Page 1CS 640: Computer NetworksAditya AkellaLecture 6 -Error/Flow Control&Intro to Switchingand Medium Access ControlError Coding• Transmission process may introduce errors into a message.– Single bit errors versus burst errors• Detection: e.g. CRC– Requires a check that some messages are invalid– Hence requires extra bits– “redundant check bits”• Correction– Forward error correction: many related code words map to the same data word– Detect errors and retry transmissionParity• Even parity– Append parity bit to 7 bits of data to make an even number of 1’s– Odd parity accordingly defined.• 1 in 8 bits of overhead?– When is this a problem?• Can detect a single error• But nothing beyond that10101001001011101010101100001010Page 22-D Parity• Make each byte even parity• Finally, a parity byte for all bytes of the packet• Example: five 7-bit character packet, even parity 01101001011010001011011101011001011101101000110 1Effectiveness of 2-D Parity• 1-bit errors can be detected, corrected• Example with even parity per byte:01101001011010000011011101011001011101101000110 1error bitodd number of 1’s• 2-bit errors can also be detected• Example:• What about 3-bit errors? >3-bit errors?01101001011010000011111101011001011101101000110 1error bitsodd number of 1’sEffectiveness of 2-D Parityeven number of 1’s - OkPage 3Cyclic Redundancy Codes(CRC)• Commonly used codes that have good error detection properties– Can catch many error combinations with a small number or redundant bits• Based on division of polynomials– Errors can be viewed as adding terms to the polynomial– Should be unlikely that the division will still work• Can be implemented very efficiently in hardware• Examples:– CRC-32: Ethernet– CRC-8, CRC-10, CRC-32: ATMLink Flow Control and Error Control• Dealing with receiver overflow: flow control.• Dealing with packet loss and corruption: error control.• Actually these issues are relevant at many layers.– Link layer: sender and receiver attached to the same “wire”– End-to-end: transmission control protocol (TCP) - sender and receiver are the end points of a connection• How can we implement flow control?– “You may send” (windows, stop-and-wait, etc.)– “Please shut up” (source quench, 802.3x pause frames, etc.)Flow Control: A Naïve Protocol• Sender simply sends to the receiver whenever it has packets.• Potential problem: sender can outrun the receiver.– Receiver too slow, small buffer overflow, ..• Not always a problem: receiver might be fast enough.SenderReceiverPage 4Adding Flow Control• Stop and wait flow control: sender waits to send the next packet until the previous packet has been acknowledged by the receiver.– Receiver can pace the sender• Drawbacks: adds overheads, slowdown for long links.SenderReceiverWindow Flow Control• Stop and wait flow control results in poor throughput for long-delay paths: packet size/ roundtrip-time.• Solution: receiver provides sender with a window that it can fill with packets.– The window is backed up by buffer space on receiver– Receiver acknowledges the a packet every time a packet is consumed and a buffer is freedSenderReceiverWindow LimitationsSenderReceiverTimeThroughput = Window SizeRoundtrip TimeRTTWindow Size = 4pktsPage 5Error Control: Stop and Wait Case• Packets can get lost, corrupted, or duplicated. • Duplicate packet: use sequence numbers.• Lost packet: time outs and acknowledgements.– Positive versus negative acknowledgements– Sender side versus receiver side timeouts• Window based flow control: more aggressive use of sequence numbers (see transport lectures).SenderReceiverWhat is Used in Practice?• No flow or error control.– E.g. regular Ethernet, just uses CRC for error detection• Flow control only.– E.g. Gigabit Ethernet• Flow and error control.– E.g. X.25 (older connection-based service at 64 Kbs that guarantees reliable in order delivery of data)Switching and Media Access Control• How do we transfer packets between two hosts connected to the a switched network?• Switches connected by point-to-point links -- store-and-forward.– Multiplexing and forwarding– Used in WAN, LAN, and for home connections– Conceptually similar to “routing”• But at the datalink layer instead of the network layer• Multiple access networks -- contention based.– Multiple hosts are sharing the same transmission medium– Used in LANs and wireless– Need to control access to the mediumPage 6A Switch-based Network• Switches are connected by “point-to-point” links.• Packets are forwarded hop-by-hop by the switches towards the destination.– Many forms of forwarding• Many datalink technologies use switching.– Virtual circuits: Frame-relay, ATM, X.25, ..– Packets: Ethernet, MPLS, …PC atHomeSwitchPoint-PointlinkPCs atWorkThree techniques for switching• Global addresses - connection-less– Routers keep next hop for destination– Packets carry destination address• Virtual circuits – connection oriented– Connection routed through network to set up state– Packets forwarded using connection state • Source routing– Packet carries pathGlobal Address ExampleReceiverPacketRSender234123412341S2S3S1RRR 3R 4R 3RPage 7Global Addresses• Advantages– Stateless – simple error recovery• Disadvantages– Every switch knows about every destination• Potentially large tables– All packets to destination take same route– Need special approach to fill tableSimplified Virtual CircuitsExampleReceiverPacketconn 5 3Sender2341conn 5 423412341conn 5 3S2S3S15 555Virtual Circuits• Advantages– Efficient lookup (simple table lookup)– Can reserve bandwidth at connection setup– Easier for hardware implementations• Disadvantages– Still need to route connection setup request– More complex failure recovery – must recreate connection state• Typical use fast router implementations– ATM – combined with fix sized cells– MPLS – tag switching for IP networksPage 8Source Routing ExampleReceiverPacketR1, R2, R3, RSender234123412341S2S3S1R2, R3, RR3, RRSource Routing• Advantages– Switches can be very simple and fast• Disadvantages– Variable (unbounded) header size– Sources must know or discover topology (e.g., failures)• Typical uses– Ad-hoc networks (DSR)– Machine room networks (Myrinet)ComparisonSource Routing Global
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