Switching

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 8-1 INTRODUCTION

 A switched network consists of a series of interlinked nodes,

called switches.
 Switches are devices capable of creating temporary connections
between two or more devices linked to the switch.
 In a switched network, some of these nodes are connected to the
end systems (computers or telephones, for example).
 Others are used only for routing.
 The end systems (communicating devices) are labeled A, B, C, D,
and so on, and the switches are labeled I, II, III, IV, and V.
 Each switch is connected to multiple links.

8.2
 Figure 8.1: Switched network

8.3
 8.8.1 Three Methods of Switching

 These are the two most common methods of

switching:
1. Circuit switching
2. Packet switching

 Packet switching can further be divided

into two subcategories,
1. Virtual-circuit approach and
2. Datagram approach

8.4
 Figure 8.2: Taxonomy of switched networks

8.5
 8.8.2 Switching and TCP/IP Layers

• At the physical layer, we can have only circuit switching.

There are no packets exchanged at the physical layer. The
switches at the physical layer allow signals to travel in one path
or another.

• At the data-link layer, we can have packet switching. However,

the term packet in this case means frames or cells. Packet
switching at the data-link layer is normally done using a virtual-
circuit approach.

• At the network layer, we can have packet switching. In this

case, either a virtual-circuit approach or a datagram approach
can be used.

8.6
 8-2 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
 When end system A needs to communicate with end system M, system A
needs to request a connection to M that must be accepted by all switches as
well as by M itself.
 This is called the setup phase; a circuit (channel) is reserved on each link,
and the combination of circuits or channels defines the dedicated path.
 After the dedicated path made of connected circuits (channels) is
established, the data-transfer phase can take place.
 After all data have been transferred, the circuits are torn down.

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 Figure 8.3: A trivial circuit-switched network

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 Circuit switching takes place at the physical layer.
 Before starting communication, the stations must make a reservation
for the resources to be used during the communication. These
resources, such as channels (bandwidth in FDM and time slots in TDM),
switch buffers, switch processing time, and switch input/output ports,
must remain dedicated during the entire duration of data transfer until
the teardown phase.
 Data transferred between the two stations are not packetized. The data
are a continuous flow sent by the source station and received by the
destination station, although there may be periods of silence.
 There is no addressing involved during data transfer. The switches
route the data based on their occupied band (FDM) or time slot (TDM).
Of course, there is end-to-end addressing used during the setup phase.
 8.2.1 Three Phases

The actual communication in a circuit-switched

network requires three phases:
• Connection setup (handshake),
• Data transfer, and
• Connection teardown

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 Setup Phase
 The end systems are normally connected through dedicated lines to the switches,
so connection setup means creating dedicated channels between the switches.
 When system A needs to connect to system M, it sends a setup request that
includes the address of system M, to switch I.
 Switch I finds a channel between itself and switch IV that can be dedicated for
this purpose.
 Switch I then sends the request to switch IV, which finds a dedicated channel
between itself and switch III.
 Switch III informs system M of system A’s intention at this time.
 In the next step to making a connection, an acknowledgment from system M
needs to be sent in the opposite direction to system A.
 Only after system A receives this acknowledgment is the connection established.
 Data-Transfer Phase
After the establishment of the dedicated circuit
(channels), the two parties can transfer data.
 Teardown Phase
When one of the parties needs to disconnect, a signal is
sent to each switch to release the resources.
 8.2.2 Efficiency

 Circuit-switched networks are not as efficient as

the other two types of networks because resources
are allocated during the entire duration of the
connection.
 These resources are unavailable to other
connections.
 In a telephone network, people normally terminate
the communication when they have finished their
conversation.
 However, in computer networks, a computer can be
connected to another computer even if there is no
activity for a long time.
 In this case, allowing resources to be dedicated
means that other connections are deprived.
8.13
 8.2.3 Delay

 Although a circuit-switched network

normally has low efficiency, the delay in
this type of network is minimal.
 During data transfer the data are not
delayed at each switch; the resources are
allocated for the duration of the connection.

