Limited availability

When customers of the PSTN make telephone calls, they commonly make use of a telecommunications network called a switched-circuit network. In a switched-circuit network, devices known as switches are used to connect the caller to the callee. Each switch has a number of inlets and outlets and by connecting a specific inlet to the correct outlet each switch helps to complete an end-to-end circuit between users [1] .

In a modern circuit-switched network, switches can connect any inlet to any outlet; this is known as Full Availability.

Due to previous technological challenges, older switches can only connect some inlets to some outlets; this is known as Limited Availability [2, 3] . In Limited Availability Switches, the circuits inside the switch are arranged into Grading groups [1] . An example of a Limited Availability Switch can be seen in "Figure 1".

"Figure 1" depicts five users whose calls are being routed through this switch. The switch contains eight inlets and 15 outlets. Not all inlets can connect to any of the outlets. The Grading Scheme divides the 15 outlets into eight individuals, four pairs, two quads and one common. To improve efficiency, incoming calls are assigned to the first available outlet, such as outlets 1, 6 and 7. If another call is received on the same inlet, then that call is assigned to the first available outlet further away. An example of this occurs in outlets 5 and 11, where the first caller was assigned to outlet 5, but the second caller was assigned to the next available outlet viz. 11. It should be noted that a particular inlet can only receive a maximum of four calls as there are only four available outlets to each inlet. This assumes that these outlets are not already in use by another inlet. Thus, this Switch has an availability of four (columns) [1] .

References

[1] Kennedy I., Lost Call Theory, Lecture Notes, ELEN5007: Teletraffic Engineering, School of Electrical and Information Engineering, University of the Witwatersrand, 2005.

[2] Akimaru H., Kawashima K., Teletraffic: Theory and Applications, Springer-Verlag London, 2nd Ed., 1999, p 6.

[3] Farr R.E., Telecommunications Traffic, Tariffs and Costs: An Introduction For Managers, Peter Peregrinus, 1988, p 90.