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Unit V: Switching Techniques and System
Switching System
The switching center receives the control signals, messages or conversations and forwards to the
required destination, after necessary modification. A switching system is a collection of switching elements arranged and
controlled in such a way as to setup a communication path between any two distant points. A switching center of a
telephone network comprising a switching network and its control and support equipment is called a central office.
In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels
through various communication channels. In computer communication, the switching technique used is known as
packet switching or message switch (store and forward switching). In telephone network the switching method used is
called circuit switching. Some practical switching systems are step-by-step, cross barred relay system, digital switching
systems, electronic switching system etc.
5.1 Message switching
Message switching is a telecommunications data transfer method that sends the entire
message as one unit. To transfer the message, the message is first stored and then transferred via hops until it reaches
the destination. Most messages are too large for random access memory (RAM), so the messages are temporarily stored
in the hard drive. Though it is slower and experiences delays when sending data, it is still used widely with
sending email. The advantages of this method are lower bandwidth costs, better management of network congestion,
and the easy transfer of messages to different destinations.
When a user sends a message using message switching, this method first saves the entire message and then designates
an address for the message, so it knows where the message is going. Once saved, the message is sent to the router,
which switches it to the first hop, or node. At each hop, the message is inspected for problems or errors before being
sent on to the next hop. The message keeps hopping around until it reaches its final destination.
Despite being an older and slower method than packet switching, message switching has some advantages. Since it is
slower, and messages can be stored at nodes, these messages can easily glide through networks with heavy congestion.
This method also is better for sending a message to several people at once, and it uses less bandwidth overall.
Advantages of Message Switching
1. It provides efficient traffic management by assigning priorities to the messages to be switched.
2. No physical connection is required between the source & destination as it is in circuit switching.
3. It reduces the traffic congestion on network because of store & forward facility. Each node can store the
message until communication channel becomes available.
4. Channels are used effectively and network devices share the data channels.
5. It supports the message length of unlimited size.
Disadvantages of Message Switching
1. As message length is unlimited, each switching node must have sufficient storage to buffer message.
2. Storing & forwarding facility introduces delay thus making message switching unsuitable for real time
applications like voice and video.
5.2 Packet switching
In packet switching data networks, all user data to be transmitted is first divided into one or more units,
called packets, by the source. These packets are of varying lengths, and each packet is assigned an address and the
necessary control information. In each switching node, packets are received, stored briefly, and passed on to the text
node. Packets include both the source and the destination network addresses. On receipt of each packet, the switching
node inspects the destination address contained in the packet. Each switching nodes contains a routing direction
specifying the outgoing links to be used for each network address. On the receipt of each packet, the switching node
forwards the packet on the appropriate link at the maximum available bit rate, which is not possible in case of circuit
switching.
To prevent unpredictably long delays and ensure that the network has a fast transit time, a maximum length is allowed
for each packet. Therefore, a message submitted to the transport layer within the DTE is divided by the transport
protocol entity into a number of small packet units before transmission.
These packets are handled in the following two ways:
Datagram
Virtual Circuit and Permanent Virtual Circuit
Datagram: A datagram is a self-contained, independent packet of data carries sufficient information to be routed from a
source to the destination without relying on earlier exchange between this source and destination and the network.
Here, each packet is treated independently that can take any practical route to reach the destination in any arbitrary
order. There may also be some packet missing at destination. It is the responsibility of DTE to reassemble the out of
order packet in sequence and request for missing packets.
Virtual Circuit and Permanent Virtual Circuit:
Packet switching was developed to replace the drawbacks offered by circuit-switched telephone network. It is clear that
a virtual circuit is created between users in a packet switched network to make use of transmission resource in the most
efficient way during the actual transfer of information. The virtual connection show created may either be connectionoriented or connection-less.
Advantages:
• Packet switching is cost effective, because switching devices do not need massive amount of secondary storage.
• Packet switching offers improved delay characteristics; because there are no long messages in the queue (maximum
packet size is fixed).
• Packet can be rerouted if there is any problem, such as, busy or disabled links.
 The advantage of packet switching is that many network users can share the same channel at the same time. Packet
switching can maximize link efficiency by making optimal use of link bandwidth.
Disadvantages:
• Protocols for packet switching are typically more complex.
• It can add some initial costs in implementation.
• If packet is lost, sender needs to retransmit the data. Another disadvantage is that packet-switched systems still can’t
deliver the same quality as dedicated circuits in applications requiring very little delay - like voice conversations or
moving images.
5.3 Circuit Switching
Circuit switching is a switching method in which a communication way in physical form between
two ways within a network is established, maintained for each communication. It has three steps establishment, data
transfer and circuit disconnect. Once the connection is created, the data transfer is transparent. The main feature of
such a connection is that it provides a fixed data rate channel, and both subscribers must operate at this rate. It is
considered inefficient as compared to packet switching because channel capacity is completely for the duration of
connection. If there is no data at any amount of time, channel capacity goes wasted. Moreover, setting up of connection
takes time.
