Review of Networking Concepts. Part 1: Switching Networks

Transcription

1 Review of Networking oncepts Part 1: Switching Networks The first few lectures of this course cover basic, but important, concepts of computer networking

2 Summary of Topics Taxonomy of switching networks Principles of circuit and packet switching Multiplexing techniques: FM, TM, statistical multiplexing atagram and V packet networks Packet forwarding in datagram and V packet networks ommunication Networks Problem: onnect end systems that want to exchange information (evice = telephone, computer, terminals, etc.) Simple Solution: onnect each pair of end system by a dedicated pointtopoint link The simple solution is sufficient if the number of end systems is small

3 ommunication Networks With a large number of end systems it is not practical to connect each pair ommunication Networks communication network provides a scalable solution to connect a large number of end systems Principles: There are two types of devices: end systems and nodes ach node is connected to at least one node Network nodes carry the information from a source to a destination end system Note: Network nodes do not generate information

4 ommunication Networks generic communication network: end system Node ommunication Network Other names for end system : station, host, terminal Other names for node : switch, router, gateway xample Networks volution of the UUNet Network UV ackbone Network Source: National Science Foundation

5 Taxonomy of Networks ommunication networks can be classified based on the way in which the nodes exchange information: ommunication Network ircuitswitched Network PacketSwitched Network Frequency ivision Multiplexing Wavelength ivision Multiplexing Time ivision Multiplexing atagram Network Virtual ircuit Network ircuit Switching In a circuitswitched network, a dedicated communication path ( circuit ) is established between two stations through the nodes of the network The dedicated path is called a circuitswitched connection or circuit circuit occupies a fixed capacity of each link for the entire lifetime of the connection. apacity unused by the circuit cannot be used by other circuits ata is not delayed at the switches

6 ircuit Switching ircuitswitched communicatiovolves three phases: 1. ircuit stablishment 2. ata Transfer 3. ircuit Termination usy Signal if capacity for a circuit not available Most important circuitswitching networks: Telephone networks ISN (Integrated Services igital Networks) ircuit Switching circuit circuit 1 7 6

7 Implementation of ircuitswitching There are two ways to implement circuits Frequency ivision Multiplexing (FM) Time ivision Multiplexing (TM) Wavelength ivision Multiplexing (WM) xample: Voice in (analog) telephone network: Needed bandwidth: 3000 Hz llocated bandwidth: 4000 Hz Therefore, a channel with 64 khz can carry 16 voice conversations Frequency ivision Multiplexing (FM) pproach: ivide the frequency spectrum into logical channels and assign each information flow one logical channel Source 1 hannel 1 (f1) 1 Source 2 Source 3 Source 4 Source 5 S w i t c h hannel 2 (f2) hannel 3 (f3) hannel 4 (f4) hannel 5 (f5) S w i t c h Source 6 hannel 6 (f6) 6

8 Frequency ivision Multiplexing (FM) ircuit Switch ircuit Switch ndsystem ndsystem ndsystem ndsystem circuit switch bundles (multiplexes) multiple voice calls on a highbandwidth link FrequencyivisionMultiplexing (FM): ach circuit receives a fixed bandwidth. The frequency of each call is shifted, so that multiple calls do not interfere Time ivision Multiplexing (TM) pproach: Multiple signals can be carried on a single transmission medium by interleaving portions of each signal in time Source 1 Source 2 Source 3 Source 4 Source 5 M U M U Source 6 6

9 Time ivision Multiplexing (TM) slots ircuit Switch ircuit Switch endsystem endsystem frames endsystem endsystem Time is divided into frames of fixed length ach frame has a fixed number of constantsized slots ach circuit obtains one or more slots per frame ircuit Switch switch fabric memory circuit switch relays a circuit from aput to aput link switch reassigns frequencies (FM) or time slot allocation (TM) No queueing or other delays are experienced

