1 Networks – basics zA network consists of: yTransmission media (wire, cable, …). yHardware devices (routers, switches, …). ySoftware components (protocol stacks, drivers, …). zTerminology: yHost: Computers and other devices that use a network. yNode: Any computer or switching device attached to a network. ySubnet: Set of interconnected nodes. zDesign issues: Performance, scalability, reliability, security, mobility, quality of service, multicasting.

2 Networks – types RangeBandwidth (Mbps)Latency (ms) LAN1-2 kms10 – – 10 WANworldwide0.010 – [1] 100 – 500 MAN2-50 kms1 – Wireless LAN km2 – 54 [2] 5 – 20 Wireless WANworldwide0.010 – 2100 – 500 Internetworldwide0.010 – 2100 – 500 [1]: OC-192 over ATM: [2]: IEEE a:

3 Networks – packet transmission zMessage: Sequence of data items of arbitrary length. zMessages subdivided into packets. zSwitching schemes: yBroadcast. yCircuit switching. yPacket switching. yATM / Frame relay.

4 Protocols – basics zProtocol: Set of rules and formats to be used for communication between processes in order to perform a given task. zShould include specification of: ySequence of messages that must be exchanged. yFormat of the data in the messages. zImplemented by a pair of software modules in the sending and receiving computers.

5 Protocols – layers Layer n Layer 2 Layer 1 Message sent Message received Communication medium SenderRecipient

6 Protocols – encapsulation and headers

7 Protocols – the ISO Open Systems Interconnection (OSI) model

8 Protocols – internetwork layers

9 Protocols – network layer routing (WAN) Hosts Links or local networks A DE B C Routers Routing algorithms, adaptive routing (congestion control) Packet delivery: Datagram or virtual circuit?

10 Internet protocols – Internetworking zInternetwork: Network which integrates a number of different subnets. zNeeds: yUnified internetwork addressing scheme (Internet: IP addresses) y Protocol defining format of internetwork packets and specifying rules for handling (Internet: IP protocol). yInterconnecting components that route packets to their destinations (Internet: Internet routers).

11 Internet protocols – the TCP/IP protocol suite Messages (UDP) or Streams (TCP) Application Transport Internet UDP or TCP packets IP datagrams Network-specific frames Message Layers Underlying network Network interface

12 Internet protocols – encapsulation and headers Application message TCP header IP header Ethernet header Ethernet frame port TCP IP

13 Internet protocols – IP zInternet Protocol. zTransmits datagrams from one host to another, if necessary via intermediate routers. zUnreliable, best-effort delivery semantics. zAddress resolution: Conversion of Internet addresses to network addresses (for a given network). zRouting: Each router in the Internet implements IP-layer software to provide a routing algorithm.

14 Internet protocols – IP packet layout and addressing

15 Internet protocols – TCP and UDP zUDP features: yTransport-level replica of IP. yNo guarantee of delivery. yNo setup cost, no acknowledgement messages. yMessage size up to 64 kbytes. zTCP features: yReliable delivery. yArbitrarily long sequences of bytes. yConnection-oriented. yMechanisms: Sequencing, flow control, retransmission, buffering, checksum.

16 Ethernet (IEEE 802.3) – Basics zCarriers Sense Multiple Access with Collision Detection (CSMA/CD) zXerox – Ethernet zRandom Access y Stations access medium randomly z Contention yStations contend for time on medium

17 Ethernet – ALOHA zSender yGo ahead and send! yRetransmit if no ACK zProblems yCollisions yLow utilization (18%) zSlotted ALOHA is an improvement (max utilization 37%)

18 Ethernet – CSMA zCarrier Sense Multiple Access zObservations yPropagation time is much less than transmission time yAll stations know that a transmission has started almost immediately zSender yFirst listen for clear medium (carrier sense) yIf medium idle, transmit yIf two stations start at the same instant, collision yWait reasonable time yRetransmit if no ACK zMax utilization depends on propagation time (medium length) and frame length

19 Ethernet – CSMA/CD zCarrier Sense Multiple Access – Collision Detection zObservation: With CSMA, collision occupies medium for duration of transmission zSender yIf medium idle, transmit yIf busy, listen for idle, then transmit yStation listens whilst transmitting yIf collision detected, transmit jam signal, then cease transmission zAfter jam, wait random time then start again yBinary exponential back off

20 Ethernet – CSMA/CD Operation

21 Ethernet – Collision Detection zBus yCollision produces much higher signal voltage than signal yCollision detected if cable signal greater than single station signal zStar yActivity on more than one input is collision ySpecial collision presence signal

22 Summary zNetworks. zProtocols. zInternet protocols (TCP/IP). zEthernet