Circuit switching has two types of transmission. These are datagram transmissions and data-stream transmissions. Data
gram transmissions have individually addressed frames. Data-stream transmissions have a stream of data for which
address checking occurs only once.
Advantages:
• The communication channel (once established) is dedicated.
Disadvantages:
• Possible long wait to establish a connection during which no data can be transmitted.
• More expensive than any other switching techniques, because a dedicated path is required for each connection.
• Inefficient use of the communication channel, because the channel is not used when the connected systems are not
using it.
5.4 Manual switching
In March 1867; Alexander Graham Bell demonstrated his telephone set and laid the foundation of
telephony. In actual telecommunication network, subscribers are not directly connected to one another; in fact they are
connected to switching system. Connection is established between the subscribers at the switching system. The function
of switching system is establishing and releasing connection. Earlier, Manual switching systems used battery such as
Local Battery (LB) or Central Battery (CB).In Local Battery system; Dry cells were used in subscriber sets to power the
microphone. In Central Battery system, a subscriber set is energizes with a powerful central battery at the exchange. But
there were certain limitation of manual switching system, due to which it lost its popularity.
Limitations of Manual switching system:
 In a manual exchange, the subscriber needs to communicate with the operator and common language becomes an
important factor.
 Sufficient Privacy is not there as an operator is involved in setting up and monitoring the call.
 Moreover, the operator takes a few minutes to establish and release of calls.
5.5 Electro mechanical switching
The limitations of manual switching led to the development of Automatic Switching
System. Basically, in automatic switching system, Electromechanical (Strowger and Crossbar) switching system came into
existence. Strowger Switching System was developed by ALMON B. STROWGER in 1889.
Strowger switching Systems are constructed using Uniselector or Two motion selector. The wiper contacts of these
selectors move in direct response to dialed pulses or signals from the subscriber telephone. The wiper moves forward by
one contact at a time and moves by as many contacts as the number of dialed pulses received.
Crossbar switching provides a matrix consisting of n*m set of contacts and selects one of the contacts. There is a set of
horizontal and vertical bars which are attached to a set of electromagnet. When an electromagnet (in horizontal
direction) is energized, the bar attached to it slightly rotates in such a way that the contact points attached to the bar
move closer to its facing point but do not make any contact. Now, if an electromagnet in the vertical direction is
energized, the corresponding bar rotates and the contact point comes in contact and a connection is established.
Advantage of Automatic switching system:
 In an automatic exchange, the subscriber does not need to communicate with the operator.
 A greater degree of privacy is obtained in automatic exchange.
 Establishment and Release of calls are faster in automatic exchanges.
5.6 Electronic Switching
In telecommunications, an electronic switching system (ESS) is a telephone switch that uses
digital electronics and computerized control to interconnect telephone circuits for the purpose of establishing telephone
calls. It is based on the principles of time division multiplexing of digitized analog signals. An electronic switching system
digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized signals to the
appropriate time slots. It may also interconnect digital data or voice circuits.
The ESS is a common control type system. It differs radically from electromechanical switching systems in the devices
that it uses as well as in the techniques that it employs. Throughout the system, solid-state electronic devices are used
extensively. Their high operating speeds permit a relatively small amount of equipment to perform all the control
functions. A major feature is the greatly reduced time required to complete connections between customers.
A switching system with major devices constructed of semiconductor components. A semi-electronic switching system
that had reed relays or crossbar matrices for its talk paths, as well as semiconductor components, was also considered
to be an ESS in the 20th Century.
5.7 Stored control Program
Stored program control (SPC) is a telecommunications technology used for telephone
exchanges controlled by a computer program stored in the memory of the switching system. SPC was the enabling
technology of electronic switching systems (ESS) developed in the Bell System in the 1950s. Electronic Switching System
(Stored Program Control) came into its existence because electromechanical component’s operational speeds were slow
and lifetime was limited.
Electronic Switching System is called Stored Program Control because the controls of switching functions are
programmed into memory and actions are executed by the controlling processor. The switching procedures in SPC
exchanges arc controlled by means of stored programs (a program is a sequence of instructions stored in the computer
memory). The SPC uses a computer as the time-shared control. The computer is defined by the program stored in its
memory. The computer is programmed to test the conditions of the inputs and old states and decide on new outputs
and states. Stored program control of switching systems is based on this principle. The computer controllers provide
the time-shared decision maker, expressed as programs, which can be used to change or extend the functions of a
control system.
There are two types of SPC (stored program control):
Centralized SPC
Distributed SPC
5.8 Space-division switching
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. Space division switching uses space to separate
channels in a communication link. These separations may be done by frequency, insulation or distance.
In space division switching, there is dedicated path established between the calling and called subscribers for the entire
duration of call. This means the speech signals are transferred between them in duration. The time format of T1 or E1,
the PCM samples occurs at 125 microseconds. Now the dedicated switching element remains unused for almost 120
microseconds. This is defect of space division switching. In terms of architecture; crossbar, matrix, contact everything
are placed separately in the board.
