The tyranny of the multitude is a multiplied tyranny.
To move beyond the DS0 into higher-bandwidth realms, additional layers of multiplexing are needed. This chapter describes how DS0s are bundled into DS1. DS1 refers to a digital signal operating at 1.544 Mbps; T1 refers specifically to a DS1 delivered over a four-wire interface. Most people simply use the term T1 to refer to any digital signal at that speed and, to avoid breaking the common convention, so does this book.
Higher levels of multiplexing are used to generate further levels of the T-carrier hierarchy, such as DS3. DS3 is different from T1, though, because the much higher speed requires different encoding methods, far more precise timing, and new network-to-router interfaces. To avoid getting lost in DS3 details, therefore, this chapter details only the DS0 to DS1 multiplexing process.
Assembling higher-speed links in the T-carrier system is conceptually easy. Take a collection of lower-speed links and bundle them together as channels in a TDM framework. When 24 DS0 streams are bundled together, the result is a higher-level digital stream: DS1. Multiple DS1s are bundled together to form DS2s, and DS2s are tied together into DS3s. Table 4-1 shows the standardized data rates in the T-carrier system. While the data rate for DS4 was standardized, most of the network interface details were not.
Table 4-1. T-carrier comparison
|
Stream type |
Speed (Mbps) |
Equivalent T1s |
Equivalent voice channels |
|---|---|---|---|
|
DS0 |
0.064 |
1/24 |
1 |
|
DS1 |
1.544 |
1 |
24 |
|
DS1C |
3.152 |
2 |
48 |
|
DS2 |
6.176 |
4 |
96 |
|
DS3 |
44.736 |
28 |
672 |
|
DS3C |
89.472 |
56 |
1344 |
|
DS4 |
274.176 |
168 |
4032 |
Due to differences in control bits and framing bits in different levels of the hierarchy, discrepancies may appear to exist. For example, DS1 is composed of 24 DS0 channels, each operating at 64 kbps, but 24 x 64 kbps = 1.536 Mbps. DS1 adds extra framing and control information that pushes the line rate to 1.544 Mbps, even though only a maximum of 1.536 Mbps is available for user data.
Within the T-carrier hierarchy, DS1C and DS3C never existed as services that could be ordered. The data rates corresponding to DS1C and DS3C are intermediate multiplexing rates that could exist in a digital cross-connect. DS2 was never widely deployed, and DS4 was never thoroughly standardized.
At the receiving end of a T1 bit stream, some method is required for distinguishing where one channel stops and another channel starts. Each channel could be individually framed with a unique header, but such an approach would add a great number of header bits and use too much of the transmission capacity of the carrier simply for control information. Instead, T1 transmits each channel in turn and adds a single framing bit at the beginning, as shown in Figure 4-1. Framing bits are used to synchronize clocks and for rudimentary error indication.
Multiple frames can be aggregated together into superframes. A complete header is constructed by using the framing bit from each frame and concatenating bits over several frames.
Tip
Use of the terms frame and superframe is different here than in the data world. I think of a frame as an entity that has a complete header, not a twelfth or a twenty-fourth of the header. If it had been up to me, I would call a succession of 192 data bits plus a framing bit a subframe, and assemble subframes into frames. Thus, my subframe is what the telco would call a frame, and my frame is what the telco calls a superframe. I obviously did not have any influence over the terms, though, so we are stuck with frame and superframe; I use the industry standard terms because the usages are entrenched.
When you first plug in a T1 cable, an alarm indicator will still complain about the lack of framing. Loss-of-framing warnings will persist for a short while as the CSU/DSU searches for the framing bits, locks on, and synchronizes its clock. Synchronization is a pretty amazing process: the CSU/DSU must identify the 193rd bit position in each frame and then identify the start of the framing sequence. After synchronization is complete, bits can be transmitted to the remote end, which locks on to the transmitted frames and clears any remote alarm conditions.
Because clock synchronization is maintained by monitoring pulse times, the T1 specifications mandate a certain pulse density so that both ends stay synchronized. Two minimums are imposed by the standards:
Overall, a 12.5% ones density.
A maximum of 15 consecutive time slots without a pulse. Modern digital repeaters can handle much longer strings of zeros, but this requirement was instituted well before such equipment was available. There is no easy way of knowing the capability of repeaters on any stretch of cable, so all commercial equipment is still built to the older specification.
When a T1 is installed, the telco technician may hook a handheld device with lots of buttons and blinking lights on it up to the new T1 jack. The testing device can perform stress tests on the new line to measure its quality and clarity. After looping back the T1, the testing device injects a specific bit stream. Returning bits are compared against the original sequence to determine the bit error rate (BER). Several common tests are used:
- QRS tests
These tests use a quasi random signal specified in ANSI T1.403. The QRS bit sequence is not guaranteed to meet pulse-density requirements. On B8ZS-encoded links, CSU/DSUs will perform zero substitution and the transmitted signal will meet pulse-density requirements without a problem. AMI-encoded links should not alter the signal to meet minimum pulse-density requirements, but misconfigured equipment on the line may be configured to do so. Errors observed in the QRS test indicate one of two things: a line that is bad, or a line that has misconfigured equipment. One common source of misconfiguration is related to pulse stuffing on the CSU/DSUs or other line components. Some components can be configured to “stuff” pulses to ensure minimum pulse density; the stuffed pulses alter digital data and are not acceptable. Poor QRS test performance may occur because pulse stuffing is mistakenly enabled on the CSU/DSU or other line equipment.
- 1-in-8 test
B8ZS-encoded links use this test, which transmits a framed sequence of bytes in which the second bit is a one (01000000). 1-in-8 is a stressful test for B8ZS links because it contains repeated strings of seven consecutive zeros, which is the longest duration of inactivity on a line that is allowed by the B8ZS line code. B8ZS zero substitution may occasionally occur with a 1-in-8 test pattern. The last channel transmitted in a frame has six consecutive zeros, and the first data bit in the first channel of the next frame is a zero. If the intervening frame bit is a zero, zero substitution will occur.
- All-ones test
Both B8ZS- and AMI-framed links may use this test, which transmits a continuously framed sequence of ones. This test checks for ringing and crosstalk. Ringing occurs when transmitted signals reflect off boundaries and the reflections bounce back and interfere with later transmitted signals. Crosstalk occurs when the cable pairs are not separated correctly, so that the transmitted signal induces a similar signal in the receive pair.
Get T1: A Survival Guide now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.
