WIDE AREA NETWORKS:CIRCUIT SWITCHING

WIDE AREA NETWORKS:CIRCUIT SWITCHING

Since the invention of the telephone, circuit switching has been the dominant

technology for voice communications, and it will remain so well into the

ISDN era. This lesson begins with an introduction to the concept of a

Switched communications network and then looks at the key characteristics of a

circuit-switching network.

Switching Network

For transmission of data1 beyond a local area, communication is typically achieved

by transmitting data from source to destination through a network of intermediate

switching nodes; this switched-network design is sometimes used to implement

LANs and MANS as well. The switching nodes are not concerned with the content

of the data; rather, their purpose is to provide a switching facility that will move the

data from node to node until they reach their destination. Figure 8.1 illustrates a

simple network. The end devices that wish to communicate may be referred to as

stations. The stations may be computers, terminals, telephones, or other communicating

devices. We will refer to the switching devices whose purpose is to provide

communication as nodes, which are connected to each other in some topology by

transmission links. Each station attaches to a node, and the collection of nodes is

referred to as a comnzunications network.

The types of networks that are discussed in this and the next three lessons

are referred to as switched communication networks. Data entering the network

from a station are routed to the destination by being switched from node to node.

For example, in Figure 8.1, data from station A intended for station F are sent to

node 4. They may then be routed via nodes 5 and 6 or nodes 7 and 6 to the destination.

Several observations are in order:

1. Some nodes connect only to other nodes (e.g., 5 and 7). Their sole task is the

internal (to the network) switching of data. Other nodes have one or more stations

attached as well; in addition to their switching functions, such nodes

accept data from and deliver data to the attached stations.

2. Node-node links are usually multiplexed, using either frequency-division

multiplexing (FDM) or time-division multiplexing (TDM).

3. Usually, the network is not fully connected; that is, there is not a direct link

between every possible pair of nodes. However, it is always desirable to have

more than one possible path through the network for each pair of stations; this

enhances the reliability of the network.

Two quite different technologies are used in wide-area switched networks: circuit

switching and packet switching. These two technologies differ in the way the

nodes switch information from one link to another on the way from source to destination.

In this lesson, we look at the details of circuit switching; packet switching

is pursued in Lesson 9. Two approaches that evolved from packet switching,

namely frame relay and ATM, are explored in Lessons 10 and 11, respectively.

Circuit Switching Network

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 logical channel is dedicated to the connection.

Communication via circuit switching involves three phases, which can be

explained with reference to Figure 8.1.

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 node 6. 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. 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. Data transfer. Information can now be transmitted from A through the network

to E. The data may be analog or digital, 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-4 link, internal switching through 4,4-5 channel, internal

switching through 5, 5-6 channel, and internal switching through 6, 6-E

link. Generally, the connection is full-duplex.

3. Circuit disconnect. After some period of data 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 percent.

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 that required for propagation 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 8.2); this is actually a collection of national networks

interconnected to form the international service. Although originally designed

and implemented to service analog telephone subscribers, the network handles

substantial data traffic via modem and is gradually 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-corporations or other large organizations

interconnecting their various sites; these usually consist 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:

* 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.

Local loop: The link between the subscriber and the network, also referred to

as the subscriber loop. Almost all local loop connections used twisted-pair

wire. The length of a local loop is typically in a range from a few kilometers

to a few tens of kilometers.

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 10' links. Rather, intermediate switching nodes are used.

* Trunks: The branches between exchanges. Trunks carry multiple voicefrequency

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 8.3. 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 a TDM 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 those applications that handle

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, as 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 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 as

a direct connection to the two attached stations; no special networking logic is

needed at either point.

Switching Concepts

The technology of circuit switching is best approached by examining the operation

of a single circuit-switched node. A network built around a single circuit-switching

node consists of a collection of stations attached to a central switching unit. The central

switch establishes a dedicated path between any two devices that wish to communicate.

Figure 8.4 depicts the major elements of such a one-node network. The

dotted lines inside the switch symbolize the connections that are currently active.

The heart of a modern system is a digital switch. The function of the digital

switch is to provide a transparent signal path between any pair of attached devices.

The path is transparent in that it appears to the attached pair of devices that there

is a direct connection between them. Typically, the connection must allow fullduplex

transmission.

