CS 640: Computer Networks. Error Coding. Parity. Page 1. Aditya Akella. Lecture 6 - Error/Flow Control & Intro to Switching and Medium Access Control

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CS 6: Computer Networks Aditya Akella Lecture 6 - Error/Flow Control & Intro to Switching and Medium Access Control Error Coding Transmission process may introduce errors into a message. Single bit errors
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CS 6: Computer Networks Aditya Akella Lecture 6 - Error/Flow Control & Intro to Switching and Medium Access Control Error 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 transmission Parity Even parity Append parity bit to 7 bits of data to make an even number of s Odd parity accordingly defined. in 8 bits of overhead? When is this a problem? Can detect a single error But nothing beyond that Page -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 Effectiveness of -D Parity -bit errors can be detected, corrected Example with even parity per byte: error bit odd number of s Effectiveness of -D Parity -bit errors can also be detected Example: error bits even number of s - Ok odd number of s What about -bit errors? -bit errors? Page Cyclic 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-: Ethernet CRC-8, CRC-, CRC-: ATM Link 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, 8.x pause frames, etc.) Flow Control: A Naïve Protocol simply sends to the receiver whenever it has packets. Potential problem: sender can outrun the receiver. too slow, small buffer overflow,.. Not always a problem: receiver might be fast enough. Page Adding Flow Control Stop and wait flow control: sender waits to send the next packet until the previous packet has been acknowledged by the receiver. can pace the sender Drawbacks: adds overheads, slowdown for long links. Window 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 acknowledges the a packet every time a packet is consumed and a buffer is freed Window Limitations Window Size = pkts RTT Time Throughput = Window Size Roundtrip Time Page Error 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 side versus receiver side timeouts Window based flow control: more aggressive use of sequence numbers (see transport lectures). What 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.5 (older connection-based service at 6 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 -- storeand-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 medium Page 5 A 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.5,.. Packets: Ethernet, MPLS, PCs at Work Switch Point-Point link PC at Home Three 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 path Global Address Example Packet R S R R S R R S R R Page 6 Global 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 table Simplified Virtual Circuits Example Packet 5 5 S conn 5 S conn 5 5 S 5 conn 5 Virtual 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 networks Page 7 Source Routing Example Packet R, R, R, R R, R, R S S R, R S R Source 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) Comparison Source Routing Global Addresses Virtual Circuits Header Size Worst OK Large address Best Router Table Size None Number of hosts Number of circuits Forward Overhead Best Table lookup Pretty Good Setup Overhead None None Error Recovery Tell all hosts Tell all switches Connection Setup Tell all switches and Tear down circuit and re-route Page 8 Most Popular: Address Lookup-based Approach Switch Address Next Hop Info B858 89CC7 - AC (,) Address from header. Absolute address (e.g. Ethernet) (IP address for routers) (VC identifier, e.g. ATM)) Next hop: output port for packet. Info: priority We will see how this table is filled (learning bridges) Multiple Access Protocols Prevent two or more nodes from transmitting at the same time over a broadcast channel. If they do, we have a collision, and receivers will not be able to interpret the signal Several classes of multiple access protocols. Partitioning the channel, e.g. frequency-division or time division multiplexing With fixed partitioning of bandwidth not flexible Taking turns, e.g. token-based, reservation-based protocols, polling based Contention based protocols, e.g. Aloha, Ethernet Next lecture Fiber Distributed Data Interface (FDDI) One token holder may send, with a time limit. known upper bound on delay. Optical version of 8.5 token ring, but multiple packets may travel in train: token released at end of frame. Mbps, km. Optional dual ring for fault tolerance. CDDI: FDDI over unshielded twisted pair, shorter range Page 9 Other Taking Turn Protocols Central entity polls stations, inviting them to transmit. Simple design no conflicts Not very efficient overhead of polling operation Stations reserve a slot for transmission. For example, break up the transmission time in contention-based and reservation based slots Contention based slots can be used for short messages or to reserve time Communication in reservation based slots only allowed after a reservation is made Issues: fairness, efficiency Page
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