Packet Switching.
Migration from a single-link network to a larger
Computer network: Via a switch.
A switch provides multiple linkages.
Network 3
Network 1
Network
Network22
Switch receives packets (or frames) and decide how
to deal with them.
Entering packets
Switch
Switch queue
Store and Forward switch (as a Router). Receive and
Transmit switch (as a Bridge). Also, Cut-through
switches. Also, hybrids between CT and SF
(Fragment-Free switching).
A number of packet or data streams concurrently
flow on a link (multiplexer) as different sessions and
then separated
(demultiplexed) by the switch.
Switch
Switch
Switch
Basic switching techniques.
Circuit switching. The entire session flows over a
dedicated set of switches. Initial setup phase sends a
probe to discover a path through switches which
would be followed by all packets (or messages) till it
ends. A dedicated channel is a must. Ideal for data
transmission in real-time. Also known as connection
oriented network.
A typical circuit-switched event.
 initiate circuit
a. User request for circuit 
setup phase. Call request signals to switches.
b. Message flows over the dedicated circuit
c. Disconnect phase.
 TDM (Time-division),
Basic circuit switching 
STDM (Statistical Time-division), FDM (Frequencydivision), WDM (Wavelength-division), CDM
(Code-division). The latter two for cellular wireless
networks.
Difference between TDM & STDM. TDM is a
dedicated time-slot based transmission. In STDM, the
time slot could be used by any so long one could
distinguish a packet from others by session, by
receiver address and data.
Packet-Switching. Packets carry instruction about
how to forward them.
Source
Address
Destination
Address
Control &
Protocol info
Data
A Packet for
transmission
A packet = analogous to a letter in an envelop
Datagram
mode
Source
Routing
Packet
switching
Virtual Circuit
mode
In Datagram mode: All packets contain destination
address, and all are sent asynchronously over the
routers. A connection-less service. Switch (or
Routers) uses a routing table to see on which port the
packet to be forwarded.
Routing table entries are obtained empirically from
packets received at a router.
In Source Routing: Each packet contains the entire
path from source to destination that has to be
followed.
In Virtual Circuit mode: A hybrid between the above
two. Has setup phase of Circuit switching.
Virtual
circuit
Permanent
Virtual Circuit
Switched
Virtual Circuit
An SVC = Temporary VC
A VC is a path between two points which appears to
be a discrete physical path, but comprises managed
pool of circuit resources from which specific circuits
emerge. A connection-oriented service.
■ Explicit connection setup and tear down phases.
■ Subsequent packets follow same route.
■ Analogy: A caravan train?
■ Each switch maintains a VC table.
LAN and VLAN.
A VLAN (Virtual LAN) provides logically a singlelink LAN even though it is comprised of different
LAN segments physically separated. VLAN is
configured through software (instead of hardware)
making it very flexible. Even when a computer is
physically moved to another location, it still may
appear on the same VLAN without any hardware
reconfiguration. More later.
LAN Switching
Problems with a single-link LANs: not scalable. Only
a limited number of stations per LAN segment.
Segment length finite.
One solution: Interconnect many LANs via LAN
switch called Bridge.
A
B
C
K
L
M
LAN 1
Bridge
LAN 2
X
Y
Z
W
Bridges are plug and play devices operating at Data
Link Layer level (next up from Physical Layer).
■ Bridges operate on promiscuous mode with a
minimum two ports.
■ Bridges learn from received frames which stations
are on what LAN segment. Using this information,
they build a table which they use to output frames
onto proper destination port.
■ If destination is ambiguous or not known, it would
broadcast received frames to all ports.
■ Network looping with bridges causes problems.
Hence, with multiple bridges (included for
redundancy) spanning tree LAN segment layout is
desired.
A
B
C
K
L
M
LAN 1
B1
B2
LAN 2
X
Y
Z
W
Either keep B1 or B2, but not both at the same time.
But, a spanning tree has to be identified by the
participating bridges themselves in a distributed
framework.
Challenge 1: Each bridge must find out which of its
ports could be safely included in the LAN
configuration so as to yield a spanning tree.
Challenge 2: In the event of any bridge failure, how
does one replace the failed bridge by another
functional one without bringing a network to a halt?
The STP (Spanning Tree Protocol IEEE 802.1d)
1. Elect one bridge as the root bridge (bridge with
lowest ID?).
2. Remaining bridges compute the shortest path to
the root bridge.
3. One bridge in each LAN segment is declared as
the designated bridge for that LAN. The
designated bridge should have shortest path to
the root bridge.
4. Each bridge will then choose a port that is the
nearest to the root bridge.
5. Each bridge will select ports that would
participate in the spanning tree. Participating
ports must be on the designated bridge.
6. All ports begin forwarding frames to the correct
LANs via the shortest loop-free path to the
destination LAN.
Other standards:
802.1s (Multiple Spanning Tree: MSTP)
802.1w (Rapid Reconfiguration Spanning Tree:
RRST)
STP is invented by Radia Perlman. She summarized
her algorithm as a poem, title “Algorhyme”.
I think that I shall never see
A graph as lovely as a tree.
A tree which must be sure to span.
So packets can reach every LAN.
First the root must be selected.
By ID, it is elected.
Least cost paths from Root are traced.
In the tree these paths are placed.
A mesh is made by folks like me.
Then bridges find a spanning tree.
More about VLAN
A VLAN is a group of machines on different LAN sections
or even same LAN segment interacting with each other as
though they are on same LAN segment. e.g. Two-floor
network shown here.
Port: 1, 2 (Switch 1), 4,5,6,7 (Switch 2) form a logical
group VLAN A.
Port: 3,4,5,6,7,8 (Switch 1), 1,2,3,8 (Switch 2) form
another logical group VLAN B.
In a port-based VLAN, system admin decides which
port goes to which group. In a MAC addressed based
VLAN, a machine cannot be member of more than
one group. IEEE 802.1q is the VLAN standard.