The network-interface element represents the functions and hardware needed

to connect digital devices, such as data processing devices and digital telephones, to

the network. Analog telephones can also be attached if the network interface contains

the logic for converting to digital signals. Trunks to other digital switches carry

TDM signals and provide the links for constructing multiple-node networks.

The control unit performs three general tasks. First, it establishes connections.

This is generally done on demand-that is, at the request of an attached device. To

establish the connection, the control unit must handle and acknowledge the request,

determine if the intended destination is free, and construct a path through the

switch. Second, the control unit must maintain the connection. Because the digital

switch uses time-division principles, this may require ongoing manipulation of the

switching elements. However, the bits of the communication are transferred transparently

(from the point of view of the attached devices). Third, the control unit

must tear down the connection, either in response to a request from one of the parties

or for its own reasons.

An important characteristic of a circuit-switching device is whether it is blocking

or nonblocking. Blocking occurs when the network is unable to connect two stations

because all possible paths between them are already in use. A blocking network

is one in which such blocking is possible. Hence, a nonblocking network

permits all stations to be connected (in pairs) at once and grants all possible connection

requests as long as the called party is free. When a network is supporting

only voice traffic, a blocking configuration is generally acceptable, as it is expected

that most phone calls are of short duration and that therefore only a fraction of the

telephones will be engaged at any time. However, when data processing devices are

involved, these assumptions may be invalid. For example, for a data-entry application,

a terminal may be continuously connected to a computer for hours at a time.

Hence, for data applications, there is a requirement for a nonblocking or "nearly

nonblocking" (very low probability of blocking) configuration.

We turn now to an examination of the switching techniques internal to a single

circuit-switching node.

Space-division Switching

Space-division switching was originally developed for the analog environment and

has been carried over into the digital realm. The fundamental principles are the

same, whether the switch is used to carry analog or digital signals. As its name

implies, a space-division switch is one in which the signal paths are physically separate

from one another (divided in space). Each connection requires the establishment

of a physical path through the switch that is dedicated solely to the transfer of

signals between the two endpoints. The basic building block of the switch is a metallic

crosspoint or semiconductor gate that can be enabled and disabled by a control

unit.

Figure 8.5 shows a simple crossbar matrix with 10 full-duplex I10 lines. The

matrix has 10 inputs and 10 outputs; each station attaches to the matrix via one

input and one output line. Interconnection is possible between any two lines by

enabling the appropriate crosspoint. Note that a total of 100 crosspoints is required.

The crossbar switch has a number of limitations:

* The number of crosspoints grows with the square of the number of attached

stations. This is costly for a large switch.

* The loss of a crosspoint prevents connection between the two devices whose

lines intersect at that crosspoint.

* The crosspoints are inefficiently utilized; even when all of the attached

devices are active, only a small fraction of the crosspoints are engaged.

To overcome these limitations, multiple-stage switches are employed. Figure

8.6 is an example of a three-stage switch. This type of arrangement has several

advantages over a single-stage crossbar matrix:

* The number of crosspoints is reduced, increasing crossbar utilization. In this

example, the total number of crosspoints for 10 stations is reduced from 100

to 48.

* There is more than one path through the network to connect two endpoints,

increasing reliability.

Of course, a multistage network requires a more complex control scheme. To

establish a path in a single-stage network, it is only necessary to enable a single gate.

In a multistage network, a free path through the stages must be determined and the

appropriate gates enabled.

A consideration with a multistage space-division switch is that it may be

blocking. It should be clear from Figure 8.5 that a single-stage crossbar matrix is

nonblocking; that is, a path is always available to connect an input to an output; that

this may not be the case with a multiple-stage switch can be seen in Figure 8.6. The

heavier lines indicate ones already in use. In this state, input line 10, for example,

cannot be connected to output line 3,4, or 5, even though all of these output lines

are available. A multiple-stage switch can be made nonblocking by increasing the

number or size of the intermediate switches, but of course this increases the cost.

Time-division Switching

The technology of switching has a long history, most of it covering an era when analog

signal switching predominated. With the advent of digitized voice and synchronous

time-division multiplexing techniques, both voice and data can be transmitted

via digital signals; this has led to a fundamental change in the design and technology

of switching systems. Instead of relatively dumb space-division systems, modern

digital systems rely on intelligent control of space- and time-division elements.

