Switched Communication Networks Switching Techniques: Circuit Switching

Module
4
Switched Communication Networks

Switching Techniques: Circuit Switching

4.2.0 Specific Instructional Objectives
At the end of this lesson the student will be able to:
•
Explain the need for packet switching
•
Explain how packet switching takes place
•
Explain different types of packet switching techniques
•
Distinguish between virtual-circuit and datagram type packet switching
•
Compare circuit switching with packet switching
4.2.1 Introduction
In the preceding lesson we have discussed about circuit switching. In circuit switching, network resources are dedicated to a particular connection. Although this satisfies the requirement of voice communication, it suffers from the following two shortcomings for data communication:
o
In a typical user/host data connection, line utilization is very low.
o
Provides facility for data transmission at a constant rate.
However, for information transmission applications, the circuit switching method is very slow, relatively expensive and inefficient. First of all, the need to establish a dedicated connection before sending the message itself inserts a delay time, which might become significant for the total message transfer time. Moreover, the total channel remains idle and unavailable to the other users once a connection is made. On the other hand once a connection is established, it is guaranteed and orderly delivery of message is ensured. Unfortunately, the data transmission pattern may not ensure this, because data transmission is bursty in nature. As a consequence, it limits the utility of the method. The problem may be overcome by using an approach known as message switching, which is discussed in Sec. 4.2.2. However, message switching suffers from various problems as discussed in Sec. 4.2.3. To overcome the limitations of message switching, another switching technique, known as packet switching was invented. Various aspects of packet switching have been discussed in Sec. 4.2.4.
4.2.2 Message Switching
In this switching method, a different strategy is used, where instead of establishing a dedicated physical line between the sender and the receiver, the message is sent to the nearest directly connected switching node. This node stores the message, checks for errors, selects the best available route and forwards the message to the next intermediate node.
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Node 1
Message
Message
Message
Node 2
Node 3
Node 4
Figure 4.2.1 Message Switching Technique
The line becomes free again for other messages, while the process is being continued in some other nodes. Due to the mode of action, this method is also known as store-and-forward technology where the message hops from node to node to its final destination. Each node stores the full message, checks for errors and forwards it.
In this switching technique, more devices can share the network bandwidth, as compared with circuit switching technique. Temporary storage of message reduces traffic congestion to some extent. Higher priority can be given to urgent messages, so that the low priority messages are delayed while the urgent ones are forwarded faster. Through broadcast addresses one message can be sent to several users. Last of all, since the destination host need not be active when the message is sent, message switching techniques improve global communications.
However, since the message blocks may be quite large in size, considerable amount of storage space is required at each node to buffer the messages. A message might occupy the buffers for minutes, thus blocking the internodal traffic.
Basic idea:
•
Each network node receives and stores the message
•
Determines the next leg of the route, and
•
Queues the message to go out on that link.
Advantages:
•
Line efficiency is greater (sharing of links).
•
Data rate conversion is possible.
•
Even under heavy traffic, packets are accepted, possibly with a greater delay in delivery.
•
Message priorities can be used, to satisfy the requirements, if any.
Disadvantages: Message of large size monopolizes the link and storage
4.2.3 Packet Switching
The basic approach is not much different from message switching. It is also based on the same ‘store-and-forward’ approach. However, to overcome the limitations of message switching, messages are divided into subsets of equal length called packets. This approach was developed for long-distance data communication (1970) and it has evolved
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over time. In packet switching approach, data are transmitted in short packets (few Kbytes). A long message is broken up into a series of packets as shown in Fig. 4.2.2. Every packet contains some control information in its header, which is required for routing and other purposes.
Figure 4.2.2 A message is divided into a number of equal length short packets
Main difference between Packet switching and Circuit Switching is that the communication lines are not dedicated to passing messages from the source to the destination. In Packet Switching, different messages (and even different packets) can pass through different routes, and when there is a "dead time" in the communication between the source and the destination, the lines can be used by other sources.
There are two basic approaches commonly used to packet Switching: virtual-circuit packet switching and datagram packet switching. In virtual-circuit packet switching a virtual circuit is made before actual data is transmitted, but it is different from circuit switching in a sense that in circuit switching the call accept signal comes only from the final destination to the source while in case of virtual-packet switching this call accept signal is transmitted between each adjacent intermediate node as shown in Fig. 4.2.3. Other features of virtual circuit packet switching are discussed in the following subsection.