5.9 Time-division switching
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. Virtually all modern circuit switches use digital time-division techniques for establishing
and maintaining "circuits."
TDM bus switching, and indeed all digital switching techniques, are based on the use of synchronous time-division
multiplexing (TDM). 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.
Time division switching in one way of doing the space separation. It may use other techniques such as TDM, FDM etc to
maintain separation so that the conversations do not collide with each other.
5.10 Space-time division switching
The space-time switch works on the same principles as the time switch. One way in which the operating speed and the
number of simultaneous connections can be increased is the space-time switching. This is a high-speed variation of the
time switch. The PCM word from one of several incoming highways can be forwarded to any time slot of one of the
several outgoing highways. The incoming highways are extended to the data memory through a multiplexer. Due to this,
the bit rate of the highways from multiplexer to data memory is several times higher than that of the incoming
highways.
Thus the bit rate of the highway from multiplexer to the data memory is four times greater than the bit rate of a single
incoming highway. The demultiplexer operates at the original bit rate to distribute the PCM words to four outgoing
highways. It has full availability without blocking.
5.11 Multiple stage switching
Multiple stages switching combines crossbar switches in several stages. Design of
multistage switch depends on the number of stages and the number of switches required in each stage. Normally, the
middle stages have fewer switches and multiple paths are available in multistage switches.
In electronics, a crossbar switch (also known as cross-point switch, crosspoint switch, or matrix switch) is
a switch connecting multiple inputs to multiple outputs in a matrix manner. Originally the term was used for a matrix
switch controlled by a grid of crossing metal bars, and later was broadened to matrix switches in general. It is one of the
principal switch architectures, together with a memory switch and a crossover switch.
The solution to limitation of the cross bar switch is the multi stage switch, which combines the crossbar switches in
several stages (normally three stage). In a single crossbar switch, only one row and one column is active for any
connection. So we need n*n crosspoints. If we can allow multiple paths inside the switch, we can decrease the number
of crosspoints. Each crosspoints in the middle stage can be accessed by multiple crosspoints in the first or third stage.
The multistage switch has one drawback-blocking during heavy traffics. The whole idea of multistage switching is to
share crosspoints in the middle stage crossbars.
In a network, a cross-bar switch is a device that is capable of channeling data between any two devices that are attached
to it up to its maximum number of ports. The paths set up between devices can be fixed for some duration or changed
when desired and each device-to-device path (going through the switch) is usually fixed for some period.
Cross-bar topology can be contrasted with bus topology, an arrangement in which there is only one path that all devices
share. Traditionally, computers have been connected to storage devices with a large bus. A major advantage of cross-bar
switching is that, as the traffic between any two devices increases, it does not affect traffic between other devices. In
addition to offering more flexibility, a cross-bar switch environment offers greater scalability than a bus environment.
5.12 Digital cross connect
A digital cross-connect system (DCS or DXC) is a piece of circuit-switched network equipment,
used in telecommunications networks, that allows lower-level TDM bit streams, such as DS0 bit streams, to be rearranged
and interconnected among higher-level TDM signals, such as DS1 bit streams. DCS units are available that operate on
both older T-carrier/E-carrier bit streams, as well as newer SONET/SDH bit streams.
DCS devices can be used for "grooming" telecommunications traffic, switching traffic from one circuit to another in the
event of a network failure, supporting automated provisioning, and other applications. Having a DCS in a circuitswitched network provides important flexibility that can otherwise only be obtained at higher cost using manual " DSX"
cross-connect patch panels.
It is important to realize that while DCS devices "switch" traffic, they are not packet switches—they switch circuits, not
packets, and the circuit arrangements they are used to manage tend to persist over very long time spans, typically
months or longer, as compared to packet switches, which can route every packet differently, and operate on micro- or
millisecond time spans.
A digital access and cross-connect system (DACS) is a telecommunication-specific circuit-switching device that is used to
route voice/data among cross-connected T1/E1 carrier lines. DACS is used in telecommunication networks to connect
various carrier channels that exist in voice and data carriers. DACS enables these carriers to connect with each other and
their specific channels as well. DACS supports connectivity among DS0 and DS1 channels used for transporting voice and
data/voice respectively. DACS also supports higher level carriers such as T3/E3, synchronous optical networking (SONET)
and synchronous digital hierarchy (SDH).
5.13 Private branch exchange
A PBX (private branch exchange) is a telephone system within an enterprise that switches
calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines.
The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company's central office.
The PBX is owned and operated by the enterprise rather than the telephone company. Private branch
exchanges used analog technology originally. Today, PBXs use digital technology (digital signals are converted to analog
for outside calls on the local loop using plain old telephone service (POTS ).
A PBX includes:
 Telephone trunk (multiple phone) lines that terminate at the PBX
 A computer with memory that manages the switching of the calls within the PBX and in and out of it
 The network of lines within the PBX
 A console or switchboard for a human operator (optional)