Download Circuit-switching networks

Circuit Switching
References
• Chapter 8, W. Stallings, Data and Computer
Communications, Prentice Hall.
K.1
Switching Networks
Introduction
– Types: circuit switching and packeting switching
– circuit switching has been the dominant technology for
voice communications.
K.2
Switching Networks
Switching Nodes
– Communication is achieved by transmitting data from
source to destination through a network of intermediate
switching nodes.
– The switching nodes are not concerned with the content
of the data.
• their purpose is to provide a switching facility that will
moves the data from node to node until they reach their
destination.
K.3
Switching Networks
Example
– transmitting data from station A to station F
• via nodes 4 - 5 -6 or via nodes 4 - 7 - 6
K.4
Circuit-switching networks
Introduction
– Communication via circuit switching implies that there is
a dedicated communication path between two stations.
– The path is a connected sequence of links between
network nodes.
– On each physical link, a logical channel is dedicated to
the connection.
K.5
Circuit-switching networks
Phases of circuit switching
– Circuit establishment
• Before any signals can be transmitted, an end-to-end
circuit must be established.
– Data Transfer
• The data may be analog or digital
• Connection is full-duplex
– Circuit Disconnect
• by the action of one of the two stations.
K.6
Circuit-switching networks
Example
– sending data from station A to station E
1. Circuit establishment
– Station A sends a request to node 4 requests a connection
to Station E.
• A to 4 is a dedicated line.
– Node 4 search the 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 (if multiplexing is
used) on that link and sends a message requesting
connection to E.
K.7
Circuit-switching networks
• Node 5 dedicates a channel to node 6 internally ties
that channel to the channel from node 4.
– Node 6 connects to E.
– A test is made to determine if E is busy or is prepared to
accept the connection.
2. Data transfer
– Information is transmitted from A through the network to
E.
3. Circuit disconnect
– By the action of one of the two stations.
– Signals must be propagated to node 4, 5 and 6 to deallocate the dedicated resources.
K.8
Circuit-switching networks
Nodes
– As the connection path is established before data
transmission begins, channel capacity 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
K.9
Circuit-switching networks
Performance
– inefficient as channel capacity is dedicated for the
duration of a connection, even if no data are being
transferred.
– Utilization for voice transmission is higher than that of
data transmission.
– There is a delay prior to signal transfer for call
establishment
– Once the circuit is established, information is transmitted
at a fixed rate with no delay other than other required for
propagation through transmission links. The delay at each
node is negligible.
K.10
Circuit-switching networks
Applications
– both voice and data transmission.
• Public telephone network
• private branch exchange within a building or office
• private networks
• data switch
K.11
Circuit-switching networks
Applications
– both voice and data transmission.
• Public telephone network
¾to service analog telephone subscribers
¾handles data traffic via modem
• Private branch exchange (PBX)
¾interconnect telephones within a building or office
• Private networks
¾consist of PBX systems at each site of a large
organization
¾sites are interconnected by dedicated, leased lines
obtained from the telephone company.
K.12
• Data switch
Circuit-switching networks
Example
K.13
Circuit-switching networks
K.14
Space division switching
Switching techniques
– switching within a circuit switching node
• Space division switching
• Time division switching
Space division switching
– developed for the analog environment
• also used for digital applications
– a space division switch is one in which the signal paths
are physically separate from one another (divided in
space)
K.15
Space division switching
– Each connection requires the establishment of a physical
path through the switch that is dedicated solely to the
transfer of signals between 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
K.16
Space division switching
Example
– a crossbar matrix with
10 full-duplex I/O lines
– interconnection is
possible between any
two lines by enabling
the appropriate
crosspoint.
– 100 crosspoints are
required.
K.17
Space division switching
– This 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
two devices whose lines intersect at that crosspoint.
• The crosspoints are inefficiently utilized. In this
example, at most 10 out of 100.
K.18
Space division switching
– To overcome these limitations, multiple-stage switches
are employed.
K.19
Space division switching
– The number of crosspoints is reduced, increasing crossbar
utilization. In this example, the total number of
crosspoints is 48.
– This is more than one path through the network to
connect two endpoints, increasing reliability.
– Requires a more complex control scheme.
• A free path through the stages must be determined.
K.20
Space division switching
– Blocking
• Example: input line 10 cannot be connected to output
line 3, 4, or 5, even though all of these output lines are
available.
K.21
Time division switching
– Virtually all modern circuit switches use digital timedivision techniques for establishing and maintaining
“circuits”
– Time division switching involves the partitioning of a
lower-speed bit stream into pieces that share a higherspeed stream with other bit streams.
– The individual pieces, or slots, are manipulated by
control logic to route data from input to output.
K.22
Time division switching
Example: TDM bus 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.
K.23
Time division switching
– Input data on each line are buffered at the gate.
– Each buffer must be cleared, by enabling the gate, quickly
enough to avoid overrun.
• Suppose there are 100 full-duplex lines at 19.2 kbps,
the data rate on the bus must be greater than 1.92
Mbps.
– 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.
K.24
Time division switching
– For that same time slot, data are switched from the
enabled input line to the enabled output line.
– During successive time slots, different input/output
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.
K.25
Time division switching
– The time slot must equal the transmission time of the
input plus the propagation delay between input and output
across the bus.
– TDM bus switching scheme can accommodate lines of
varying data rates.
K.26