8.14
 Figure 8.6: Delay in a circuit-switched network

Data transfer

8.15
 CONTROL SIGNALING
• In a circuit-switched network, control signals are the means by which the
network is managed and by which calls are established, maintained,
and terminated.
• Both call management and overall network management require that
information be exchanged between subscriber and switch, among
switches, and between switch and network management center.
Signaling Functions

 As networks become more complex, the number of functions performed by control signaling
necessarily grows.
 The following functions, are among the most important:
1. Audible communication with the subscriber, including dial tone, ringing tone, busy signal,
and so on.
2. Transmission of the number dialed to switching offices that will attempt to complete a
connection.
3. Transmission of information between switches indicating that a call cannot be completed.
4. Transmission of information between switches indicating that a call has ended and that the
path can be disconnected.
5. A signal to make a telephone ring.
6. Transmission of information used for billing purposes.
7. Transmission of information giving the status of equipment or trunks in the network. This
information may be used for routing and maintenance purposes.
 Signaling can also be classified functionally as supervisory, address, call-
information, and network-management.
 The term supervisory is generally used to refer to control functions that have a
binary character (true/false; on/off), such as request for service, answer, alerting,
and return to idle.
 Supervisory control signals are used to determine if a needed resource is
available and, if so, to seize it; they are also used to communicate the status of
requested resources.
 Address signals identify a subscriber.
 Initially, an address signal is generated by a calling subscriber when dialing a
telephone number.
 The resulting address may be propagated through the network to support the
routing function and to locate and ring the called subscriber's phone.
 The term call-information refers to those signals that provide information to the
subscriber about the status of a call.
 Call information signals are audible tones that can be heard by the caller or an
operator with the proper phone set.
 Supervisory, address, and call-information control signals are directly involved in the
establishment and termination of a call.
 In contrast, network management signals are used for the maintenance,
troubleshooting, and overall operation of the network.
 Such signals may be in the form of messages, such as a list of preplanned routes
being sent to a station to update its routing tables.
Location of Signaling

 Control signaling needs to be considered in two contexts: signaling between a

subscriber and the network, and signaling within the network.
 The signaling between a telephone or other subscriber device and the switching
office to which it attaches is, to a large extent, determined by the characteristics
of the subscriber device and the needs of the human user.
 Signals within the network are entirely computer-to-computer.
In Channel Signaling

 With in channel signaling, the same channel is used to carry control

signals as is used to carry the call to which the control signals relate.
 Such signaling begins at the originating subscriber and follows the same
path as the call itself.
 This process has the merit that no additional transmission facilities are
needed for signaling; the facilities for voice transmission are shared with
control signaling.
 Two forms of in-channel signaling are in use: inband and out-of-band.
 Inband signaling uses not only the same physical path as the call it serves; it also uses
the same frequency band as the voice signals that are carried.
 In band signaling advantages
 Because the control signals have the same electromagnetic properties as the voice
signals, they can go anywhere that the voice signals go.
 Out-of-band signaling advantages
 Out-of-band signaling takes advantage of the fact that voice signals do not use the full
4-kHz bandwidth allotted to them.
 A separate narrow signaling band within the 4 kHz is used to send control signals.
 The major advantage of this approach is that the control signals can be sent whether
or not voice signals are on the line, thus allowing continuous supervision and control
of a call.
 However, an out-of-band scheme needs extra electronics to handle the signaling band,
and the signaling rates are slower because the signal has been confined to a narrow
bandwidth.
Drawbacks of inchannel signaling
The information transfer rate is quite limited with inchannel signaling.
With out-of-band signals, a very narrow bandwidth is available.
A second drawback of inchannel signaling is the amount of delay from the time a
subscriber enters an address (dials a number) to when the connection is established.
Both of these problems can be addressed with common-channel signaling, in which
control signals are carried over paths completely independent of the voice channels.
One independent control signal path can carry the signals for a number of subscriber
channels, and, hence, is a common control channel for these subscriber channels.
Common-channel signaling

 The signal path for common channel signaling is physically separate from the
path for voice or other subscriber signals.
 The common channel can be configured with the bandwidth required to carry
control signals for a rich variety of functions.
 The control signals are messages that are passed between switches as well as
between a switch and the network management center.
 Two modes of operation are used in common-channel signaling.
 In the associated mode, the common channel closely tracks along its entire
length the inter-switch trunk groups that are served between endpoints.
 A more complex, but more powerful, mode is the non-associated mode; with
this, the network is augmented by additional nodes, known as signal transfer
points.
 There are now two separate networks, with links between them so that the
control portion of the network can exercise control over the switching nodes that
are servicing the subscriber calls.
 8-3 PACKET SWITCHING

 If the message is going to pass through a packet-switched network, it

needs to be divided into packets of fixed or variable size.
 The size of the packet is determined by the network and the governing
protocol.
 In packet switching, there is no resource allocation for a packet.
 This means that there is no reserved bandwidth on the links, and there
is no scheduled processing time for each packet.
 Resources are allocated on demand.
 The allocation is done on a first-come, first-served basis.
 When a switch receives a packet, no matter what the source or destination
is, the packet must wait if there are other packets being processed.

