Download Switching

Switching
When we have a lot of devices that
want to communicate, how do we set
up connections between them?
How to create point-to-point
connections?
• On small networks, it’s okay to have direct
connections between all devices (mesh topology)
or connections to a central device (star topology).
• That gets obviously unwieldly and expensive
rather quickly.
• Switching is the mechanism by which we create
these connections with limited physical links
available.
Circuit-switching
• In a circuit switched
network, a connection
between two devices is
made through a series
of physical links
connected by switches.
• When the connection is
created, a set of
dedicated channels is
set aside on each link
so that a dedicated
connection is formed.
QuickTime™ and a
decompressor
are needed to see this picture.
Complications
• What complications will arise when we add
switches to the network?
– How do we pick one pathway from the many
available?
– How do we communicate to the sender and
receiver what that pathway is?
– How do we reserve the pathway while it’s
being used?
– How do we free the pathway when we’re
finished with it?
Phases of communication
• Setup
• The sender, S, sends a request to the nearest switch with the address
of the receiver, R.
• That switch finds a dedicated channel between itself and the next
switch in the sequence, and forwards the request through that channel.
• The next switch creates a dedicated channel between itself and the
next switch down the line…..
• etc., until the switch that R is connected to is found.
• R sends an acknowledgement back to S
• Once S receives the acknowledgement, a connection is established
and S can start sending data.
• Data transfer
• Teardown: A signal is sent to each switch freeing up the
channels
Packet switching
• Reserving a dedicated link between two
devices is wasteful, especially if they don’t
release the link when they’re not using it.
• Packet switching allows for maximal
utilization of the physical links available on
a network.
• This type of switching is typically software
based, and done at the network level (so
we’ll talk about it more there).
Virtual-circuit switching
• An attempt to blend the speed of circuit switching
with the efficiency of packet switching
• As in circuit-switching, a unique pathway through
the network is established.
• However, instead of this pathway consisting of
dedicated channels, it consists of dedicated
routing.
• So, whereas in a packet switched network,
different packets can take different paths between
S and R, in a virtual-circuit network, all packets
between S and R take the same path.
• Setup request phase:
– In the setup phase, switches operate essentially as
packet switches.
– A request frame for a connection to R is sent
from S to the first switch containing the virtualcircuit ID that S is assigning to this circuit.
– The routing table says that packets from S to R
go out through a particular port, so an entry in
the virtual-circuit table is created.
– Same thing happens at subsequent switches until
the request reaches R, at which point R assigns a
virtual-circuit ID from it’s end.
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decompressor
are needed to see this picture.
• Acknowledgement phase
– In order to complete the setup, R must acknowledge the
request from S and communicate its VCI.
– R sends an acknowledgement frame to its switch with
the VCI it assigned to its virtual circuit with S.
– That switch sends the packet to the previous switch, etc.
until the acknowledgement gets back to S.
QuickTime™ and a
decompressor
are needed to see this picture.