Flashcards Collections. Documents Last activity. Add to Add to collection s Add to saved. T1 Multiplexing 2 Under the simplest conditions, a medium can carry only one signal at any moment in time.
For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth. Synchronous time division multiplexing, and 2. An advantage to T1 connections is that they are able to maintain and provide a reliable connection that remains constantly accessible and readily available when data transmission is needed.
A T1 connection line relies on 2 wire pairs that serve separate functions: 1 data transmission, 2 data receiving. A multiplexer is similar to a digital switch that accepts 24 individual channels and has a singular multiplexed output connected to the data transmission line. These digital signals are multiplexed over different time slots, enabling several channels to be carried on the same line. This is why each channel is also called a timeslot. Each channel that goes into the TDM, is initially created from analog signals that are encoded and translated via a PCM.
This data stream is carried at a rate of 64 Kbps. Most of us are familiar with a normal business or residential line from the phone company. A normal phone line like this is delivered on a pair of copper wires that transmit your voice as an analog signal.
When you use a normal modem on a line like this, it can transmit data at perhaps 30 kilobits per second 30, bits per second. The phone company moves nearly all voice traffic as digital rather than analog signals. Your analog line gets converted to a digital signal by sampling it 8, times per second at 8-bit resolution 64, bits per second. Nearly all digital data now flows over fiber optic lines, and the phone company uses different designations to talk about the capacity of a fiber optic line.
If your office has a T1 line, it means that the phone company has brought a fiber optic line into your office a T1 line might also come in on copper.
For example, a kbps FT1 is a T1 with only 4 of the 24 time slots active. 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:. 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:. The QRS bit sequence is not guaranteed to meet pulse-density requirements.
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. When used on AMI-encoded links, the 3-in test sends the framed pattern , which meets both the minimum pulse density and maximum zero length, and stresses the link by sending the lowest density signal allowed by the specification.
B8ZS-encoded links use this test, which transmits a framed sequence of bytes in which the second bit is a one B8ZS zero substitution may occasionally occur with a 1-in-8 test pattern.
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