Review of Networking Concepts
Part 1: Switching Networks
© Jörg Liebeherr, 2000-2003
CS757
Summary of Topics
•
•
•
•
•
Taxonomy of switching networks
Principles of circuit and packet switching
Multiplexing techniques: FDM, TDM, statistical multiplexing
Datagram and Virtual-circuit packet networks
Packet forwarding in datagram and VC packet networks
© Jörg Liebeherr, 2000-2003
CS757
Communication Networks
• Problem: Connect end systems that want to
exchange information
(Device = telephone, computer, terminals, etc.)
• Simple Solution: Connect each pair of end system by
a dedicated point-to-point link
– The simple solution is sufficient if the number of
end systems is small
© Jörg Liebeherr, 2000-2003
CS757
Communication Networks
• With a large number of end systems it is not practical to
connect each pair
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CS757
Communication Networks
• A communication network provides a scalable solution to
connect a large number of end systems
• Principles:
– There are two types of devices: end systems and
nodes
– Each node is connected to at least one node
– Network nodes carry the information from a source to
a destination end system
– Note: Network nodes do not generate information
© Jörg Liebeherr, 2000-2003
CS757
Communication Networks
• A generic communication network:
end system
Node
Communication
Network
Other names for “end system”: station, host, terminal
Other names for “node”: switch, router, gateway
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CS757
Example Networks
• Evolution of the UUNet Network 1996-2000
• UVA Backbone Network
Source: National Science Foundation
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CS757
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
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Packet-Switched
Network
Datagram
Network
Time Division
Multiplexing
Wavelength
Division
Multiplexing
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Virtual Circuit
Network
Circuit Switching
• In a circuit-switched network, a dedicated
communication path (“circuit”) is established
between two stations through the nodes of the
network
• The dedicated path is called a circuit-switched
connection or circuit
• A circuit occupies a fixed capacity of each link for the
entire lifetime of the connection. Capacity unused by
the circuit cannot be used by other circuits
• Data is not delayed at the switches
© Jörg Liebeherr, 2000-2003
CS757
Circuit Switching
•
•
•
Circuit-switched communication involves three phases:
1. Circuit Establishment
2. Data Transfer
3. Circuit Release
“Busy Signal” if capacity for a circuit not available
Most important circuit-switching networks:
• Telephone networks
• ISDN (Integrated Services Digital Networks)
© Jörg Liebeherr, 2000-2003
CS757
Circuit Switching
circuit 2
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A
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circuit 1
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Implementation of Circuit-Switching
• There are two ways to implement circuits
– Frequency Division Multiplexing (FDM)
– Time Division Multiplexing (TDM)
– Wavelength Division Multiplexing (WDM)
• Example: Voice in (analog) telephone network:
Needed bandwidth:
3000 Hz
Allocated bandwidth:
4000 Hz
Therefore, a channel with 64 kHz can carry 16 voice conversations
© Jörg Liebeherr, 2000-2003
CS757
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
© Jörg Liebeherr, 2000-2003
Channel 2 (f2)
S
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Channel 3 (f3)
Channel 4 (f4)
Channel 5 (f5)
Channel 6 (f6)
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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
© Jörg Liebeherr, 2000-2003
CS757
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
3
Source 4
U
Source 5
X
12345612
U
X
Source 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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CS757
Circuit Switch
switch
fabric
memory
•A circuit switch relays a circuit from an input to an output link
•A switch may reassign frequencies (FDM) or time slot allocation (TDM)
•No queueing delays are experienced
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CS757
Packet Switching
•
•
Data are sent as formatted bit-sequences, so-called packets
Packets have the following structure:
Header
Data
Trailer
• Header and Trailer carry control information
• 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
© Jörg Liebeherr, 2000-2003
CS757
A Packet Switch
input
queues
output
queues
switch
fabric
memory
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CS757
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
Packets from different
streams
Transmission
line
1
2
1
N
2
output buffer
N
© Jörg Liebeherr, 2000-2003
CS757
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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
© Jörg Liebeherr, 2000-2003
CS757
Virtual-Circuit Packet Switching
• Virtual-circuit packet switching is a hybrid of circuit switching
and packet switching
– All data is transmitted as packets
– Emulates a circuit-switched network
• All packets from one packet stream are sent along a preestablished path (=virtual circuit)
– Guarantees in-sequence delivery of packets
– Note: Packets from different virtual circuits may be
interleaved
© Jörg Liebeherr, 2000-2003
CS757
Virtual-Circuit Packet Switching
•
Communication with virtual circuits (VC) takes place in three
phases:
1. VC Establishment
2. Data Transfer
3. VC Disconnect
•
Note: Packet headers don’t need to contain the full
destination address of the packet
•
Circuit-switched and virtual-circuit packet-switched networks
are said to provide a connection-oriented service.
© Jörg Liebeherr, 2000-2003
CS757
Packet Forwarding and Routing
•
There are two parts to the routing problem:
1. How to pass a packet from an input interface to the output
interface of a router (packet forwarding)?
2. How to calculate routes (routing algorithm)?
•
Packet forwarding is done differently in datagram and virtualcircuit packet networks
•
Route calculation is similar in datagram and virtual-circuit
packet networks
© Jörg Liebeherr, 2000-2003
CS757
Datagram Packet Switching
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Virtual-Circuit Packet Switching
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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
© Jörg Liebeherr, 2000-2003
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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
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© Jörg Liebeherr, 2000-2003
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Forwarding
Datagrams
To
Next
hop
To
A
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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
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B
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D
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© Jörg Liebeherr, 2000-2003
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Next
hop
Next
hop
A
B
B
B
C
B
D
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-
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
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© Jörg Liebeherr, 2000-2003
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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
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Forwarding with VCs
Part 1: VC setup
from X to E
nin
Vin
nout
Vout
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nin
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Vout
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Forwarding with VCs
Part 2: Forwarding
the packet
nin
Vin
nout
Vout
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5
nin
Vin
D
5
nout
Vout
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Comparison
Datagram Packet
Switching
Circuit Switching


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




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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
© Jörg Liebeherr, 2000-2003









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