Data Link Networks. Hardware Building Blocks. Nodes & Links. CS565 Data Link Networks 1

Transcription

1 Data Link Networks Hardware Building Blocks Nodes & Links CS565 Data Link Networks 1

2 PROBLEM: Physically connecting Hosts 5 Issues 4 Technologies Encoding - encoding for physical medium Framing - delineation of bit stream Error Detection Network Card - identify frame errors Reliable Delivery - link integrity despite errors Media Access Control - multiple host access Point-to-point Links CSMA (Carrier Sense Multiple Access) - Ethernet - IEEE Token Ring - FDDI - IEEE Wireless - IEEE CS565 Data Link Networks 2

3 Nodes CPU general-purpose computers; e.g., desktop workstations, special- purpose hardware, PC Cache Network adaptor (To network) Memory I/O bus Finite memory Connects to network via a network adaptor Fast processor, slow memory CS565 Data Link Networks 3

4 Network Node Memory Moore s Law Doubling processor speeds in 18 months Memory Latency Only 7% improvement each year Network nodes run at memory speeds, not CPU speeds Memory accesses needed to be considered carefully Two scarce resources: bandwidth and memory CS565 Data Link Networks 4

5 Links f (Hz) Radio Microwave Infrared UV X ray Gamma ray Coax Satellite Fiber optics AM FM TV Terrestrial microwave Electromagnetic Spectrum CS565 Data Link Networks 5

6 Links Sometimes you install your own Category 5 twisted pair Mbps, 1 50-ohm coax (ThinNet) Mbps, 2 75-ohm coax (ThickNet) Mbps, 5 Multimode fiber 100Mbps, 2km Single-mode fiber Mbps Sometimes leased from the phone company Service to ask for ISDN T1 T3 STS-1 STS-3 STS-12 STS-24 STS-48 Bandwidth you get 64 Kbps Mbps Mbps Mbps Mbps Mbps Gbps Gbps (Note: T1 also called DS1, STS-1 also called OC-1) CS565 Data Link Networks 6

7 Last-Mile Links From home to the network service provider. Service POTS (Plain Old Telephone Service) ISDN (Integrated Services Digital Network) xdsl (Digital Subscriber Line) CATV (CAble TV) Bandwidth Kbps Kbps 16 Kbps-55.2Mbps Mbps CS565 Data Link Networks 7

8 Point-to-Point Links Encoding Framing Error Detection Reliable Transmission CS565 Data Link Networks 8

9 Encoding Signals propagate over a physical medium modulate electromagnetic waves by varying the voltage Network adaptor handles encoding Encoded bits to signals (sending) Decodes signals to bits (receiving) CS565 Data Link Networks 9

10 Adaptors Signalling component Node Adaptor Signal Adaptor Node Bits Signal travel between signalling components; Bits flow between adaptors CS565 Data Link Networks 10

11 Modem and Codec Modem = Modulator + Demodulator Codec = Encoder + Decoder Encoder Modulator Demodulator Decoder Media CS565 Data Link Networks 11

12 NRZ Encoding Encode binary data onto signals e.g., 0 as low signal and 1 as high signal known as Non-Return to zero (NRZ) Bits NRZ Problem: Consecutive 1s or 0s Low signal (0) may be interpreted as no signal High signal (1) leads to baseline wander Unable to recover clock CS565 Data Link Networks 12

13 Alternative Encodings Non-return to Zero Inverted (NRZI) make a transition from current signal to encode a one; stay at current signal to encode a zero solves the problem of consecutive ones Manchester transmit XOR of the NRZ encoded data and the clock only 50% efficient. CS565 Data Link Networks 13

14 4-Bit Symbol bit Code Others: idle dead Encodings (cont) 4B/5B every 4 bits of data encoded in a 5-bit code 5-bit codes selected to have no more than one leading 0 and no more than two trailing 0s thus, never get more than three consecutive 0s resulting 5-bit codes are transmitted using NRZI achieves 80% efficiency CS565 Data Link Networks 14

15 Encodings (cont) Bits NRZ Clock Manchester NRZI CS565 Data Link Networks 15

16 Framing Node A Bits Adaptor Adaptor Node B Packet-switched networks Break sequence of bits into frames (blocks of data) What set of bits constitute a frame? Where the frame begins? Where the frame ends? Frames Central challenge - Use different protocols Typically implemented by network adaptor Adaptor fetches (deposits) frames out of (into) host memory CS565 Data Link Networks 16

17 Framing Protocol Byte-oriented View each frame as a collection of bytes (characters) Sentinel approach BISYNC (Binary Synchronous Communication) protocol - IBM Byte counting DDCMP ( Digital Data Communication Message Protocol) protocol - DEC Bit-oriented HDLC (High-Level Data Link Control) Protocol IBM and then ISO Clock-based SONET (Synchronous Optical Network) Bellcore and then ANSI CS565 Data Link Networks 17

18 Byte-Oriented - Sentinel Approach Frame begins at SYN (Synchronization) Sentinel values between body STX = Start of text ETX = End of text CRC (Cycle Redundancy Check) checks for errors SYN SYN SOH Header STX Body BISYNC frame format (Binary Synchronous Communication) IBM ETX CRC problem: ETX character might appear in the data portion of the frame solution: Character stuffing Escape the ETX character with a DLE (data line escape) character in BISYNC CS565 Data Link Networks 18

