Computer Networks Chapter 8 – Circuit Switching versus Packet Switching

Spring 2006Computer Networks2 Switches  What is a switch?  A central device usually used with a star topology  Can be built in hardware and/or software  Used to provide temporary connections between any two devices connected to the switch  A network of switches can be made if a very large number of devices spread in a large geographic space need to be connected

Spring 2006Computer Networks3 Telephone network – Local Switch  The telephone network uses switches  Every subscriber ( telephone jack in a house) has a twisted-pair wire connected to the closest telephone exchange. They are called local switches or local exchanges. subscriber    switch           This cannot provide connection to subscribers connected to another local switch.

Spring 2006Computer Networks4 A Circuit Switch  Device with a number of inputs and outputs  Creates temporary physical connection between an input and output link   Subscribers connected to the same swich  The local switch can connect each telephone with each other

Spring 2006Computer Networks5 Telephone network – Connecting Local Switches  It is expensive to connect each local switch with every other in a fully meshed topology The trafic between the switches is multiplexed              Higher order switches are introduced to save on cables  Redundancy is introduced to provide reliability Local switch (1 st order switch) 2 nd order switch  

Spring 2006Computer Networks6 Subscribers Connected to Different Local Switches  The connection goes through several switches.  If any part of the connection cannot be reserved, the connection is not established  

Spring 2006Computer Networks7 Circuit Switching  Three phases of the connection:  Circuit establishment  Data transfer  Circuit disconnect  The bandwidth is guaranteed during the connection  The bandwidth cannot be used by anyone else, even if it is not needed at certain moment (no flexibility)

Spring 2006Computer Networks8 Circuit Switching –cont.  The resources (for example, the bandwith) are divided into pieces  Pieces are allocated to “calls”.  No sharing: no two calls can share the same piece of bandwidth.  A piece is regarded “idle” if not assigned to any calls.  A call is rejected if no ”idle” piece exists on any part of the connection.

Spring 2006Computer Networks9 Telephone calls – Typical Example for Circuit Switching

Spring 2006Computer Networks10 Types of Switches  Space switches  Crossbar  Multistage – A way to save on the cross points  Blocking – input point cannot be connected to the output due to all cross points being used by others  Time-division switches  Time-slot interchange  TDM bus  Combined switches

Spring 2006Computer Networks11 Characteristics of the Switches  Space switches  The advantage is that if a cross point is available, the connection is almost instantaneous  The disadvantage is the need for many cross points which is expensive  Time switches  Advantage is that it does not need cross points  Disadvantage is the time necessary for processing each time slot.  Combined switches combine the advantages of both types

Spring 2006Computer Networks12 Hierarchy of the Telephone Network subscriber lines (local loops) local exchanges (toll offices) International gateway exchange National tandem exchanges regional tandem exchanges local tandem exchanges local network International network trunk network Tandem offices

Spring 2006Computer Networks13 Computer Networks  Telephone network has been developing since the end of the 19 th century  Computer networks started their development late in the 20 th century (ARPANet has emerged in the late 60s and early 70s)  Circuit switching was not convinient for computer traffic because  Data traffic is bursty – a lot of traffic might need to be transmitted at one moment and nothing in another (dedicated bandwidth will then be wasted)  Data traffic is delicate – if binary file is transmiited, every bite must be correct for the program to work (for voice this is not too important)

Spring 2006Computer Networks14 Packets – A New Paradigm  The message is splitted in smaller chunks called packets.  Introducing packets is important for the following reasons:  Errors can appear only in a few packets. Then these packets need to be retransmitted again. However, retransmitting several packets instead of the whole message saves bandwidth.  A very long message means monopolizing bandwidth by a single user. Using smaller packets provides better sharing.

Spring 2006Computer Networks15 Message versus Packets Header Data Padding if necessary Whole message Each packet carries the destination address in the header and is independent from the other packets in the message - datagram

Spring 2006Computer Networks16 Store-and-forward Switching  Instead of establishing a circuit and making reservation on every hop of the path, packet switching is actually store- and-forward switching.  In store-and-forward switching every packet is switched (forwarded) as it comes to the switch.  If the switch is busy, the packet waits in the input queue to be processed  Once the switch is idle it checks the header of the packet and based upon that information forwards it to the output line which is closer to the packet’s destination.  If the output line is busy, the packet waits in the output queue until it is transmitted to the next store-and-forward switch.  The switch that is directly connected to the desination will deliver the packet.

Spring 2006Computer Networks17 Router – A Store-and-Forward Switch Link 1 Link 2 Link 3 router incoming links outgoing links

Spring 2006Computer Networks18 Statistical multiplexing  The physical link is shared over time (like in STDM)  Data are transmitted on demand rather than during the predetermined time slot (unlike in STDM)  Defines a cost-effective way for multiple users to share network resources in fine-grained manner Buffering (queues)

Spring 2006Computer Networks19 Packet Switching  Two types of packet switching  Datagram switching  The store-and-forward switch forwards the packet to the output link by looking only at its destination address.  Packets belonging to the same message can travel different paths and their ordering is done at the receiver  Example technologies: IP  Virtual circuit switching

Spring 2006Computer Networks20 Datagram Switching Each router forwards the datagram solely based upon the destination address. Packets can arrive out of order.

Spring 2006Computer Networks21 Delays with packet Switching  Packet experiences delay on each hop:  transmission delay (length of message/through.)  propagation delay (distance/speed of prop.)  nodal processing  quieuing A B propagation transmission nodal processing queueing