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 four typical switching techniques
available for digital traffic.
*
Circuit Switching
*
Packet Switching
*
Message Switching
*
Cell 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 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.
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.
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.
* 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'').
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
* 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.
* 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.
* 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
• 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. 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:
• 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:
• 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.
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.
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.
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
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
Cell Switching
Cell Switching is
similar to packet switching, except that the switching does not necessarily
occur on packet boundaries. This is ideal for an integrated environment and is
found within Cell-based networks, such as ATM. Cell-switching can handle both
digital voice and data signals.