8.29
 8-3 PACKET SWITCHING

 Packet switched networks are classified as

a) Datagram Networks
b) Virtual circuit Networks

8.30
 8.3.1 Datagram Networks

 In a datagram network, each packet is treated independently of all

others.
 Even if a packet is part of a multipacket transmission, the network treats
it as though it existed alone.
 Packets in this approach are referred to as datagrams.
 Datagram switching is normally done at the network layer.
 The switches in a datagram network are traditionally referred to as
routers.
 There are no setup or teardown phases.
 Each packet is treated the same by a switch regardless of its source or
destination.

8.31
 Figure 8.7: A Datagram network with four 3-level switches (routers)

3 1
4 3 2 1
4
1

2 3
1
4
2 2 3 4 1

8.32
 8.3.1 Datagram Networks

Routing Table
 If there are no setup or teardown phases, how are the packets
routed to their destinations in a datagram network?
 In this type of network, each switch (or packet switch) has a
routing table which is based on the destination address.
 The routing tables are dynamic and are updated periodically.
 The destination addresses and the corresponding forwarding
output ports are recorded in the tables.

8.33
 Figure 8.8: Routing table in a datagram network

8.34
 Figure 8.9: Delays in a datagram network

8.36
 8.3.2 Virtual-Circuit Networks

 A virtual-circuit network is a cross between a circuit-switched network and a

datagram network. It has some characteristics of both.
1. As in a circuit-switched network, there are setup and teardown phases in addition to
the data transfer phase.
2. Resources can be allocated during the setup phase, as in a circuit-switched network,
or on demand, as in a datagram network.
3. As in a datagram network, data are packetized and each packet carries an address
in the header. However, the address in the header has local jurisdiction (it defines
what the next switch should be and the channel on which the packet is being
carried), not end-to-end jurisdiction.
4. As in a circuit-switched network, all packets follow the same path established during
the connection.

8.37
 Figure 8.10: Virtual-circuit network

8.38
 8.3.2 Virtual-Circuit Networks
Addressing
 In a virtual-circuit network, two types of addressing are involved: global
and local (virtual-circuit identifier).
Global Addressing
 A source or a destination needs to have a global address—an address that
can be unique in the scope of the network or internationally if the network
is part of an international network.
 However, a global address in virtual-circuit networks is used only to create
a virtual-circuit identifier.
Virtual-Circuit Identifier
 The identifier that is actually used for data transfer is called the virtual-
circuit identifier (VCI) or the label.
 A VCI, unlike a global address, is a small number that has only switch
scope; it is used by a frame between two switches.
 When a frame arrives at a switch, it has a VCI; when it leaves, it has a
different VCI.
 VCI does not need to be a large number since each switch can use its own
unique set of VCIs.

8.39
 Figure 8.11: Virtual-circuit identifier

8.40
 8.3.2 Virtual-Circuit Networks

Three Phases:
As in a circuit-switched network, a source and destination need to go through
three phases in a virtual-circuit network: setup, data transfer, and teardown.
In the setup phase, the source and destination use their global addresses to
help switches make table entries for the connection.
In the teardown phase, the source and destination inform the switches to
delete the corresponding entry.
Data transfer occurs between these two phases.