4.2.3.1 Virtual Circuit Packet Switching Networks
An initial setup phase is used to set up a route between the intermediate nodes for all the packets passed during the session between the two end nodes. In each intermediate node, an entry is registered in a table to indicate the route for the connection that has been set up. Thus, packets passed through this route, can have short headers, containing only a virtual circuit identifier (VCI), and not their destination. Each intermediate node passes the packets according to the information that was stored in it, in the setup phase. In this way, packets arrive at the destination in the correct sequence, and it is guaranteed that essentially there will not be errors. This approach is slower than Circuit Switching, since different virtual circuits may compete over the same resources, and an initial setup phase is needed to initiate the circuit. As in Circuit Switching, if an intermediate node fails, all virtual circuits that pass through it are lost. The most common forms of Virtual Circuit
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networks are X.25 and Frame Relay, which are commonly used for public data networks (PDN). Request Packet Node 1 Node 2 Node 3 Node 4 PacketsCall- Call-acknowledgement p 3 2 1 3 2 1 3 1 Call-accept packet
Figure 4.2.3 Virtual circuit packet switching technique
4.2.3.2 Datagram Packet Switching Networks
This approach uses a different, more dynamic scheme, to determine the route through the network links. Each packet is treated as an independent entity, and its header contains full information about the destination of the packet. The intermediate nodes examine the header of the packet, and decide to which node to send the packet so that it will reach its destination. In the decision two factors are taken into account:
•
The shortest ways to pass the packet to its destination - protocols such as RIP/OSPF are used to determine the shortest path to the destination.
•
Finding a free node to pass the packet to - in this way, bottlenecks are eliminated, since packets can reach the destination in alternate routes.
Thus, in this method, the packets don't follow a pre-established route, and the intermediate nodes (the routers) don't have pre-defined knowledge of the routes that the packets should be passed through.
Node 1
Node 2
Node 3
Figure 4.2.4 Datagram Packet switching
Packets can follow different routes to the destination, and delivery is not guaranteed (although packets usually do follow the same route, and are reliably sent). Due to the nature of this method, the packets can reach the destination in a different order
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than they were sent, thus they must be sorted at the destination to form the original message. This approach is time consuming since every router has to decide where to send each packet. The main implementation of Datagram Switching network is the Internet, which uses the IP network protocol.
Advantages:
•
Call setup phase is avoided (for transmission of a few packets, datagram will be faster).
•
Because it is more primitive, it is more flexible.
•
Congestion/failed link can be avoided (more reliable).
Problems:
•
Packets may be delivered out of order.
•
If a node crashes momentarily, all of its queued packets are lost.
4.2.3.3 Packet Size
In spite of increase in overhead, the transmission time may decreases in packet switching technique because of parallelism in transmission as shown in Fig. 4.2.5.
Figure 4.2.5 Reduction of transmission time because of parallelism in transmission in packet switching technique
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However, question arises about the optimal size of size of a packet. As packet size is decreased, the transmission time reduces until it is comparable to the size of control information. There is a close relationship between packet size and transmission time as shown in Fig. 4.2.6. In this case it is assumed that there is a virtual circuit from station X to Y through nodes a and b. Times required for transmission decreases as each message is divided into 2 and 5 packets. However, the transmission time increases if each message is divided into 10 packets.
Figure 4.2.6 Variation of transmission time with packet size
4.2.3.4 Virtual Circuit Versus Datagram Packet Switching
Key features of the virtual circuit packet switching approach is as follows:
•
Node need not decide route
•
More difficult to adopt to congestion
•
Maintains sequence order
•
All packets are sent through the same predetermined route
On the other hand, the key features of the datagram packet switching are as follows:
•
Each packet is treated independently
•
Call set up phase is avoided
•
Inherently more flexible and reliable
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4.2.3.5 External and Internal Operations
There are two dimensions to the problem of whether to use virtual circuit or datagram in a particular situation:
•
At the interface between a station and a network node, we may have connection-oriented or connectionless service.
•
Internally, the network may use virtual circuits or datagrams.
This leads us to four different scenarios using different VC/DG combinations, which are discussed below.
Scenario 1: External virtual circuit, Internal virtual circuit
In this case a user requests a virtual circuit and a dedicated route through the network is constructed. All packets follow the same route as shown in Fig. 4.2.7.
Figure 4.2.7 External virtual circuit and internal virtual circuit
Scenario 2: External virtual circuit, Internal datagram
In this case, the network handles each packet separately. Different packets for the same external virtual circuit may take different routes as shown in Fig. 4.2.8. The network buffers packets, if necessary, so that they are delivered to the destination in the proper order.
Scenario 3: External datagram, Internal datagram
In this case each packet is treated independently from both the user’s end and the network’s point of view as shown in Fig. 4.2.9.
