Switching techniques

Switching Techniques
In large networks there might be multiple paths linking sender
and receiver. Information may be switched as it travels through
various communication channels. There are three typical
switching techniques available for digital traffic.
• Circuit Switching
• Message Switching
• Packet Switching

Circuit Switching
• Circuit switching is a technique that directly connects the
sender and the receiver in an unbroken path.
• Telephone switching equipment, for example, establishes a
path that connects the caller's telephone to the receiver's
telephone by making a physical connection.
• With this type of switching technique, once a connection is
established, a dedicated path exists between both ends
until the connection is terminated.
• Routing decisions must be made when the circuit is first
established, but there are no decisions made after that time.

Circuit Switching
• Circuit switching in a network operates almost the same
way as the telephone system works.
• A complete end-to-end path must exist before
communication can take place.
• The computer initiating the data transfer must ask for a
connection to the destination.
• Once the connection has been initiated and completed to the
destination device, the destination device must acknowledge
that it is ready and willing to carry on a transfer.

Circuit switching
Advantages:
• The communication channel (once established) is dedicated.
Disadvantages:
• Possible long wait to establish a connection, (10 seconds,
more on long- distance or international calls.) during which
no data can be transmitted.
• More expensive than any other switching techniques,
because a dedicated path is required for each connection.
• Inefficient use of the communication channel, because the
channel is not used when the connected systems are not
using it.

Message Switching
• With message switching there is no need to establish a
dedicated path between two stations.
• When a station sends a message, the destination address is
appended to the message.
• The message is then transmitted through the network, in its
entirety, from node to node.
• Each node receives the entire message, stores it in its entirety
on disk, and then transmits the message to the next node.
• This type of network is called a store-and-forward network.

Message Switching
A message-switching node is typically a general-purpose
computer. The device needs sufficient secondary-storage
capacity to store the incoming messages, which could be long.
A time delay is introduced using this type of scheme due to
store- and-forward time, plus the time required to find the next
node in the transmission path.

Message Switching
Advantages:
• Channel efficiency can be greater compared to circuit-
switched systems, because more devices are sharing the
channel.
• Traffic congestion can be reduced, because messages may be
temporarily stored in route.
• Message priorities can be established due to store-and-forward
technique.
• Message broadcasting can be achieved with the use of
broadcast address appended in the message.

Message Switching
Disadvantages
• Message switching is not compatible with interactive
applications.
• Store-and-forward devices are expensive, because they
must have large disks to hold potentially long messages.

Packet Switching
• Packet switching can be seen as a solution that tries to combine the
advantages of message and circuit switching and to minimize the
disadvantages of both.
• There are two methods of packet switching: Datagram
and virtual circuit.

Packet Switching
• In both packet switching methods, a message is broken into
small parts, called packets.
• Each packet is tagged with appropriate source and destination
addresses.
• Since packets have a strictly defined maximum length, they
can be stored in main memory instead of disk, therefore access
delay and cost are minimized.
• Also the transmission speeds, between nodes, are optimized.
• With current technology, packets are generally accepted onto
the network on a first-come, first-served basis. If the network
becomes overloaded, packets are delayed or discarded
(``dropped'').

Packet size
• The size of the packet can vary from 180 bits, the size for the
Datakit® virtual circuit switch designed by Bell Labs for
communications and business applications; to 1,024 or
2,048 bits for the 1PSS® switch, also designed by Bell Labs
for public data networking; to 53 bytes for ATM switching,
such as Lucent Technologies' packet switches.

Packet switching
• In packet switching, the analog signal from your phone is
converted into a digital data stream. That series of digital
bits is then divided into relatively tiny clusters of bits,
called packets. Each packet has at its beginning the digital
address -- a long number -- to which it is being sent. The
system blasts out all those tiny packets, as fast as it can,
and they travel across the nation's digital backbone
systems to their destination: the telephone, or rather the
telephone system, of the person you're calling.
• They do not necessarily travel together; they do not travel
sequentially. They don't even all travel via the same route.
But eventually they arrive at the right point -- that digital
address added to the front of each string of digital data --
and at their destination are reassembled into the correct
order, then converted to analog form, so your friend can
understand what you're saying.

Packet Switching: Datagram
• Datagram packet switching is similar to message switching in
that each packet is a self-contained unit with complete
addressing information attached.
• This fact allows packets to take a variety of possible paths
through the network.
• So the packets, each with the same destination address, do not
follow the same route, and they may arrive out of sequence at
the exit point node (or the destination).
• Reordering is done at the destination point based on the
sequence number of the packets.
• It is possible for a packet to be destroyed if one of the nodes on
its way is crashed momentarily. Thus all its queued packets may
be lost.

Packet Switching:Virtual Circuit
• In the virtual circuit approach, a preplanned route is established
before any data packets are sent.
• A logical connection is established when
 a sender send a "call request packet" to the receiver and
 the receiver send back an acknowledge packet "call accepted
packet" to the sender if the receiver agrees on conversational
parameters.
• The conversational parameters can be maximum packet sizes,
path to be taken, and other variables necessary to establish and
maintain the conversation.
• Virtual circuits imply acknowledgements, flow control, and error
control, so virtual circuits are reliable.
• That is, they have the capability to inform upper-protocol layers
if a transmission problem occurs.

Packet Switching:Virtual Circuit
• In virtual circuit, the route between stations does not mean that
this is a dedicated path, as in circuit switching.
• A packet is still buffered at each node and queued for output over
a line.
• The difference between virtual circuit and datagram approaches:
 With virtual circuit, the node does not need to make a routing
decision for each packet.
 It is made only once for all packets using that virtual circuit.

Packet Switching: Virtual Circuit
VC's offer guarantees that
 the packets sent arrive in the order sent
 with no duplicates or omissions
 with no errors (with high probability)
regardless of how they are implemented internally.

Advantages of packet switching
Advantages:
• Packet switching is cost effective, because switching
devices do not need massive amount of secondary
storage.
• Packet switching offers improved delay characteristics,
because there are no long messages in the queue
(maximum packet size is fixed).
• Packet can be rerouted if there is any problem, such as,
busy or disabled links.
• The advantage of packet switching is that many
network users can share the same channel at the same
time. Packet switching can maximize link efficiency by
making optimal use of link bandwidth.

Disadvantages of packet switching
Disadvantages:
• Protocols for packet switching are typically more complex.
• It can add some initial costs in implementation.
• If packet is lost, sender needs to retransmit the data.
• Another disadvantage is that packet-switched systems still
can’t deliver the same quality as dedicated circuits in
applications requiring very little delay - like voice
conversations or moving images.

Switching techniques

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