Switching Networks
ECE 466
Communication Networks
• A communication network provides a scalable solution to
connect a large number of end systems
end system
node
Communication
Network
Other names for “end system”: station, host, terminal
Other names for “node”: switch, router, gateway
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Taxonomy of Networks
• Communication networks can be classified based on
the way in which the nodes exchange information:
Communication
Network
Circuit-Switched
Network
Frequency
Division
Multiplexing
Packet-Switched
Network
Datagram
Network
Time Division
Multiplexing
Wavelength
Division
Multiplexing
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Virtual Circuit
Network
Circuit Switching
• A dedicated communication path (“circuit”) is established
• Each circuit occupies a fixed capacity
– Unused capacity cannot be used by other circuit
– “Busy Signal” if capacity for a circuit not available
• Circuit-switched communication involves three phases:
Circuit Establishment  Data Transfer  Circuit Release
• Circuit-switched multiplexing
– FDM, WDM
– TDM
• Data is not formatted (“raw bit stream”)
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Circuit Switching
circuit 2
C
B
D
1
2
3
5
4
A
circuit 1
7
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6
E
Timing in Circuit Switching
Host
Node
1
Host
Node
2
3
Circuit
Establishment
Data
Transmission
DATA
Circuit
Termination
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Processing delay
4
Implementation of Circuit-Switching
• There are three ways to implement circuits
– Frequency Division Multiplexing (FDM)
– Time Division Multiplexing (TDM)
– Wavelength Division Multiplexing (WDM)
ECE 466
Frequency Division Multiplexing (FDM)

Approach: Divide the frequency spectrum into logical
channels and assign each information flow one logical
channel
Channel 1 (f1)
Source 1
1
Source 2
Source 3
Source 4
Source 5
Source 6
Channel 2 (f2)
S
w
i
t
c
h
Channel 3 (f3)
Channel 4 (f4)
Channel 5 (f5)
Channel 6 (f6)
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2
S
w
i
t
c
h
3
4
5
6
Frequency Division Multiplexing (FDM)
Endsystem
Endsystem
Circuit
Switch
Circuit
Switch
Endsystem
Endsystem
• A circuit switch bundles (multiplexes) multiple voice calls on a highbandwidth link
• Frequency-Division-Multiplexing (FDM): Each circuit receives a fixed
bandwidth. The frequency of each call is shifted, so that multiple calls do
not interfere
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Time Division Multiplexing (TDM)

