STRUCTURE OF A SWITCH

We use switches in circuit-switched and packet switched networks. In this section, we discuss the structures of the switches used in each type of network.

Topics discussed in this section:

• Structure of Circuit Switches
• Structure of Packet Switches

Structure of Circuit Switches

Circuit switching today can use either of two technologies: the space-division switch or the time-division switch.

Space-Division Switch

In space-division switching, the paths in the circuit are separated from one another spatially.

This technology was originally designed for use in analog networks but is used currently in both analog and digital networks.

Time-Division Switch

Time-division switching uses time-division multiplexing (TDM) inside a switch. The most popular technology is called the time-slot interchange (TSI).

Crossbar Switch

A crossbar switch connects n inputs to m outputs in a grid, using electronic microswitches (transistors) at each crosspoint.

The major limitation of this design is the number of crosspoints required.

To connect n inputs to m outputs using a crossbar switch requires n x m crosspoints.

Such a switch is also inefficient because statistics show that, in practice, fewer than 25 percent of the crosspoints are in use at any given time. The rest are idle.

Crossbar switch with three inputs and four outputs

Multistage Switch

• The solution to the limitations of the crossbar switch is the multistage switch, which combines crossbar switches in several (normally three) stages.
• In a single crossbar switch, only one row or column (one path) is active for any connection. So we need N x N crosspoints.
• If we can allow multiple paths inside the switch, we can decrease the number of crosspoints. Each crosspoint in the middle stage can be accessed by multiple crosspoints in the first or third stage.

Multistage switch

Note

In a three-stage switch, the total

number of crosspoints is

2kN + k(N/n)2

which is much smaller than the number of

crosspoints in a single-stage switch (N2).

Drawback

• The multistage switch has one drawback-blocking during periods of heavy traffic:
• Blocking refers to times when one input cannot be connected to an output because there is no path available between them-all the possible intermediate switches are occupied.
• In a single-stage switch, blocking does not occur because every combination of input and output has its own crosspoint; there is always a path.
• Clos investigated the condition of nonblocking in multistage switches and came up with the following formula. In a nonblocking switch, the number of middle-stage switches must be at least 2n - 1. In other words, we need to have k >2n - 1.

Note

According to the Clos criterion:

n = (N/2)1/2

k > 2n – 1

Crosspoints ≥ 4N [(2N)1/2 – 1]

Time-Division Switch

• A multistage switch that uses the Clos criteria and a minimum number of crosspoints still requires a huge number of crosspoints.
• For example, to have a 100,000 input/output switch, we need something close to 200 million crosspoints (instead of 10 billion). The number can be reduced if we accept blocking.
• Time-division switching uses time-division multiplexing (TDM) inside a switch. The most popular technology is called the timeslot interchange (TSI).

Time-Slot Interchange

• A TSI consisting of random access memory (RAM) with several memory locations.
• The size of each location is the same as the size of a single time slot. The number of locations is the same as the number of inputs.
• The RAM fills up with incoming data from time slots in the order received.Slots are then sent out in an order based on the decisions of a control unit.

Time-slot interchange

Time- and Space-Division

Switch Combinations

We combine space-division and time-division technologies to take advantage of the best of both. Combining the two results in switches that are optimized both physically (the number of crosspoints) and temporally (the amount of delay). Multistage switches of this sort can be designed as time-space-time (TST) switch.

Time-space-time switch

Packet switch components

Input port

Output port

A banyan switch

Examples of routing in a banyan switch