must carry 8000 bytes, or 64 Kbps. This amount of bandwidth -- called a level 0 digital signal, or DS0 -- has become the fundamental unit of connectivity in the digital phone system.
As with analog carrier lines, it doesn't make sense for the system to devote a separate pair of wires to each connection from end to end. This is where the hierarchical architecture and time-division multiplexing come in: The data streams from 24 DS0 channels can be grouped together and carried by a DS1 (also known as T1) line. The signals from multiple T1 lines can then be multiplexed into digital trunks with still more bandwidth (i.e., DS2, DS3, and so on).
To allow multiple calls to share a trunk without interfering with one another, the Bell Labs scientists developed a time-division multiplexing scheme, called the T1 frame. Each frame of data on a T1, or DS1, line consists of 1 byte of data from each of 24 DS0 channels (each carrying
a voice call or a data connection) plus a framing bit that indicates the beginning of the next frame (see the figure
"T1 Frames Manage Time-Critical Data"
).
Multiplying 193 bits (24 bytes plus the framing bit) times 8000 frames per second gives you 1.544 Mbps -- the clock rate of a T1 line. Note that this scheme is ideal for voice transmission because it's isochronous; that is, each channel is guaranteed a fixed number of regularly spaced "turns" during which its data is transmitted. (Ethernet, and other computer networking protocols, do not have this property; there's no guarantee that data will arrive on time to avoid an ugly gap in the audio signal.)
Of course, bandwidth is wasted if the party on the sending end is not saying anything or sending data. But nothing special needs to be done when all parties are talking at once. This makes the scheme simpler to manage than statistical multiplexing, which tries to make use of the extra bandwidth.
To sort out the data from
an incoming T1 signal, the receiver must determine where each frame begins and ends. But since the frames are sent back-to-back in a continuous stream, and the data itself might not provide any clue about which bits belong to which channel, the sender needs to tell the receiver the location of the frame boundary. This is the framing bit's job.
The framing bit, which the sender transmits at the end of every T1 frame, varies from frame to frame according to a predetermined pattern. The receiver knows the expected pattern and scans the rush of incoming data to see which bit (out of every group of 193) is varying the right way. (It's always possible, of course, that another bit will also happen to fit the pattern, but the probability of this becomes minuscule over a large number of frames.)
Robbing Peter to Pay Paul
Designed to eke out every last bit of bandwidth from a digital trunk, the DS1, or T1, multiplexing scheme is also able to carry a small amount of additional data (e.g.,
billing and connection information) using a technique called bit robbing. In this technique, at every sixth frame the system "borrows" the least significant bit of one of the 24 bytes of data and uses it to carry internal switching information rather than data. The human ear isn't sensitive enough to miss such a small change in the audio signal.
But of course this isn't true of digital devices. Because that last bit isn't certain to come through reliably, the effective throughput of a data channel is cut from 64 Kbps to 56 Kbps. Thus, many digital leased lines have a bandwidth of only 56 Kbps. (Some equipment used for digital leased lines has been retrofitted to disable or hide bit robbing and to allow 64 Kbps of throughput on each channel -- a facility called clear channel T1.)
Up the Hierarchy
At first, a DS0 connection -- already faster than today's fastest dial-up modems -- seemed like more bandwidth than any user could reasonably want. Use of the upper levels of the digita
l hierarchy was confined entirely to the phone company's internal equipment. But after the famous Carterfone decision, in which the courts ruled that users could connect their own equipment to the phone company's lines, businesses became interested in renting larger digital pipelines and creating their own internal phone systems and WANs.
Divestiture, the breakup of the Bell System, and the entrance of competitors into the long-distance phone market have all speeded the move toward private voice and data networks. Equipment manufacturers, such as Stratacom (which was recently acquired by Cisco Systems), leaped into the breach, providing equipment that allowed a company to run its telephone systems, WANs, or both over the same rented T1 connection.
illustration_link (32 Kbytes)
The Advantage: Each channel receives a regularly spaced frame for transmitting data to ensure there are no gaps in the audio signal.
Brett Glass is a writer, computer consultant, and teacher in Laramie, Wyoming. You can contact him at
rogue@well.com
.
How Bell Ran in Digital Communications
