Computer Networks Chapter 8 Circuit Switching versus Packet

Computer Networks Chapter 8 – Circuit Switching versus Packet Switching Spring 2006 Computer Networks

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 2006 Computer Networks 2

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. § This cannot provide connection to subscribers connected to another local switch. subscriber switch Spring 2006 Computer Networks 3

A Circuit Switch § Device with a number of inputs and outputs § Creates temporary physical connection between an input and output link § The local switch can connect each telephone with each other Subscribers connected to the same swich Spring 2006 Computer Networks 4

Telephone network – Connecting Local Switches § It is expensive to connect each local switch with every other in a fully meshed topology § Higher order switches are 2 nd order introduced to save switch on cables The trafic between the § Redundancy is introduced to switches is multiplexed provide reliability Local switch (1 st order switch) Spring 2006 Computer Networks 5

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 2006 Computer Networks 6

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 2006 Computer Networks 7

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 2006 Computer Networks 8

Telephone calls – Typical Example for Circuit Switching Spring 2006 Computer Networks 9

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 2006 Computer Networks 10

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 2006 Computer Networks 11

Hierarchy of the Telephone Network International network International gateway exchange National tandem exchanges regional tandem exchanges trunk network Tandem offices local tandem exchanges local network local exchanges (toll offices) subscriber lines (local loops) Spring 2006 Computer Networks 12

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 60 s and early 70 s) § 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 2006 Computer Networks 13

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 2006 Computer Networks 14

Message versus Packets Header Data Whole message Padding if necessary Each packet carries the destination address in the header and is independent from the other packets in the message - datagram Spring 2006 Computer Networks 15

Store-and-forward Switching § Instead of establishing a circuit and making reservation on every hop of the path, packet switching is actually storeand-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 2006 Computer Networks 16

Router – A Store-and-Forward Switch incoming links router outgoing links Link 1 Link 2 Link 3 Spring 2006 Computer Networks 17

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) Buffering (queues) § Defines a cost-effective way for multiple users to share network resources in fine-grained manner Spring 2006 Computer Networks 18

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 2006 Computer Networks 19

Datagram Switching Each router forwards the datagram solely based upon the destination address. Packets can arrive out of order. Spring 2006 Computer Networks 20

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 transmission A propagation B Spring 2006 nodal processing Computer Networks queueing 21