4.4 Time Delay Switching
The writing and reading, of all time-slots in a frame, has to be completed within one frame time period (before the start of the next frame). A TS of incoming PCM may, therefore, get delayed by a time period ranging from 1 TS to 31 TS periods, before being transmitted on outgoing PCM. For example, consider a case when TS6 of incoming PCM is to be switched to TS5 in outgoing PCM. In this case, switching can be completed in two consecutive frames only, i.e., 121 microseconds for a 32 channel PCM system. However, this delay is imperceptable to human beings.
4.5 Non-Blocking feature of a Time Switch
In a Time Switch, there are as many memory locations in the control and speech memories as there are time-slots in the incoming and outgoing PCM highways, i.e., corresponding to each time-slot in incoming highway, there is a definite memory location available in the speech and control memories. Similarly, corresponding to each time-slot in the outgoing highway there is a definite memory location available in the control and speech memories. This way, corresponding to free incoming and outgoing time-slots, there is always a free path available to interconnect them. In other words, there is no blocking in a time switch.
4.6 Two Dimensional Switching
Though the electronic cross points are not so expensive, the cost of accessing and selecting them from external pins in a Space Switch becomes prohibitive as the switch size increases. Similarly, the memory location requirements rapidly go up as a Time Switch is expanded, making it uneconomical. Hence, it becomes necessary to employ a number of stages, using small switches as building blocks to build a large network. This would result in the necessity of changing both the time-slot and highway in such a network. Hence, the network, usually, employs both types of switches viz., space switch and time switch, and. therefore, is known as a two-dimensional network. These networks can have various combinations of the two types of switches and are denoted as TS, STS, TSST, etc.
Though to ensure full availability, it may be desirable to use only T stages. However, the networks having the architecture of TT, TTT, TTTT, etc., are uneconomical, considering the acceptability of tolerable limits of blocking, in a practical network. Similarly, a two-stage two-dimensional network, TS or ST, is basically suitable for very low capacity networks only. The most commonly used architecture has three stages, viz., STS or TST. However, in certain cases, their derivatives, viz., TSST, TSSST, etc., may also be used.
An STS network has relatively simpler control requirements and hence, is still being favored for low capacity networks, viz., PBX exchanges. As the blocking depends mainly on the outer stages, which are space stages, it becomes unsuitable for high capacity systems.
A TST network has lesser blocking constraints as the outer stages are timed stages that are essentially non-blocking and the space stage is relatively smaller. It is, therefore, most cost-effective for networks handling high traffic, However, for still higher traffic handling capacity networks, e.g., tandom exchanges, it may be desirable to use TSST or TSSST architecture.
The choice of a particular architecture is dependent on other factors also, viz., implementation complexity, modularity, testability, expandability, etc. As a large number of factors favor the TST structure, it is most widely used.
4.7 TST Network
As the name suggests, in a TST network, there are two-time stages separated by a space stage. The former carries out the function of time-slot changing, whereas the latter performs highway jumping. Let us consider a network having n input and n output PCM highways. Each of the input and output time stages will have n time switches and the space stage will consist of an n x n cross point matrix. The speech memory, as well as the control memory of each time switch and each column of a control memory of the space switch, will have m locations, corresponding to m time-slots in each PCM. Thus, it is possible to connect any TS in I/C PCM to any TS in O/G PCM.
In the case of local exchange, the network will be of the folded type, i.e., the O/G PCM highways, via a suitable hybrid. Whereas, for a transit exchange, the network will be non-folded, having complete isolation of I/C and O/G PCM highways. However, a practical local exchange will have a combination of both types of networks.
For the sake of explanation, let us assume that there are only four I/C and O/G PCM highways in the network. Hence, there will be only four-time switches in each of the T-stages and the space switch will consist of a 4×4 matrix. let us consider an objective of connecting two subscribers through this switching network of local exchange, assuming that the CC assigns TS4 on HWY0 to the calling party and TS6 on HWY3 to the called party
The speech samples of the calling party have to be carried from TS4 of I/C HWY 0 and to TS6 of O/G HWY3 and those of the called party from TS6 of I/C HWY 3 to TS4 of O/G HWY 0, with the help of the network. The CC establishes the path, through the network in three steps. To introduce greater flexibility, it uses an intermediate time-slot, TSx, which is also known as internal time-slot. The three switching steps for transfer of speech sample of the calling party to the called party are as under:
Step 1 Input Time Stage (IT) TS4 HWY0 to TSx HWY0
Step 2 Space stage (S)Tsx HWY0 to Tsx HWY3
Step 3 Output Time Stage (OT)Tsx HWY3 to TS6 HWY3
As the message can be conveyed only in one direction through this path, another independent path, to carry the message in the other direction is also established by the CC, to complete the connection. Assuming the internal time-slots to be TS10 and TS11, the connection may be established as shown in fig 6.
Let us now consider the detailed switching procedure making some more assumptions for the sake of simplicity. Though practical time switches can handle 256 time-slots in parallel mode, let us assume serial working and that there are only 32 time-slots in each PCM. Accordingly, the speech and control memories in time switches and control memory columns in the space switch will contain 32 locations each.