1. Layered Architecture of Communication Networks: Circuit Switching & Packet Switching

© Tallal Elshabrawy 2 Agenda Packet Concept Packet Transmission time Packet Propagation time Circuit Switching Packet Switching Virtual Circuit Packet Switching

© Tallal Elshabrawy 3 Packet Concepts … … … … … A packet is a small block of data bits. Packets are sent individually All the bits in a given packet are treated as single entity

© Tallal Elshabrawy 4 Why Packets Coordination helps transmitter and receiver determine which section of data has been received correctly Low overhead due to Retransmission When bit error occurs, retransmission of the erroneous packet only is needed

© Tallal Elshabrawy 5 Headers and Trailers For a layer N+1, layer N does not understand the message content So layer N adds bits (Headers and Trailers) that represents its language (i.e., protocol) The header and trailer for each layer may represent Messages to help the receiver side understand what to do Parameters to help the receiver side perform its service correctly

© Tallal Elshabrawy 6 Headers = Overhead TH: Transport Header NH: Network Header DH: Data link Header DT: data link Trailer L bits Application Data THApplication Data H T bits Transport DataNH H N bits Network DataDHDT H D bitsT D bits Data Link Data T D + H D +H N +H T +L bits We want to send L bits We actually send T D +H D +H N +H T +L bits Header and/or trailer added by layer N is independent of the packet length from layer N+1 WHY?

© Tallal Elshabrawy 7 Throughput Efficiency For an Error Free Channel Throughput Efficiency reflects the percentage of information bits to the total number of bits required for the information packet to arrive safely at the destination

© Tallal Elshabrawy 8 Packet Transmission Time It is the time required to put all packet bits over the physical channel It is dependent on the modem transmission rate Example If Transmission Rate is 1 Mbps and Packet Size is 8000 bits then Transmission Time is: T t = 8000/1*10 6 =8 ms

© Tallal Elshabrawy 9 Packet Propagation Time It is the time it takes the electrical signals to travel from one machine to the other It is dependent on the speed of light and the distance between the two machines Example Assume the speed of light over cable is 2.3*10 8 m/s and the distance between two machines is 1000 Km, then Propagation Time is: T p = 1000*10 3 /2.3*10 8 =4.345ms

© Tallal Elshabrawy 10 Illustrative Figure Machine A Machine B Physical Channel Time T p =4.345 ms T t =8 ms

© Tallal Elshabrawy 11 Circuit Switching The concept of circuit-switching comes from old telephone technology For each connection, physical switches are set in the telephone network to create a physical “circuit” An end-to-end path MUST be set up before any data can be sent

© Tallal Elshabrawy 12 Phases of Circuit Switching Circuit Establishment A circuit is established between two end users with the help of intermediate switches Information Transfer Information is transmitted using the circuit established in the set-up phase Circuit Disconnect Circuit is terminated Dedicated resources are released

© Tallal Elshabrawy 13 Physical copper connection set up when call is made Switching offices 1. Circuit Establishment 2. Information Transfer 3. Circuit Disconnect Switch A Switch B Switch C Switch D Circuit Switching - Example

© Tallal Elshabrawy 14 Event Timing (Circuit Switching) Time Circuit Establishment Data Transfer Circuit Disconnect Switch ASwitch B Switch D Switch C TtTt 5T p TpTp

© Tallal Elshabrawy 15 Switches are set up at the beginning of the connection (call) and maintained throughout the connection Network resources are reserved and dedicated from sender to receiver Propagation time is the only source of delay during data transfer Not a very efficient strategy A connection “holds” a physical line even during “silence” periods (when there is nothing to transmit) Circuit Switching (cont’d)

© Tallal Elshabrawy 16 Packet Switching Networks No physical connection is established in advance between the sender and receiver Each packet is sent independently Each packet is stored in a router, processed and then forwarded to the next hop STORE AND FORWARD Operation

© Tallal Elshabrawy 17 Router Congested Router A Router B Router C Router D Router E Packet Switching - Example Router F

© Tallal Elshabrawy 18 1 Event Timing (Packet Switching) Time 1 Data Transfer Router ARouter B Router D Router C T Pr TtTt TpTp T Pr  Processing Time

© Tallal Elshabrawy 19 Circuit switching Must set up a connection (initial delay). Resources are dedicated –Therefore they may be used inefficiently! But, performance is predictable as resources are reserved. Packet switching Very small set-up delay. Efficient shared use of resources. Possible congestion and consequent packet dropping Performance is unpredictable and is a function of current traffic conditions. Circuit Switching vs Packet Switching

© Tallal Elshabrawy 20 Virtual Circuit Packet Switching Compromise between circuit switching and packet switching Preplanned route established before any packets sent Connection Establishment Phase is required Each packet contains a virtual circuit identifier instead of destination address No routing decisions required for each packet Connection release is required after data transfer phase is completed

© Tallal Elshabrawy 21 Router A Router B Router C Router D Router E 1. Connection Establishment2. Information Transfer 3. Circuit Disconnect Virtual Circuit Packet Switching-Example Router F

© Tallal Elshabrawy 22 1 Time 1 Data Transfer Router ARouter B Router D Router C T Pr Connection Establishment Connection Release Event Timing (Virtual Circuit Packet Switching)

© Tallal Elshabrawy 23 Virtual Circuit vs. Packet Switching Virtual Circuits No packet reordering occurs because all packets follow the same reserved path Packets are forwarded more quickly Less processing time because no routing decisions are necessary Requires connection set-up and release times If an intermediate router is down an new connection establishment must be performed Packet Switching No Set/Release Phases More flexible Can recover from down time of intermediate routers by selecting alternate routes without the need for any new connection establishment