DS1: The Ever-Popular T1
The T1, in typical American trunk line voice/data networking terminology, is the bulwark of the phone system. Prior to 1983, T1 lines were available to Bell Telephone only internally, but with the breakup of AT&T, these lines became available to the public.
The bit rate of a T1 line is 1.544Mbps, 1.536Mbps of which is available for payload; the remaining 8Kbps is used for framing. A T1 is the aggregate of 24 64Kbps DS0s.
Although the T1 line consists of only two sets of twisted pairs (one transmit and one receive, upon which there can be zero bridge taps), data is transmitted as though the T1 were actually physically made up of a group of 24 DS0s (referred to now as channels) that are time-domain multiplexed, meaning that data is first sent to one logical DS0, then to the second, and so on, based upon the clock sequence.
The same basic rules of the 56Kbps DDS apply to the T1. It uses the same return to zero bipolar pulse stream that the 56Kbps DS0 uses.
Each T1 channel is like a line of soldiers marching in a parade. A drill instructor at the telco can use a DS splitter to have some of the solders march off of their T1 and join another unit (this happens frequently when several partial T1s are combined). To make sure that all the data arrives at the destination at the right time and in the right order, it's imperative not only that each DS1 be synchronized with its own channels, but also that each be synchronized with the channels of all the other DS1s. The synchronization must be consistent throughout the entirety of the telco. If each of the troops is marching to a different drummer, none of them will arrive at the same time, and their formation will suffer, troops will get lost, or they'll show up after the battle or before the food. For this reason, a single clock in Hillsboro, MO, pounds out a 1.544MHz drum beat that all the DS1 traffic in the country listens to, sending in as nearly perfect synchronization with it as is machinely possible.
The T1 Framing Format: Two Standards
This is the reserved 8Kbps control overhead that turns your 1.544Mbps T1 into a 1.536Mbps line. There are two framing format standards—D4 (superframe) and extended superframe.
Superframe (D4)
The control overhead in a D4-configured T1 circuit is purely to provide frame synchronization. A dozen frames make up a "superframe." The framing bits line up in a particular pattern (100011011100). The recipient uses this known pattern to discern which 8 bits belong to which channel.
Extended Superframe (ESF)
One thing vendors never seem to grasp when naming technology (perhaps because sales departments end up naming products) is that you should never call anything "super" or "advanced," or anything else with a similarly inflated moniker, because a year later it's going to be yesterday's trash. Superframe was soon replaced by extended superframe, which added some new features. ESF has error correction in the form of cyclic redundancy check (CRC6—specifically, the 6-bit version; CRC32 in TCP/IP is normally 32-bit). Additionally, extended superframe allocates 4Kbps of the overhead as a facility data link. The facility data link gives the phone company the capability to interrogate the customer premises equipment for error statistics and line performance to aid in predicting and preventing line outages.