Scenario 4: External datagram, Internal virtual circuit
In this case, an external user does not see any connections - it simply sends packets one at a time as shown in Fig. 4.2.10. The network sets up a logical connection between stations for packet delivery. May leave such connections in place for an extended period, so as to satisfy anticipated future needs.
A comparison of different switching techniques is given in Table 4.2.1
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Figure 4.2.8 External virtual circuit and internal datagram
Figure 4.2.9 External datagram and internal datagram
Figure 4.2.10 External datagram and internal virtual circuit
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Table 4.2.1 Comparison of the three switching techniques
Fill In the Blanks:
1
. A switched virtual circuit involves _____________.
2.
A permanent virtual circuit involves ____________.
3.
Two basic approaches are common to Packet Switching are _____________ packet switching and _________ packet switching.
4.
X.25 is a standard for _________ communications.
Ans:
1.
creation of link as and when needed
2.
permanent link
3.
virtual circuit … datagram
4.
packet switched communication
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Short Answer Questions
Q-1. How the drawback of circuit switching is overcome in message switching?
Ans: Message switching is based on store and forward technique. Instead of establishing a dedicated path, the message is sent to the nearest directly connected node. Each node stores the message, checks for error and forwards it. It allows more devices to share the network bandwidth and one message can be sent to several users. Destination host need not be on at the time of sending message.
Q-2. What is the drawback of message switching? How is it overcome in packet switching?
Ans.: In message switching, large storage space is required at each node to buffer the complete message blocks. On the other hand, in packet switching, messages are divided into subset of equal length, which are generated in the source node and reassembled to get back the initial complete message in destination node. Moreover, to transmit a message of large size, link is kept busy for a long time leading to increase in delay for other messages.
Q-3. What are the key differences between datagram and virtual-circuit packet switching?
Ans: In datagram, the packets are routed independently and it might follow different routes to reach the destination in different order. In virtual-circuit packet switching, first a virtual connection is being established, and all the packets are sent serially through the same path. In this case, packets are received in order.
Q-4. Distinguish between circuit switching and virtual-circuit packet switching.
Ans: - In circuit switching, a dedicated path is established. Data transmission is fast and interactive. Nodes need not have storage facility. However, there is a call setup delay. In overload condition, it may block the call setup. It has fixed bandwidth from source to destination and no overhead after the call setup.
In virtual-circuit packet switching, there is no dedicated path. It requires storage facility and involves packet transmission delay. It can use different speed of transmission and encoding techniques at different segments of the route.
Q-5. How packet size affects the transmission time in a packet switching network?
Ans: Initially, transmission time decreases as packet size is reduced. But, as packet size is reduced and the payload part of a packet becomes comparable to the control part, transmission time increases.
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Switched Communication Networks Switching Techniques: Circuit Switching

Module
4
Switched Communication Networks

Switching Techniques: Circuit Switching

Specific Instructional Objectives
At the end of this lesson the student will be able to:
•
Understand the need for circuit switching
•
Specify the components of a switched communication network
•
Explain how circuit switching takes place
•
Explain how switching takes place using space-division and time-division switching
•
Explain how routing is performed
•
Explain how signalling in performed
4.1.1 Introduction
When there are many devices, it is necessary to develop suitable mechanism for communication between any two devices. One alternative is to establish point-to-point communication between each pair of devices using mesh topology. However, mesh topology is impractical for large number of devices, because the number of links increases exponentially (n(n-1)/2, where n is the number of devices) with the number of devices. A better alternative is to use switching techniques leading to switched communication network. In the switched network methodology, the network consists of a set of interconnected nodes, among which information is transmitted from source to destination via different routes, which is controlled by the switching mechanism. A basic model of a switched communication is shown in Fig. 4.1.1. The end devices that wish to communicate with each other are called stations. The switching devices are called nodes. Some nodes connect to other nodes and some are to connected to some stations. Key features of a switched communication network are given below:
•
Network Topology is not regular.
•
Uses FDM or TDM for node-to-node communication.
•
There exist multiple paths between a source-destination pair for better network reliability.
•
The switching nodes are not concerned with the contents of data.
• Their purpose is to provide a switching facility that will move data from node to node until they reach the destination.
The switching performed by different nodes can be categorized into the following three types:
•
Circuit Switching
•
Packet Switching
•
Message Switching
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Figure 4.1.1 Basic model of a switched communication network
In this lesson we shall discuss various aspects of circuit switching and discuss how the Public Switched Telephone Network (PSTN), which is based on circuit switching, works.
