Packet-Switched Networks
© Jörg Liebeherr
ECE 1545
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
ECE 1545
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
ECE 1545
Virtual Circuit
Network
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
ECE 1545
A Packet Switch
input
queues
output
queues
switch
fabric
memory
ECE 1545
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
ECE 1545
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
ECE 1545
A.2
6
E
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
ECE 1545
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
ECE 1545
6
E
Packet Switching Technologies
• Both packet switching technologies are used today:
• Datagram packet switching:
– IP routers (Internet)
– Ethernet Switches (Switched LANs)
• Virtual-circuit packet switching
– Asynchronous Transfer Mode (ATM)
– Multi-protocol label switching (MPLS)
ECE 1545
Packet Switch Architectures
ECE 1545
Packet Switches
• Different types of packet switches:
– IP routers
– ATM switches
– MPLS switches
– Ethernet (LAN) switches
– Frame Relay
• All types of packet switches have very similar characteristics
ECE 1545
Switch Components
• Hardware components of a
router:
– Network interfaces
– Interconnection network
– Processor with a memory
and CPU
Processor
Memory
CPU
Interconnection Network
Interface Card
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Interface Card
Interface Card
Basic Architectural Components
Per-packet processing
Routing
Table
Routing
Decision
Routing
Table
Forwarding
Decision
Routing
Table
Forwarding
Decision
Switch Fabric
Output
Scheduling
Functional Components
routing
protocol
Routing
functions
routing
protocol
routing table
updates
Control
routing
table
Datapath:
routing table
lookup
per-packet
processing
Forwarding
Incoming
packets
outgoing
packets
ECE 1545
Routing and Forwarding
Routing functions include:
– route calculation
– maintenance of the routing table
– execution of routing protocols
Forwarding is per-packet processing
• On high-end packet switches routers, forwarding is highly
parallelized, and most work is done on the interface cards
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IP Router
• Lookup packet destination address in forwarding table.
– If known, forward to correct port.
– If unknown, drop packet.
• Decrement TTL, update header checksum.
• Forward packet to outgoing interface.
• Transmit packet onto link.
© Jörg Liebeherr, 2005
ECE 1545
ATM Switch
•
•
•
•
Look up VCI/VPI of cell in VC table.
Replace old VCI/VPI with new.
Forward cell to outgoing interface.
Transmit cell onto link.
© Jörg Liebeherr, 2005
ECE 1545
Ethernet Switch
• Lookup frame destination address in forwarding table.
– If known, forward to correct port.
– If unknown, broadcast to all ports.
• Learn source address of incoming frame.
• Forward frame to outgoing interface.
• Transmit frame onto link.
© Jörg Liebeherr, 2005
ECE 1545