Chapter 8Switching

Switching at the physical layer in the traditional telephone network uses the circuit-switching approach.

Figure 8.2 Taxonomy of switched networks

Figure 8.1 Switched network

CIRCUIT-SWITCHED NETWORKSCIRCUIT-SWITCHED NETWORKS• A circuit-switched network consists of a set of switches

connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.

• has three phases– Establish– Transfer– Disconnect

• inefficient– channel capacity dedicated for duration of connection– if no data, capacity wasted

• set up (connection) takes time• once connected, transfer is transparent

Figure 8.3 A trivial circuit-switched network

A circuit-switched network is made of a set of switches connected by physical links, in which each link is divided into n channels.

Public Circuit Public Circuit Switched Switched NetworkNetwork

Circuit EstablishmentCircuit Establishment

Packet Switching

• circuit switching was designed for voice• packet switching was designed for data• transmitted in small packets• packets contains user data and control info

– user data may be part of a larger message– control info includes routing (addressing) info

• packets are received, stored briefly (buffered) and past on to the next node

In a packet-switched network, there is no resource reservation; resources are allocated on demand.

Packet Switching

Packet Switching Datagram Approach

Packet Switching Virtual Circuit Approach

Blocking or Non-blocking

• blocking network– may be unable to connect stations because

all paths are in use– used on voice systems

• non-blocking network– permits all stations to connect at once– used for some data connections

8-4 STRUCTURE OF A 8-4 STRUCTURE OF A SWITCHSWITCH

Figure 8.17 Crossbar switch with three inputs and four outputs

Circuit Switch

Elements

Figure 8.18 Multistage switch

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).

Design a three-stage, 200 × 200 switch (N = 200) with k = 4 and n = 20.

SolutionIn the first stage we have N/n or 10 crossbars, each of size 20 × 4. In the second stage, we have 4 crossbars, each of size 10 × 10. In the third stage, we have 10 crossbars, each of size 4 × 20. The total number of crosspoints is 2kN + k(N/n)2, or 2000 crosspoints. This is 5 percent of the number of crosspoints in a single-stage switch (200 × 200 = 40,000).

Example 8.3

According to the Clos criterion: n = (N/2)1/2

k > 2n – 1 Crosspoints ≥ 4N [(2N)1/2 – 1]

Redesign the previous three-stage, 200 × 200 switch, using the Clos criteria with a minimum number of crosspoints.

SolutionWe let n = (200/2)1/2, or n = 10. We calculate k = 2n − 1 = 19. In the first stage, we have 200/10, or 20, crossbars, each with 10 × 19 crosspoints. In the second stage, we have 19 crossbars, each with 10 × 10 crosspoints. In the third stage, we have 20 crossbars each with 19 × 10 crosspoints. The total number of crosspoints is 20(10 × 19) + 19(10 × 10) + 20(19 ×10) = 9500.

Example 8.4

Figure 8.19 Time-slot interchange

• modern digital systems use intelligent control of space & time division elements

• use digital time division techniques to set up and maintain virtual circuits

• partition low speed bit stream into pieces that share higher speed stream

• individual pieces manipulated by control logic to flow from input to output

Time Division Switching

Figure 8.20 Time-space-time switch

Figure 8.21 Packet switch components

Figure 8.22 Input port

Figure 8.23 Output port

Figure 8.24 A banyan switch

Figure 8.25 Examples of routing in a banyan switch

Figure 8.26 Batcher-banyan switch

In Channel Signaling

• Use same channel for signaling and call– Requires no additional transmission facilities

• Inband– Uses same frequencies as voice signal– Can go anywhere a voice signal can– Impossible to set up a call on a faulty speech path

• Out of band– Voice signals do not use full 4kHz bandwidth– Narrow signal band within 4kHz used for control– Can be sent whether or not voice signals are present– Need extra electronics– Slower signal rate (narrow bandwidth)

Drawbacks of In Channel Signaling

• Limited transfer rate• Delay between entering address (dialing) and

connection• Overcome by use of common channel signaling

Common Channel Signaling

• Control signals carried over paths independent of voice channel

• One control signal channel can carry signals for a number of subscriber channels

• Common control channel for these subscriber lines• Associated Mode

– Common channel closely tracks interswitch trunks

• Disassociated Mode– Additional nodes (signal transfer points)

– Effectively two separate networks

Common v. In Channel Signaling

Common Channel Signaling Modes