10 Packet Switching ata are sent as formatted bitsequences, socalled packets Packets have the following structure: Header ata Trailer Header and Trailer carry control information ach packet is passed through the network from node to node along some path (Forwarding/Routing) t each node the entire packet is received, stored briefly, and then forwarded to the next node (StoreandForward Networks) Packet transmission is never interrupted (no preemption) No capacity is allocated for packets Packet Switch input queues output queues switch fabric memory

11 Statistical Multiplexing Packet transmission on a link is referred to as statistical multiplexing There is no fixed allocation of packet transmissions Packets are multiplexed as they arrive Packets from different streams 1 Transmission line 2 1 N 2 1 output buffer N atagram Packet Switching The network nodes process each packet independently If Host sends two packets backtoback to Host over a datagram packet network, the network cannot tell that the packets belong together In fact, the two packets can take different routes Packets are called datagrams Implications of datagram packet switching: sequence of packets can be received in a different order than it was sent ach packet header must contain the full address of the destination

12 Virtualircuit Packet Switching Virtualcircuit packet switching is a hybrid of circuit switching and packet switching ll data is transmitted as packets ll packets from one packet stream are sent along a preestablished path (=virtual circuit) Guarantees insequence delivery of packets However: Packets from different virtual circuits may be interleaved Virtualircuit Packet Switching ommunication with virtual circuits (V) takes place in three phases: 1. V stablishment 2. ata Transfer 3. V isconnect Note: Packet headers don t need to contain the full destination address of the packet

13 Packet Forwarding and Routing There are two parts to the routing problem: 1. How to pass a packet from aput interface to the output interface of a router (packet forwarding)? 2. How to calculate routes (routing algorithm)? Packet forwarding is done differently in datagram and virtualcircuit packet networks Route calculation is similar in datagram and virtualcircuit packet networks atagram Packet Switching

14 Virtualircuit Packet Switching V V Packet Forwarding of atagrams Recall: In datagram networks, each packet must carry the full destination address ach router maintains a routing table which has one row for each possible destination address The lookup yields the address of the next hop (nexthop routing) Routing Table of node v to via (next hop) w v n d d n x

15 Packet Forwarding of atagrams When a packet arrives at acoming link, The router looks up the routing table 2. The routing table lookup yields the address of the next node (next hop) 3. The packet is transmitted onto the outgoing link that goes to the next hop Routing Table of node v to via d d (next hop) w v n d d n x Forwarding atagrams To Next hop To Next hop To Next hop To Next hop To Next hop To Next hop

16 Packet Forwarding with Virtual ircuits Recall: In V networks, the route is setup in the connection establishment phase uring the setup, each router assigns a V number (V#) to the virtual circuit The V# can be different for each hop V# is writteto the packet headers Routing Table of node v from V# to V# 2 3 w v n path of virtual circuit 1 d w 2 v 3 x Packet Forwarding of Virtual ircuits When a packet with in header arrives from router, The router looks up the routing table for an entry with (, ) 2. The routing table lookup yields (, ) 3.The router updates the V# of the header to and transmits the packet to Routing Table of node v from V# to V# w v n path of virtual circuit d w 2 v 3 x

17 Forwarding with Vs Part 1: V setup from to Forwarding with Vs Part 2: Forwarding the packet

18 omparison ircuit Switching edicated transmission path ontinuous transmission Path stays fixed for entire connection all setup delay Negligible transmission delay No queueing delay usy signal overloaded network Fixed bandwidth for each circuit No overhead after call setup atagram Packet Switching No dedicated transmission path Transmission of packets Route of each packet is independent No setup delay Transmission delay for each packet Queueing delays at switches elays increase in overloaded networks andwidth is shared by all packets Overhead in each packet V Packet Switching No dedicated transmission path Transmission of packets Path stays fixed for entire connection all setup delay Transmission delay for each packet Queueing delays at switches elays increase in overloaded networks andwidth is shared by all packets Overhead in each packet