4.1.2 Circuit switching Technique
Communication via circuit switching implies that there is a dedicated communication path between the two stations. The path is a connected through a sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. Circuit switching is commonly used technique in telephony, where the caller sends a special message with the address of the callee (i.e. by dialling a number) to state its destination. It involved the following three distinct steps, as shown in Fig. 4.1.2.
Circuit Establishment: To establish an end-to-end connection before any transfer of data.
Some segments of the circuit may be a dedicated link, while some other segments may be shared.
Data transfer:
•
Transfer data is from the source to the destination.
•
The data may be analog or digital, depending on the nature of the network.
•
The connection is generally full-duplex.
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Circuit disconnect:
•
Terminate connection at the end of data transfer.
• Signals must be propagated to deallocate the dedicated resources. Acknowledgement signalNode 1 Node 2 Node 3 MessageNode 4 TimeCall-request Signal Call-accept Signal
Figure 4.1.2 Circuit Switching technique
Thus the actual physical electrical path or circuit between the source and destination host must be established before the message is transmitted. This connection, once established, remains exclusive and continuous for the complete duration of information exchange and the circuit becomes disconnected only when the source wants to do so.
4.1.3 Switching Node
Let us consider the operation of a single circuit switched node comprising a collection of stations attached to a central switching unit, which establishes a dedicated path between any two devices that wish to communicate.
Major elements of a single-node network are summarized below:
•
Digital switch: That provides a transparent (full-duplex) signal path between any pair of attached devices.
•
Network interface: That represents the functions and hardware needed to connect digital devices to the network (like telephones).
•
Control unit: That establishes, maintains, and tears down a connection.
The simplified schematic diagram of a switching node is shown in Fig. 4.1.3. An important characteristic of a circuit-switch node is whether it is blocking or non-blocking. A blocking network is one, which may be unable to connect two stations because all possible paths between them are already in use. A non-blocking 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. For a network that supports only voice traffic, a blocking configuration may be acceptable, since most phone calls are of short duration. For data applications, where a connection may remain active for hours, non-blocking configuration is desirable.
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Figure 4.1.3 Schematic diagram of a switching node.
Circuit switching uses any of the three technologies: Space-division switches, Time-division switches or a combination of both. In Space-division switching, the paths in the circuit are separated with each other spatially, i.e. different ongoing connections, at a same instant of time, uses different switching paths, which are separated spatially. This was originally developed for the analog environment, and has been carried over to the digital domain. Some of the space switches are crossbar switches, Multi-stage switches (e.g. Omega Switches). A crossbar switch is shown in Fig. 4.1.4. Basic building block of the switch is a metallic crosspoint or semiconductor gate that can be enabled or disabled by a control unit.
Figure 4.1.4 Schematic diagram of a crossbar switch
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Example: Xilinx crossbar switch using FPGAs. It is based on reconfigurable routing infrastructure. It is a high-speed high capacity nonblocking type switch with sizes varying from 64X64 to 1024X1024 and data rate of 200 Mbps.
Limitations of crossbar switches are as follows:
•
The number of crosspoints grows with the square of the number of attached stations.
•
Costly for a large switch.
• The failure of a crosspoint prevents connection between the two devices whose lines intersect at that crosspoint.
•
The crosspoints are inefficiently utilized.
•
Only a small fraction of crosspoints are engaged even if all of the attached devices are active.
Some of the above problems can be overcome with the help of multistage space division switches. By splitting the crossbar switch into smaller units and interconnecting them, it is possible to build multistage switches with fewer crosspoints.
Figure 4.1.5 A three-stage space division switch
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Figure 4.1.6 Block nature of the switch
Figure 4.1.5 shows a three-stage space division switch. In this case the number of crosspoints needed goes down from 64 to 40. There is more than one path through the network to connect two endpoints, thereby increasing reliability. Multistage switches may lead to blocking. The problem may be tackled by increasing the number or size of the intermediate switches, which also increases the cost. The blocking feature is illustrated in Fig. 4.1.6. As shown in Fig. 4.1.6, after setting up connections for 1-to-3 and 2-to-4, the switch cannot establish connections for 3-to-6 and 4-to-5.
Time Division Switching
Both voice and data can be transmitted using digital signals through the same switches. All modern circuit switches use digital time-division multiplexing (TDM) technique for establishing and maintaining circuits. Synchronous TDM allows multiple low-speed bit streams to share a high-speed line. A set of inputs is sampled in a round robin manner. The samples are organized serially into slots (channels) to form a recurring frame of slots. During successive time slots, different I/O pairings are enabled, allowing a number of connections to be carried over the shared bus. To keep up with the input lines, the data rate on the bus must be high enough so that the slots recur sufficiently frequently. For 100 full-duplex lines at 19.200 Kbps, the data rate on the bus must be greater than 1.92 Mbps. The source-destination pairs corresponding to all active connections are stored in the control memory. Thus the slots need not specify the source and destination addresses. Schematic diagram of time division switching is shown in Fig. 4.1.7.
