Chapter 8
Switching
Switching at the physical layer in the traditional
telephone network uses the circuit-switching approach.
Figure 8.1 Switched network
Figure 8.2
Taxonomy of
switched
networks
CIRCUIT-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
Switched
Network
Circuit Establishment
Packet Switching
In a packet-switched network, there is no resource
reservation; resources are allocated on demand.
•
•
•
•
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
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
SWITCH
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).
Example 8.3
Design a three-stage, 200 × 200 switch (N = 200) with
k = 4 and n = 20.
Solution
In 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).
According to the Clos criterion:
n = (N/2)1/2
k > 2n – 1
Crosspoints ≥ 4N [(2N)1/2 – 1]
Example 8.4
Redesign the previous three-stage, 200 × 200 switch, using the Clos
criteria with a minimum number of crosspoints.
Solution
We 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.
Figure 8.19 Time-slot interchange
Time Division Switching
• 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
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