1. Packet Switching Outline Store-and-Forward Switches
Bridges and Extended LANs
Cell Switching
Segmentation and Reassembly
CS380
Int. to Comp. Networks
Switching – Part I

2. Packet Switching Problem: “Not all networks are Directly Connected”
Directly connected networks have two limitations:
- number of hosts that can be accommodated
- geographic limitation
Goal – networks that are global in scale
Like Telephone using (circuit) switches,
we use packet switches(store-and-forward) that take packets that arrive on an input and forward (switch) them to the right output. Two ways to do it:
- connection
- connectionless
CS380
Int. to Comp. Networks
Switching – Part I

3. Packet Switching Key problems that a switch must deal with:
Contention – packet arrival exceeds packet dispatch (in this chapter)
Congestion – packets discards (due to running out of buffer space) too frequently (in future chapter)
Key issues covered
- forwarding
- contention
Two switching technologies
- LAN switching – Popular in LAN, evolved from
Ethernet bridging
- Asynchronous Transfer Mode(ATM) – popular in WAN
CS380
Int. to Comp. Networks
Switching – Part I

4. Star Topology Large numbers of switches can be connected
Connecting switches/hosts using Point-to-Point links
Scaling doesn’t always mean performance  (switches designed with enough aggregate capacity)
CS380
Int. to Comp. Networks
Switching – Part I

5. Scalable Networks Switch Advantages
forwards packets from input port to output port, this is referred to as either switching or forwarding. In terms of the OSI architecture, it is the main function of the network layer.
port selected based on address in packet header
Advantages
cover large geographic area (tolerate latency)
support large numbers of hosts (scalable bandwidth)
Input
ports T3
STS-1
Switch
Output
CS380
Int. to Comp. Networks
Switching – Part I

6. Switches Q. How does the switch determine output port?
By looking at an identifier in the packet header
Three approaches:
Virtual Circuit (connection-oriented)
Datagram (connectionless)
Source Routing
CS380
Int. to Comp. Networks
Switching – Part I

7. Virtual Circuits Explicit connection setup (and tear-down) phase
Switch 1 3 1 2
Switch 2 2 3 1 5 11
Explicit connection setup (and tear-down) phase
Subsequence packets follow same circuit
Sometimes called connection-oriented model
Host A
Two types:
- PVC Permanent Virtual Circuit
Need network administrator configure the state
- SVC “Signalled” Virtual Circuit
Send a message into the network(signalling)
Analogy: phone call
Each switch maintains a VC table 7
Switch 3 1 3 4
Host B 2
CS380
Int. to Comp. Networks
Switching – Part I

8. Virtual Circuits Tables
VC Table
for Switch 1
Incoming Incoming Outgoing Outgoing
Interface VCI Interface VCI
. . .
Switch 1 3 1 2 2 3 1 5 11
VC Table for Switch 2
Switch 2
Incoming Incoming Outgoing Outgoing
Interface VCI Interface VCI
Host A
VCI Virtual Circuit Identifier (0, 1, 2, …)
combined with incoming/outgoing interface (e.g. 0, 1, 2, 3)can uniquely identify the virtual connection (VC)
assigned whenever a new connection is created
not a globally significant identifier for the VC; rather, only on a given link 7
Switch 3 1 3 4
Host B 2
CS380
Int. to Comp. Networks
Switching – Part I

9. Example: Give Virtual Circuit Tables for all switches
Host C
Host E
Switch 1
Host D
Switch 3 2
Host F 3 1 3 1 1
Switch 2
Host B 2 2
Host G 3
Host A
Host H
Incoming
Interface#
VCI#
Outgoing 1 2
Connections
Established
in the order of
(a) to (d)
Switch
Incoming
Interface# VCI#
Outgoing
(a) Host A connects to Host E 1 2 3
(b) Host C connects to Host G
(c) Host G connects to Host A
(d) Host D
connects to Host E
VC Table
for Switch 1
Incoming
Interface#
VCI#
Outgoing 1 2 3
VC Table
for Switch 2
Incoming
Interface#
VCI#
Outgoing 3 1 2
VC Table
for Switch 3
CS380
Int. to Comp. Networks
Switching – Part I

10. Virtual Circuit Model
Typically wait full RTT for connection setup before sending first data packet.
While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small.
If a switch or a link in a connection fails, the connection is broken and a new one needs to be established.
Connection setup provides an opportunity to reserve resources(QoS reservations).
ATM utilizes virtual circuits
CS380
Int. to Comp. Networks
Switching – Part I

11. Datagrams
Idea – provide just enough information for the switch to forward the packet
Host D
Switch 1
Host E
Host F 3 1 2
Switch 2
Host C 2 3 1
No setup time
Independent forwarding packets
Analogy – postal system
Each switch maintains a forwarding (routing) table.
More routing in the next chapter
Host A
destination
Port A 3 B C D E 2 F 1 G H
Forwarding table for Switch 2
Host G
Switch 3
Host B 1 3 2
Host H
CS380
Int. to Comp. Networks
Switching – Part I

12. Workstation Used As a Switch
Main problem: all packets must pass through a single point of contention. (I/O bus, read to/write from the main memory)
CS380
Int. to Comp. Networks
Chapter 3, Figure 9Switching – Part I

13. Forwarding Table for NodesC E
Give the datagram forwarding table for each node:
Node A Node B Node C Node D Node E
Destination
Next Hop B C D E
Destination
Next Hop A C D E
Destination
Next Hop A B D E
Destination
Next Hop A C B E
Destination
Next Hop A C B D
CS380
Int. to Comp. Networks
Switching – Part I

14. Datagram Model
There is no round trip time delay for connection setup; a host can send data as soon as it is ready.
Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up.
Since packets are treated independently, it is possible to route around link and node failures.
Since every packet must carry the full address of the destination, the overhead per packet is higher than for the connection-oriented model.
CS380
Int. to Comp. Networks
Switching – Part I

15. Source Routing
Does not need to use either VCs or Datagrams although can be used in combination with
IP for instance uses datagrams but has a source routing option
Can be used for VC setup
Source host contains all
information
- rotates data
CS380
Int. to Comp. Networks
Switching – Part I