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Figure 4.1.7 Schematic diagram of a time division multiplexing system
Figure 4.1.8 TDM with Switching using TSI
Time-division switching uses time-division multiplexing to achieve switching, i.e. different ongoing connections can use same switching path but at different interleaved time intervals. There are two popular methods of time-division switching namely, Time-Slot Interchange (TSI) and the TDM bus. TSI changes the ordering of the slots based on desired connection and it has a random-access memory to store data and flip the time slots as shown in Fig. 4.1.8. The operation of a TSI is depicted in Fig. 4.1.9. As shown in the figure, writing can be performed in the memory sequentially, but data is read selectively. In TDM bus there are several input and outputs connected to a high-speed bus. During a time slot only one particular output switch is closed, so only one connection at a particular instant of time as shown in Fig. 4.1.10.
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Figure 4.1.9 Operation of a TSI
Figure 4.1.10 TDM bus switching
4.1.4 Public Switched Telephone Networks
Public switched telephone network (PSTN) is an example of circuit-switched network. It’s also known as Plain Old Telephone Service (POTS). The switching centres used for the switching are organised in different levels, namely: Regional offices (class 1), Section offices (class 2), primary offices (class 3), Toll offices (class 4) and finally End offices
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(class 5) as shown in Fig. 4.1.11. Level 1 is at the highest level and Level 5 is the lowest level. Subscribers or the customers are directly connected to these end offices. And each office is connected directly to a number of offices at a level below and mostly a single office at higher level.
Subscriber Telephones are connected, through Local Loops to end offices (or central offices). A small town may have only one end office, but large cities have several end offices. Many end offices are connected to one Toll office, which are connected to primary offices. Several primary offices are connected to a section office, which normally serves more than one state. All regional offices are connected using mesh topology. Accessing the switching station at the end offices is accomplished through dialling. In the past, telephone featured rotary or pulse dialling, in which digital signals were sent to the end office for each dialled digit. This type of dialling was prone to errors due to inconsistency in humans during dialling. Presently, dialling is accomplished by Touch-Tone technique. In this method the user sends a small burst of frequency called dual tone, because it is a combination of two frequencies. This combination of frequencies sent depends on the row and column of the pressed pad.
End Office
Sectional Office
Figure 4.1.11 Basic organization of a Public Switched Telephone Network (PSTN)
The connections are multiplexed when have to send to a switching office, which is one level up. For example, Different connections will be multiplexed when they are to be forwarded from an end-office to Toll office. Figure 4.1.12 shows a typical medium distance telephone circuit.
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Figure 4.1.12 Typical medium distance telephone circuit
Fill In the Blanks:
1
. ________ uses the entire capacity of the link.
2
. In ________ switching, each packet of a message need not follow the same path from sender to receiver.
3.
In ________ switching all the datagrams of a message follows the same channel of a path.
4.
PSTN is an example of _______________ network.
5.
PSTN is also known as ____________.
Ans:
1.
Circuit switching
2.
Datagram packet
3.
virtual circuit
4.
circuit switching
5.
plain old telephone service (POTS)
Short Answer Questions
Q-1. What are the three basic steps involved in data communication through circuit switching?
Ans: The steps are:
i)
Circuit establishment (before data transfer)
ii
) Circuit maintenance (When data transfer is going on)
ii
i) Circuit disconnect (When data transfer is over)
Q-2. Mention the key advantages and disadvantages of circuit switching technique.
Ans: Advantages:
i)
After path is established, data communication without delay.
ii)
Very suitable for continuous traffic.
ii
i) It establishes a dedicated path.
iv)
No overhead after call setup.
v) I
t is transparent and data passes in order.
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Disadvantages:
i)
Provide initial delay for setting up the call.
ii
) Inefficient for bursty traffic.
ii
i) Data rate should be same because of fixed bandwidth.
iv)
When load increases, some calls may be blocked.
Q-3. Why data communication through circuit switching is not efficient?
Ans: In data communication, traffic between terminal and server are not continuous. Sometimes more data may come or sometimes there is no data at all. Circuit switching is not efficient because of its fixed bandwidth.
Q-4. Compare the performance of space-division single-stage switch with multi-stage switch.
Ans: Space-division single-stage switch requires more number of crosspoints, nonblocking in nature but provides no redundant path. On the other hand multi-stage switches require lesser number of crosspoints, blocking in nature but provides redundant paths.
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