Virtually all modern circuit switches use digital time-division techniques for

establishing and maintaining "circuits." Time-division switching involves the partitioning

of a lower-speed bit stream into pieces that share a higher-speed stream with

other bit streams. The individual pieces, or slots, are manipulated by control logic

to route data from input to output. There are a number of variations on this basic

concept. To give the reader some feel for time-division switching, we examine one

of the simplest but most popular techniques, referred to as TDM bus switching.

TDM bus switching, and indeed all digital switching techniques, are based on

the use of synchronous time-division multiplexing (TDM). As we saw in Figure 7.6,

synchronous TDM permits multiple low-speed bit streams to share a high-speed

line. A set of inputs is sampled in turn. The samples are organized serially into slots

(channels) to form a recurring frame of slots, with the number of slots per frame

equal to the number of inputs. A slot may be a bit, a byte, or some longer block. An

important point to note is that with synchronous TDM, the source and destination

of the data in each time slot are known. Hence, there is no need for address bits in

each slot.

Figure 8.7 shows a simple way in which this technique can be adapted to

achieve switching. Each device attaches to the switch through a full-duplex line.

These lines are connected through controlled gates to a high-speed digital bus. Each

line is assigned a time slot for providing input. For the duration of the slot, that

line's gate is enabled, allowing a small burst of data onto the bus. For that same time

slot, one of the other line gates is enabled for output. Thus, during that time slot,

data are switched from the enabled input line to the enabled output line. During

successive time slots, different inputloutput pairings are enabled, allowing a number

of connections to be carried over the shared bus. An attached device achieves

full-duplex operation by transmitting during one assigned time slot and receiving

during another. The other end of the connection is an I10 pair for which these time

slots have the opposite meanings.

Let us look at the timing involved more closely. First, consider a nonblocking

implementation of Figure 8.7. For a switch that supports, for example, 100 devices,

there must be 100 repetitively occurring time slots, each one assigned to an input and

an output line. One iteration for all time slots is referred to as a frame. The input

assignment may be fixed; the output assignments vary to allow various connections.

When a time slot begins, the designated (enabled) input line may insert a burst of

data onto the line, where it will propagate to both ends past all other output lines.

The designated (enabled) output line, during that time, copies the data, if present,

as they go by. The time slot, therefore, must equal the transmission time of the input

plus the propagation delay between input and output across the bus. In order to

keep successive time slots uniform, time-slot length is defined as transmission time

plus the end-to-end bus propagation delay.

To keep up with the input lines, the data rate on the bus must be high enough

that the slots recur sufficiently frequently. For example, consider a system connecting

100 full-duplex lines at 19.2 kbps. Input data on each line are buffered at the

gate. Each buffer must be cleared, by enabling the gate, quickly enough to avoid

overrun. Thus, the data rate on the bus in this example must be greater than

1.92 Mbps. The actual data rate must be high enough to also account for the wasted

time due to propagation delay.

The above considerations determine the traffic-carrying capacity of a blocking

switch, as well, where there is no fixed assignment of input lines to time slots; they

are allocated on demand. The data rate on the bus dictates how many connections

can be made at a time. For a system with 200 devices at 19.2 kbps and a bus at

2 Mbps, about half of the devices can be connected at any one time.

The TDM bus-switching scheme can accommodate lines of varying data rates.

For example, if a 9600-bps line gets one slot per frame, a 19.2-kbps line would get

two slots per frame. Of course, only lines of the same data rate can be connected.

Figure 8.8 is an example that suggests how the control for a TDM bus switch

can be implemented. Let us assume that the propagation time on the bus is

0.01 pec. Time on the bus is organized into 30.06-psec frames of six 5.01-psec time

slots each. A control memory indicates which gates are to be enabled during each

time slot. In this example, six words of memory are needed. A controller cycles

through the memory at a rate of one cycle every 30.06 psec. During the first time

slot of each cycle, the input gate from device 1 and the output gate to device 3 are

enabled, allowing data to pass from device 1 to device 3 over the bus. The remaining

words are accessed in succeeding time slots and treated accordingly. As long as

the control memory contains the contents depicted in Figure 8.8, connections are

maintained between 1 and 3, 2 and 5, and 4 and 6.