Instruction manual

DS1 Facility Interface
When digital signals are transmitted over a network of digital communications links, switching
nodes, multiplexers, and transmission interfaces, all entities must be synchronized. This is
referred to as network synchronization.
For synchronous transmission,
information is transmitted to the transmission facility at a
fixed rate. Each bit occupies a fixed unit time interval. All significant transitions must
correspond to multiples of the fixed unit time interval.
Message and frame synchronization
are achieved by using special characters at the beginning and end of the message and by
knowing the number of bits contained in each frame.
If the average transmit rate is faster than the average receive rate, the buffer of the receiving
unit will overrun.
If the average receive rate is faster than the average transmit rate, the
buffer of the receiving unit will underrun.
It is necessary to control the overruns, (frame
deletions) and underruns (frame repetitions) and, when necessary, only allow an overrun or
underrun in 1-frame increments. The deletion or repetition of a single frame is termed a slip
or a controlled slip.
Controlling the slip rate is accomplished by synchronizing the clocks associated with the
switching equipment so that all transmissions have the same average line rate. The AT&T
digital network synchronization is based on a hierarchy of clocks. Four strata are used with
stratum 1 clock being the most accurate (+/- 0.00001 ppm) and stratum 4 clock the least
accurate (+/- 32 ppm). The stratum 1 clock is the AT&T Reference Frequency clock. AT&T
4ESS™ toll switches maintain stratum 2 clocks while AT&T 5ESS® central office digital
switches and Digital Access and Cross Connects maintain stratum 3 clocks. The System 25
meets accuracy requirements for a stratum 4 clock. If possible, the System 25 should be
synchronized to a higher stratum clock.
T1 Facility Network Connections
The System 25 can be connected to T1 transmission facilities with endpoints on private
networks and public networks.
Private Network Connections
Private network endpoints include connections to other PBXs, or computers which use
internal modems, and off-premises stations.
AT&T Digital PBX Endpoints
The T1 facility can provide digital tie trunk service between System 25, System 75, System
85, and DEFINITY™ Communications System, Generic 1 and Generic 2. The PBXs can be
co-located or geographically disbursed.
When co-located, any combination of framing,
signaling, and line coding can be used. However, each PBX must be administered the same.
If the PBXs are separated, the carrier facility providing the T1 facility can place limitations on
the permitted options. In either case, when two or more PBXs are connected together, one
must be selected as the timing master and the others are administered as timing slaves.
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