Virtual Circuits

All communications across a data network are carries out over previously established virtual circuits. These can be set up either on demand by user or permanently by network management. With on-demand connections, the user device sends a request for a switched virtual connection (SVC) to be set up to a central control unit known as the Signaling Control Point (SCP).

The SCP then determines the availability of both the required destination and the transmission bandwidth appropriate to call across the network.

With Permanent Virtual Circuits (PVC), the route is already determined when demand is made. The PVC is a logical predefined path or link through a carrier network.

Value Added Network

A Value Added Network (VAN) is a private network provider (sometimes called a turnkey communication line) that is hired by a company to facilitate electronic data interchange. The VAN services are such as legal research or access to a specialised database, for a fee.

Telephone Traffic Definitions

The estimating of the switching quantities for an automatic telephone exchange is based primarily on the amount of traffic which it will be required to handle. Calculations of the probable average, and maximum amount of traffic are necessary to determine the optimum number of switches required in the various switching ranks.

The more important quantities used in traffic calculations are:

Traffic Unit (T.U): The traffic unit is used to measure the traffic flow. The number of traffic units carried by a group of switches is numerically equal to the product of the number of calls per hour and the average duration of the calls in hours. The traffic flow in traffic units is equal to the average number of simultaneous calls.

Busy Hour: This is defined as the hour of the day in each exchange when the originating traffic carried is the greatest.

Busy Hour Calling Rate: This is the average number of calls initiated per subscriber during the busy hour.

Grade of Service: The flow of traffic initiated by subscribers varies widely throughout the day, and from one day to the next. If during any traffic peak the number of outlets from a switching stage are all occupied, additional calls at that instant cannot be extended and completed and will thus be lost. The proportion of calls that are lost to the total number of calls in the busy hour determines the quantity known as the grade of service for that particular switching stage.

Availability: The number of trunks to which a switch has access is known as the availability. If a subscriber’s uniselector has, say 25 outlets, each of which is coupled to the first selector, then the availability of the uniselector is 25

Full-Availability Models

A full-availability system is one in which there are S sources of traffic and N outlets and any free source can seize any free outlet, regardless of the state of the system. The simplest example is a matrix with SXN cross-points as shown below.

Figure 16: SXN Matrix

Source: J. Dunlop and D. G. Smith (1998). Telecommunications Engineering

Probability of Blocking

In order to find out if the system will perform as it should, some measure of performance is required. The most often used measure is the probability of blocking, which is the proportion of calls, in the long term that are rejected. If we consider the general case of a two stage link system in which the corresponding outlets on the second-stage switches form a route as indicated in the diagram below.

Figure 17: 2 Stage Link System

Source: J. Dunlop and D. G. Smith (1998). Telecommunications Engineering

There are two ways in which a call may be blocked.

1. There may be on free route circuit

2. There may be no path available between the inlet carrying the incoming call and the free route circuits.
   

The diagram shows both of these conditions; route 3 has no free circuits available, whereas if inlet A on first-stage switch 1 is wanting to be connected to route 2, on which there are free route circuits, it cannot be, because there is no available link.