Circuit switching

Circuit switching

Circuit switching has been the dominant technology for both voice and data communications.

Communication via circuit switching implies that there is a dedicated

communication path between two stations. That path is a connected sequence

of links between network nodes. On each physical link, a channel is dedicated to

the connection. The most common example of circuit switching is the telephone

network.

1. Circuit establishment. Before any signals can be transmitted, an end-to-end

(station-to-station) circuit must be established. For example, station A sends a

request to node 4 requesting a connection to station E. Typically, the link from A

to 4 is a dedicated line, so that part of the connection already exists. Node 4 must

find the next leg in a route leading to E. Based on routing information and measures

of availability and perhaps cost, node 4 selects the link to node 5, allocates

a free channel (using frequency division multiplexing, FDM, or time division

multiplexing,TDM) on that link, and sends a message requesting connection to

E. So far, a dedicated path has been established from A through 4 to 5. Because

a number of stations may attach to 4, it must be able to establish internal paths

from multiple stations to multiple nodes. How this is done is discussed later in

this section.The remainder of the process proceeds similarly. Node 5 dedicates a

channel to node 6 and internally ties that channel to the channel from node 4.

Node 6 completes the connection to E. In completing the connection, a test is

made to determine if E is busy or is prepared to accept the connection.

2. Information transfer. Information can now be transmitted from A through the

network to E. The transmission may be analog voice, digitized voice, or binary

data, depending on the nature of the network. As the carriers evolve to fully integrated

digital networks, the use of digital (binary) transmission for both voice and

data is becoming the dominant method. The path is A-4Iink, internal switching

through 4, 4-5 channel, internal switching through 5,5-6 channel, internal switching

through 6, 6-E link. Generally, the connection is full duplex, and signals may

be transmitted in both directions simultaneously.

3. Circuit disconnect. After some period of information transfer, the connection

is terminated, usually by the action of one of the two stations. Signals must be

propagated to nodes 4,5, and 6 to deallocate the dedicated resources.

Note that the connection path is established before data transmission begins.

Thus, channel capacity must be reserved between each pair of nodes in the path and

each node must have available internal switching capacity to handle the requested

connection. The switches must have the intelligence to make these allocations and

to devise a route through the network.

Circuit switching can be rather inefficient. Channel capacity is dedicated for

the duration of a connection, even if no data are being transferred. For a voice connection,

utilization may be rather high, but it still does not approach 100%. For a terminal-

to-computer connection, the capacity may be idle during most of the time of

the connection. In terms of performance, there is a delay prior to signal transfer for

call establishment. However, once the circuit is established, the network is effectively

transparent to the users. Information is transmitted at a fixed data rate with

no delay other than the propagation delay through the transmission links. The delay

at each node is negligible.

Circuit switching was developed to handle voice traffic but is now also used for

data traffic. The best-known example of a circuit-switching network is the public

telephone network (Figure 3.4). This is actually a collection of national networks

interconnected to form the international service. Although originally designed and

implemented to service analog telephone subscribers, it handles substantial data

traffic via modem and iswel1 on its way to being converted to a digital network.

Another well-known application of circuit switching is the private branch exchange

(PBX), used to interconnect telephones within a building or office. Circuit switching

is also used in private networks. Typically, such a network is set up by a corporation

or other large organization to interconnect its various sites. Such a network usually

consists of PBX systems at each site interconnected by dedicated, leased lines

obtained from one of the carriers, such as AT&T. A final common example of the

application of circuit switching is the data switch. The data switch is similar to the

PBX but is designed to interconnect digital data processing devices, such as terminals

and computers.

A public telecommunications network can be described using four generic

architectural components:

1II Subscribers: The devices that attach to the network. It is still the case that most

subscriber devices to public telecommunications networks are telephones, but

the percentage of data traffic increases year by year.

1II Subscriber line: The link between the subscriber and the network, also

referred to as the local loop. Almostall subscriber line connections use twisted

pair wire. The length of a subscriber line is typically in a range from a few kilometers

to a few tens of kilometers. The subscriber line is also known as a

subscriber loop, or a local loop.

• Exchanges: The switching centers in the network. A switching center that

directly supports subscribers is known as an end office. Typically, an end

office will support many thousands of subscribers in a localized area. There

are over 19,000 end offices in the United States, so it is clearly impractical for

each end office to have a direct link to each of the other end offices; this

would require on the order of 2 X 108 links. Rather, intermediate switching

nodes are used.

• Thunks: The branches between exchanges. Trunks carry multiple voice-frequency

circuits using either FDM or synchronous TDM. Earlier, these were referred to

as carrier systems.

Subscribers connect directly to an end office, which switches traffic between

subscribers and between a subscriber and other exchanges. The other exchanges

are responsible for routing and switching traffic between end offices. This distinction

is shown in Figure 3.5. To connect two subscribers attached to the same end

office, a circuit is set up between them in the same fashion as described before. If

two subscribers connect to different end offices, a circuit between them consists of

a chain of circuits through one or more intermediate offices. In the figure, a connection

is established between lines a and b by simply setting up the connection

through the end office. The connection between c and d is more complex. In c's end

office, a connection is established between line c and one channel on aTDM trunk

to the intermediate switch. In the intermediate switch, that channel is connected to

a channel on a TDM trunk to d's end office. In that end office, the channel is connected

to line d.

Circuit-switching technology has been driven by its use to carry voice traffic.

One of the key requirements for voice traffic is that there must be virtually no

transmission delay and certainly no variation in delay. A constant signal transmission

rate must be maintained, because transmission and reception occur at the

same signal rate. These requirements are necessary to allow normal human conversation.

Further, the quality of the received signal must be sufficiently high to

provide, at a minimum, intelligibility.

Circuit switching achieved its widespread, dominant position because it is well

suited to the analog transmission of voice signals. In today's digital world, its inefficiencies

are more apparent. However, despite the inefficiency, circuit switching is

and will remain an attractive choice for both local area and wide area networking.

One of its key strengths is that it is transparent. Once a circuit is established, it

appears like a direct connection to the two attached stations; no special networking

logic is needed at the station.