of link with this amount of bandwidth.
The next highest bandwidth commonly delivered to users is a DS-3, often referred to as a T3. Again, T3 is specific to transmission over copper wires. A DS-3 connection is a collection of 672 DS-0 circuits, which gives a total throughput of 43,008 Kbps. The actual circuit spe
ed is somewhat faster, but some effective bandwidth is lost to synchronization traffic.
What happened to a DS-2? A DS-2 consists of 4 DS-1 channels; seven DS-2 channels make a DS-3. However, DS-2 service is not commonly available.
Why is the throughput for a T1 line often listed at 1.544 Mbps? A T1 always has a bandwidth of 1.544 Mbps, but 8 Kbps of that bandwidth is never available. It is lost to housekeeping tasks, such as tracking which packets belong to which channel. Therefore, the effective usable bandwidth of a T1 circuit is 1.536 Mbps.
Digital Communications
Let's look at using a regular point-to-point T1 connection
, possibly as part of a backbone WAN or as a high-speed LAN-to-LAN connection on a campus. Typically, the telephone company will deliver the T1 on an RJ connector, to which you attach a CSU/DSU and a router.
This is the simplest way to deploy a T1. You configure the appropriate settings in the CSU/DSU and then connect it to the router via a V.35 DTE cable. In this configuration, the router takes its clock signal from the CSU/DSU and sees one link with an effective bandwidth of 1.536 Mbps.
|
REPORTS
ANALYTICS
Follow 
