Circuit vs. Packet Switching

Transcription

1 Circuit vs. Packet Switching 1 Taxonomy of Switched Wide Area Networks WAN Telecommunication Systems intelligent core dumb nodes Circuit-Switched Networks (e.g. telephone networks) dumb core intelligent nodes connectionless Packet-Switched Networks connection-oriented Datagram Networks (e.g. the Internet) Virtual Circuit (e.g. ATM) connection-oriented service (TCP) connectionless service (UDP)

2 Circuit vs. Packet Switching (cont.) 2 Network Core mesh of routers/switches that interconnect end systems two fundamental approaches to building a WAN core: (1) circuit switching (example: telephone networks) a sequence of links (communication path) between two communicating nodes is determined ahead of the actual communication on each physical link, a channel is dedicated to the connection data is sent as a stream of bits through the network (2) packet switching (example: the Internet) data is sent through network in short blocks packets network links are dynamically shared by many packets; each packet uses full link bandwidth LAN 1 WAN Core Network LAN 2

3 Circuit vs. Packet Switching (cont.) 3 Circuit-Switched Networks (e.g. telephone networks)

4 Circuit vs. Packet Switching (cont.) 4 Packet Switching: Datagram Networks (e.g. the Internet)

5 Circuit Switching 5 Communication via involves three phases: Circuit Switching (1) circuit establishment before any data is transmitted, an end-to-end circuit must be established, i.e. network resources on path/ links between end-devices must be reserved (2) data transfer data transmission and signaling may each be digital or analog (3) circuit disconnect after some period of data transfer, the connection is terminated, by action of one of two stations, and dedicated resources are released Vancouver Toronto

6 Circuit Switching (cont.) 6 Multiplexing in Circuit-Switched Networks each link can be shared among (up to) n circuits each circuit gets a fraction 1/n of the link s bandwidth multiplexing = set of techniques that allows simultaneous transmission of multiple signals across a single data link frequency division multiplexing (FDM) = each circuit continuously gets a fraction of the link s bandwidth time division multiplexing (TDM) = each circuit gets all of the bandwidth periodically during brief intervals of time FDM frequency TDM time time frequency

7 Circuit Switching (cont.) 7 Advantages of Circuit Switching guaranteed Quality of Service data are transmitted at fixed (guaranteed) rate; delay at nodes is negligible Disadvantages of Circuit Switching inefficient use of capacity channel capacity is dedicated for the duration of a connection, even if no data is being transferred (example: silent periods in a phone call) circuit establishment delay circuit establishment introduces initial delay network complexity end-to-end circuit establishment and end-to-end bandwidth allocation is complicated and requires complex signaling software to coordinate operation of switches

8 Packet Switching 8 Communication via Packet Switching (1) message segmentation longer message is broken up into series of packets packets contain user s data + control data control data (header) contains information that network requires to route the packet (2) data transfer intermediate nodes perform following operations: (a) receive entire packet (b) determine next node and link on route (c) queue packet to go out on that link when link is available, packet is transmitted to next node

9 Packet Switching (cont.) 9 Further Details of Packet Switching each packet is treated independently with no reference to packets that have gone before! each packet contains the full (IP) address its destination as well as its source each packet switch has a forwarding (routing) table that maps destination addresses to an output link when packet arrives at a packet switch, the switch examines packet s destination address and chooses the next node on packet s path based on current traffic, line failure, etc. packets with the same destination address do not necessarily follow the same route packets may arrive out of sequence at the destination! if packets arrive out of order, resequencing must be performed at the destination

10 Packet Switching (cont.) 10 Main Principle of Packet Switching statistical multiplexing ( ) on-demand rather than pre-allocated sharing of resources link capacity is shared on packet-to-packet basis only among those users who have packets that need to be transmitted over the link (1) router buffers packets and arranges them in a queue (2) as the transmission line becomes available, packets are transmitted one by one Bandwidth division into pieces Dedicated allocation Resource reservation A B queue of packets waiting for output statistically multiplexed packets: packets are interleaved based on the statistics of the senders store-and-forward ( ) switch must receive entire packet before it can begin to transmit the first bit of the packet onto the outbound link

11 Packet Switching (cont.) 11 Example [ circuit switching vs. packet switching ] N=35 users share a 1 Mbps link each user generates 100kbps when active each user is active 10% of time How many users can be supported with circuit and how many with packet switching? N users 100 kbps 100 kbps 1 Mbps link Circuit Switching With circuit switching, 100kbps must be reserved for each user at all times. Hence, the output link can support 1Mbps/100kbps = 10 simultaneous users. Packet Switching 10 or fewer simultaneously active users aggregate rate 1 Mbps users packets flow through output link without delay, as in case of circuit switching more than 10 simultaneously active users aggregate rate exceeds output capacity With 35 users, probability of 10 or less simultaneously active users = Thus, packet switching can support all 35 users with virtually no delay!

12 Packet Switching (cont.) 12 Advantages of Packet Switching greater line efficiency node-to-node link dynamically shared by many packets / connections data rate conversion two stations of different data rates can exchange packets, because each connects to its node at its proper data rate nodes act as buffers no blocked calls packets are accepted even under heavy traffic, but delivery delay increases Disadvantages of Packet Switching transmission delay each time a packet passes through a packet-switching node, it incurs a delay not present in circuit switching = the time it takes to absorb the packet into an internal buffer variable delay each node introduces additional variable delay due to processing and queueing overhead to route packets through a packet-switching network, overhead information including the address of destination and/or sequence information must be added to each packet