Wide Area Systems & Services - What's Cooking With T1 Bandwidth?

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 fractional 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 transmitted 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," below).

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 strictly 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 speed is somewhat faster, but some effective bandwidth is lost to synchronization traffic.

What happened to a DS-2? A DS-2 consists of 28 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.

There are two key setup parameters for the CSU/DSU in this configuration. The first is to define the line code as Alternate Mark Inversion (AMI) or Bipolar 8 Zero Substitution (B8ZS). The former is a standard related to Dataphone Digital Service, the oldest data service still available that uses the 64-Kbps channel for data. This service gives you only 56 Kbps of available throughput. The additional 8 Kbps is not available for data transfer and is used to ensure synchronization between the two ends of the DDS circuit by the AMI line-encoding mechanism.

If you buy a T1 with 24 channels, each of them loses 8 Kbps—a 192-Kbps problem. You need a way to regain that lost bandwidth. The solution is to use a smarter encoding technique that can maintain synchronization without the loss of 8 Kbps on each channel. In many locations, 64-Kbps lines are available through the use of B8ZS encoding, which replaces AMI. This 64-Kbps service is known as Clear Channel Capability or Clear 64.

On a practical level, all you need to make sure of in your CSU configuration is that it has AMI encoding for 56-Kbps channel services and B8ZS for 64-Kbps channel service when selecting the line-encoding options.

The second configuration you must define is the T1 frame format, which is usually Extended Super Frame (ESF). Occasionally the telephone company may define the frame type as D4 framing—older implementations used the Super Frame format. Whatever it is, your telephone company will let you know.

Checking the Channels

AMI and B8ZS cover synchronization within the channel. But if that T1 circuit uses time-division multiplexing (TDM) to put 24 channels on one four-wire circuit, how is the beginning of the T1 rotation marked? And how do we identify which channel is which? This is where ESF comes in. It identifies the first channel in the 24-channel rotation (see "ESF Framing for TDM," below).

Using a channelized T1 to connect a central site to multiple remote locations is a little different than the point-to-point case. Previously, the implementation of multiple WAN connections at a central site meant each line had its own dedicated CSU/DSU device and physical router port. Now, with more sophisticated devices, such as Cisco Systems CT1 card, a channelized T1 (which is plugged into the CT1 card) can be used to supply 24 individual channels, each of which can be terminated in an individual circuit in a different geographic location. The benefit of this arrangement is that there are no CSU/DSU devices or associated cabling at the central site.

Given that the T1 connects directly to the router in this case, some additional configuration is necessary for the router. The T1 controller in the router must be configured for ESF framing and B8ZS line code. Once this is done, there should be 24 64-Kbps channels that the telephone company can "groom" out to up to 24 locations, typically using a piece of equipment called a Digital Access Cross Connect (DACC). At the central site router, the 24 channels appear as virtual interfaces on the one physical line; each virtual interface can receive its own configuration as if it were a separate physical connection.

If we go with this arrangement and one of the locations we want to connect to the central site is serviced only by 56-Kbps lines using AMI, what do we do? Well, the good news is that you have configured the central site to cope with a 64-Kbps connection, and as long as you configure the channel connected to the 56 Kbps appropriately, all will be well. The telco will use AMI encoding for the remote end of the channel and B8ZS for the central site end of the channel; all you have to do is enter the correct command for your brand of router to tell it that the channel will only receive or send data at 56 Kbps. This does not affect the operation of other channels that are connected at 64 Kbps through to the remote site.

In fact, the Cisco default is for a channel to be set up for 56 Kbps throughput, the "speed 64" command must be entered manually for a channel (represented as a subinterface in the Cisco configuration) to get it to work at 64-Kbps throughput.

A channelized T1 can also be used as an efficient way to deliver analog phone services, but to do this, an additional piece of equipment, a channel bank, is needed to convert the digital T1 signals to 24 analog telephone lines. This can be useful if you need to configure many centrally located dial-up ports, for roving users with analog modems.

The Cisco AS-5200 router has a built-in channel bank and modems so that just by connecting a single T1 connector to it, you can have up to 24 modem calls answered simultaneously. The AS-5200 also has a built-in T1 multiplexer, giving it hybrid functionality with regard to call answering. If you connect a T1 configured as a PRI to an AS-5200, the AS-5200 will autodetect if the incoming call is from an ISDN or analog caller, and answer the call with the appropriate equipment.

ISDN Anyone?

With ISDN, each channel is usually 64 Kbps. However, we have the same concerns regarding 56-Kbps channels in areas where Clear 64 is not supported. Some additional setups on a router need to be completed in order for it to use a PRI service. On a T1 delivering a PRI service, 23 channels are delivered as "B" channels for data, and one of the 24 channels is reserved for Q.931 signaling, the "D" channel.

To configure PRI services, you have to set the router to use the correct ISDN switch type and service provider identification (SPID) number, both of which will be supplied by the telephone company.

Using a T1 service to deliver some level of bandwidth between a DS-0 and a DS-1 has become popular with the availability of the fractional T1 services. If you order a 128-Kbps or 256-Kbps line, a T1 circuit with only the appropriate number of DS-0 channels is installed. Typically this fractional T1 service is terminated in a CSU/DSU that presents the appropriate clock rate to the router DTE interface. However, just as a full T1 can be connected directly to a router, so can a fractional T1. In this case, the only additional piece of router configuration you need is to identify the channels that will be active in the router configuration.

One final note on T1 technology. The T1 configuration is peculiar to the United States and Japan. If your network starts to grow internationally, you should be aware that in Europe and most other countries, multiple DS-0 services are delivered on an E1, which comprises 32 DS-0 channels, giving 2,048-Kbps throughput. The only real difference this makes to router and CSU/DSU configurations is that the E1 uses High Density Bipolar 3 (HDB3) line coding instead of B8ZS. AMI is not used internationally.

Chris Lewis is vice president of international operations at ILX Systems. He can be reached at chrisl@ilx.com.

Copyright © 1997 CMP Media Inc., 600 Community Drive, Manhasset, NY 11030.

Reprinted from Network Computing with permission.

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