19 Byte-Oriented Byte-Counting Approach COUNT field specifies how many bytes contained in a frame SYN SYN Class Count Header Body CRC DDCMP frame format ( Digital Data Communication Message Protocol) - DEC CS565 Data Link Networks 19

20 Bit-Oriented Denote the beginning/end of a frame with the distinguished bit sequence Beginning sequence Header Body CRC Ending sequence HDLC frame format (High-level Data Link Control) IBM and then ISO problem: the pattern could appear anywhere in the body of the frame solution: Bit Stuffing - When it is located in the body, it is preceded with an escape sequence of bits (like an escape character in C) CS565 Data Link Networks 20

21 Clock-Based each frame is 125 s long At STS-1 (= Mbps) rate, 810B long e.g., SONET: Synchronous Optical Network ITU standard for transmission over fiber STS-n(STS-1 = Mbps) Overhead Payload Hdr STS-1 Hdr STS-1 Hdr STS-1 9 rows 90 columns Each frame is 810 bytes long Hdr STS-3c c - concatenated CS565 Data Link Networks 21

22 Error Long history of dealing with bit errors Hamming Reed/Solomon Detecting Error is only one part of the problem, the other part is correcting errors Two methods of error correction Have the message retransmitted Error-correcting codes (algorithms that all the recipient to reconstruct the correct message) CS565 Data Link Networks 22

23 Error Detection Basic idea add extra (redundant) bits to a frame that can be used to determine if errors have been introduced. Ethernet: 1500B data requires only 32-bits (CRC-32) Sender applies algorithm to the message to come up with the extra bits Receiver uses the same algorithm to check if the calculation comes up with the same result Common error-detecting codes Two-dimensional parity (ASCII) (link-level) Checksum (internet) (not link-level) CRC, Cyclic Redundancy Check, (link-level) CS565 Data Link Networks 23

24 Two-Dimensional Parity Catch all 1,2,3-bit and most 4-bit errors In this example, use 14 redundant bits for a 42-bit message, which is much better than the obvious way of sending two copies of the same data Data Parity byte Parity bits Used by BISYNC protocol (IBM) to transmitting ASCII characters CS565 Data Link Networks 24

25 Internet Checksum Algorithm Not used in link-level (unlike parity and CRC) Sender adds up all the word and then transmit the result of that sum (Checksum) Received adds up all the words and compares its checksum to the sender s checksum Algorithm for the Internet 1. Treat the data as a sequence of 16-bit integers. Add the 16-bit integers using 16-bit ones complement arithmetic 2. take the ones complement of the result. That 16-bit number is the checksum. CS565 Data Link Networks 25

26 CRC - Cyclic Redundancy Check Add k bits of redundant data to an n-bit message want k << n e.g., Ethernet: k = 32 and n = 12,000 (1500 bytes) Represent n-bit message as n-1 degree polynomial e.g., MSG= as M(x) = x 7 + x 4 + x 3 + x 1 Let k be the degree of some divisor polynomial e.g., C(x) = x 3 + x when k = 3 CS565 Data Link Networks 26

27 CRC - Cyclic Redundancy Check Transmit polynomial P(x) that is evenly divisible by C(x) shift left k bits, i.e., M(x)x k subtract remainder of M(x)x k / C(x) from M(x)x k Receiver polynomial P(x) + E(x) (E(x) error in the transmission) E(x) = 0 implies no errors Divide (P(x) + E(x)) by C(x); remainder zero if: E(x) was zero (no error), or E(x) is exactly divisible by C(x) CS565 Data Link Networks 27

28 CRC Example: k=3 Generator C(x) 1101 Perform logical XOR Once the reminder is obtained, subtract it from M(x)x k, this can be accomplished with the XOR = Send this message Recipient divides received message by C(x), if the reminder is 0 no error (most likely) Original Message M(x) Message & k bits of M(x)x k Remainder M(x)x k / C(x) CS565 Data Link Networks 28

29 Selecting C(x) All single-bit errors, as long as the x k and x 0 terms have non-zero coefficients. All double-bit errors, as long as C(x) contains a factor with at least three terms Any odd number of errors, as long as C(x) contains the factor (x + 1) Any burst error (i.e., sequence of consecutive error bits) for which the length of the burst is less than k bits. Most burst errors of larger than k bits can also be detected CS565 Data Link Networks 29

30 Common CRC Divisor Polynomials CRC CRC-8 (ATM) CRC-10 (ATM) CRC-12 CRC-16 CRC-CCITT (HDLC) CRC-32 (Ethernet) C(x) x 8 +x 2 +x 1 +1 x 10 +x 9 +x 5 +x 4 +x 1 +1 x 12 +x 11 +x 3 +x 2 +x 1 +1 x 16 +x 15 +x 2 +1 x 16 +x 12 +x 5 +1 x 32 +x 26 +x 23 +x 22 +x 16 +x 12 +x 11 +x 10 +x 8 +x 7 +x 5 +x 4 +x 2 +x+1 CSC 550 Data Link Networks 30