8.41
Data-Transfer Phase
 To transfer a frame from a source to its destination, all switches
need to have a table entry for this virtual circuit.
 The table, in its simplest form, has four columns.
 This means that the switch holds four pieces of information for
each virtual circuit that is already set up.
 The data-transfer phase is active until the source sends all its
frames to the destination.
 Figure 8.12: Switch and table for a virtual-circuit network

8.43
 Figure 8.13: Source-to-destination data transfer in a circuit-switch network

8.44
 Setup Phase
• In the setup phase, a switch creates an entry for a
virtual circuit.
• For example, suppose source A needs to create a
virtual circuit to B.
• Two steps are required: the setup request and the
acknowledgment.
1. Setup Request
• A setup request frame is sent from the source to the
destination.
 Figure 8.14: Setup request in a virtual-circuit network
All nodes have a VCI

8.46
 Virtual Circuit Networks

2. Acknowledgment
• A special frame, called the acknowledgment
frame, completes the entries in the
switching tables.

8.47
 Figure 8.15: Setup acknowledgment in a virtual-circuit network

8.48
Teardown Phase
 In this phase, source A, after sending all
frames to B, sends a special frame called a
teardown request.
 Destination B responds with a teardown
confirmation frame.
 All switches delete the corresponding entry
from their tables.
Efficiency
 Resource reservation in a virtual-circuit network can be made during the
setup or can be on demand during the data-transfer phase.
 In the first case, the delay for each packet is the same; in the second
case, each packet may encounter different delays.
 The source can check the availability of the resources, without actually
reserving it.
 Figure 8.16: Delay in a virtual-circuit network

8.52
CONGESTION CONTROL IN PACKET-SWITCHING
NETWORKS
A number of control mechanisms for congestion control in packet-switching networks have been
suggested and tried. The following are examples:
1.Send a control packet from a congested node to some or all source nodes. This choke
packet will have the effect of stopping or slowing the rate of transmission from sources and hence
limit the total number of packets in the network. This approach requires additional traffic on the
network during a period of congestion.
2.Rely on routing information. Routing algorithms, such as ARPANET’s, provide link delay
information to other nodes, which influences routing decisions. This information could also be used
to influence the rate at which new packets are produced. Because these delays are being
influenced by the routing decision, they may vary too rapidly to be used effectively for congestion
control.
3.Make use of an end-to-end probe packet. Such a packet could be timestamped to measure the
delay between two particular endpoints. This has the disadvantage of adding overhead to the
network.
4.Allow packet-switching nodes to add congestion information to packets as they go by.
There are two possible approaches here. A node could add such information to packets going in
the direction opposite of the congestion. This information quickly reaches the source node, which
can reduce the flow of packets into the network. Alternatively, a node could add such information
to packets going in the same direction as the congestion. The destination either asks the source to
adjust the load or returns the signal back to the source in the packets (or acknowledgments) going
in the reverse direction.
 X.25
 One technical aspect of packet-switching networks remains to be examined: the
interface between attached devices and the network.
 We have seen that a circuit-switching network provides a transparent
communications path for attached devices that makes it appear that the two
communicating stations have a direct link.
 However, in the case of packet-switching networks, the attached stations must
organize their data into packets for transmission.
 This requires a certain level of cooperation between the network and the
attached stations.
 The standard used for traditional packet-switching networks is X.25.
 X.25 is an ITU-T (International Telecommunication
Union Telecommunication Standardization Sector)
standard that specifies an interface between a host
system and a packet-switching network.
 The functionality of X.25 is specified on three levels:
• Physical level
• Link level
• Packet level
 The physical level deals with the physical interface between an attached station
(computer, terminal) and the link that attaches that station to the packet-switching node.
 It makes use of the physical-level specification in a standard known as X.21.
 The link level provides for the reliable transfer of data across the physical link, by
transmitting the data as a sequence of frames.
 The link level standard is referred to as LAPB (Link Access Protocol–Balanced).
 The packet level provides a virtual circuit service.
 This service enables any subscriber to the network to set up logical connections, called
virtual circuits, to other subscribers.
 User data are passed down to X.25 level 3, which appends control information as
a header, creating a packet.
 This control information serves several purposes, including
1. Identifying by number a particular virtual circuit with which this data is to be
associated
2. Providing sequence numbers that can be used for flow and error control on a
virtual circuit basis
 The entire X.25 packet is then passed down to the LAPB entity, which appends
control information at the front and back of the packet, forming a LAPB frame.
 Again, the control information in the frame is needed for the operation of the
LAPB protocol.
 Each X.25 data packet includes send and receive sequence numbers.
 The send sequence number, P(S), is used to number sequentially all outgoing
data packets on a particular virtual circuit.
 The receive sequence number, P(R), is an acknowledgment of packets received
on that virtual circuit.