Approach: Multiple signals can be carried on a single
transmission medium by interleaving portions of each
signal in time
Source 1
1
Source 2
Source 3
M
M
2
3
Source 4
U
Source 5
X
12345612
U
X
Source 6
4
5
6
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Time Division Multiplexing (TDM)
endsystem
Circuit
Switch
endsystem
endsystem
slots
Circuit
Switch
endsystem
frames
• Time is divided into frames of fixed length
• Each frame has a fixed number of constant-sized “slots”
• Each circuit obtains one or more “slots” per frame
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Packet Switching
•
Data are sent as formatted bit-sequences, so-called packets
Header
Data
• Each packet is passed through the network from node to node along some
path (Forwarding/Routing)
• At each node, the entire packet is received, stored briefly, and then
forwarded to the next node (Store-and-Forward Networks)
• Packet transmission is never interrupted (no preemption)
• No capacity is allocated for packets
• Multiplexing: Statistical TDM
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A Packet Switch
input
queues
output
queues
switch
fabric
memory
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Statistical Multiplexing
• Packet transmission on a link is referred to as statistical
multiplexing
– There is no fixed allocation of packet transmissions
– Packets are multiplexed as they arrive
• Packet switches require buffering
Packets from different
streams
Transmission
line
1
2
1
N
2
output buffer
N
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1
Datagram Packet Switching
• The network nodes process each packet independently
If Host A sends two packets back-to-back to Host B over a datagram
packet network, the network cannot tell that the packets belong
together In fact, the two packets can take different routes
• Packets are called datagrams
• Implications of datagram packet switching:
• A sequence of packets can be received in a different
order than it was sent
• Each packet header must contain the full address of the
destination
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Timing of Datagram Packet Switching
Host
Node
1
Transmission
delay
Host
Node
2
3
4
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
Packet 1
Packet 2
Packet 3
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Datagram Packet Switching
C
B
C.2
C.1
C.1
C.2
1
2
C.1
C.2
5
A.1
A.3 A.2
4
A
A.1
A.2
A.3
A.3 C.2
A.1
A.2
A.3
D
3
A.1
A.2
A.2
7
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A.2
6
E
Virtual-Circuit Packet Switching
• Virtual-circuit packet switching is a hybrid of circuit switching
and packet switching:
– All data is transmitted as packets
– All packets from one packet stream are sent along a preestablished path (=virtual circuit)
• Communication with virtual circuits (VC) takes place in three
phases:
1. VC Establishment
2. Data Transfer
3. VC Release
ECE 466
Virtual-Circuit Packet Switching
•
•
Guarantees in-sequence delivery of packets, but packets
from different virtual circuits may be interleaved
Packet headers don’t need to contain the full destination
address of the packet
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Timing of VC Packet Switching
Host
Node
1
Host
Node
2
3
4
VC
Establishment
Transmission
delay
Pkt1
Pkt2
Pkt3
Pkt1
Pkt2
Pkt3
Pkt1
Pkt2
Pkt3
VC
Termination
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Virtual-Circuit Packet Switching
C.2
VC 2
B
C
C.1
C.1
C.2
1
3
C.1
5
A A.1
VC 1
D
2
C.2
4
A.1
A.2
A.3
C.2
A.3 A.2
A.1 C.1
A.1
A.3 A.2
7
A.2
A.3
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6
E
Packet Forwarding and Routing
•
Datagram networks organize their routing table
according to destination addresses. All packets with same
destination are handled in the same way
•
VC packet networks set up routing table for each virtual
circuit during the call-establishment phase. Each switch
has a routing table entry for each virtual circuit going
through this switch.
ECE 466
Packet Forwarding of Datagrams
• Recall: In datagram networks, each packet must carry the full
destination address
• Each router maintains a routing table which has one row for
each possible destination address
• The lookup yields the address of the next hop (next-hop
routing)
Routing Table of node v
to
via
(next hop)
w
v
n
x
d
n
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d
Packet Forwarding of Datagrams
• When a packet arrives at an incoming link, ...
1. The router looks up the routing table
2. The routing table lookup yields the address of the next
node (next hop)
3. The packet is transmitted onto the outgoing link that goes
to the next hop
Routing Table of node v
to
via
(next hop)
d
w
d
v
n
x
d
n
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d
Forwarding
Datagrams
To
Next
hop
To
A
-
A
A
B
B
B
-
C
C
C
D
D
C
D
D
E
B
E
E
X
C
X
D
A
To
Next
hop
Next
hop
To
E
E
B
E
E
E
E
E
A
C
B
C
C
C
D
C
E
C
A
A
A
B
X
-
B
-
B
B
C
D
C
C
D
D
D
-
E
D
E
B
X
E
X
C
X
C
To
D
To
Next
hop
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Next
hop
Next
hop
A
B
B
B
C
B
D
B
E
-
X
B
Packet Forwarding with Virtual Circuits
• Recall: In VC networks, the route is setup in the connection
establishment phase
• During the setup, each router assigns a VC number (VC#) to
the virtual circuit
• The VC# can be different for each hop
• VC# is written into the packet headers
Routing Table of node v
from
VC#
to
path of virtual
circuit
VC#
2
w
3
1
v
n
x
w
2
d
3
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d
Packet Forwarding of Virtual Circuits
• When a packet with VCin in header arrives from router nin, ...
1. The router looks up the routing table for an entry with
(VCin, nin)
2. The routing table lookup yields (VCout, nout)
3. The router updates the VC# of the header to VCout and
transmits the packet to nout
Routing Table of node v
from
VC#
to
2
VC#
2
w
3
path of virtual
circuit
1
3
1
v
n
x
w
2
n
3
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d
Forwarding with VCs
Part 1: VC setup
from X to E
nin
Vin
nout
Vout
-
-
C
5
nin
Vin
D
5
nout
Vout
E
3
A
B
nin
X
C
nin
X
E
Vin
5
Vin
B
3
nout
Vout
B
5
D
nout
Vout
D
3
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nin
C
Vin
3
nout
Vout
-
-
Forwarding with VCs
Part 2: Forwarding
the packet
nin
Vin
nout
Vout
-
-
C
5
nin
Vin
D
5
nout
Vout
E
2
A
2
B
5
5
X
3
C
nin
X
Vin
5
E
nin
Vin
B
3
nout
Vout
B
5
D
nout
Vout
D
3
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nin
C
Vin
3
nout
Vout
-
-
Comparison
Datagram Packet
Switching
Circuit Switching









Dedicated
transmission path
Continuous
transmission
Path stays fixed for
entire connection
Call setup delay
Negligible
transmission delay
No queueing delay
Busy signal
overloaded network
Fixed bandwidth for
each circuit
No overhead after
call setup









No dedicated
transmission path
Transmission of
packets
Route of each packet
is independent
No setup delay
Transmission delay
for each packet
Queueing delays at
switches
Delays increase in
overloaded networks
Bandwidth is shared
by all packets
Overhead in each
packet
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VC Packet
Switching









No dedicated
transmission path
Transmission of
packets
Path stays fixed for
entire connection
Call setup delay
Transmission delay
for each packet
Queueing delays at
switches
Delays increase in
overloaded networks
Bandwidth is shared
by all packets
Overhead in each
packet
Packet Switching Technologies
• Both packet switching technologies are used.
• Datagram packet switching:
– Internet
– Switched LANs
• Virtual-circuit packet switching
– Asynchronous Transfer Mode (ATM)
– Multi-protocol label switching (MPLS)
ECE 466