Outline Encoding Framing Error Detection Reliable Transmission Ethernet (802.3)

1. Direct Link Networks Outline Encoding Framing Error Detection Reliable Transmission Ethernet (802.3) Token Rings (802.5, FDDI) Wireless (802.11) Network Adaptors

2. Problem: Physically Connecting Hosts • Five issues: • Encoding • Framing • Error detection • Reliable delivery • Access Mediation • Goal: • Survey the available network technology • Explore these five fundamental issues

3. Problem: Physically Connecting Hosts • These five functions are usually implemented in a network adaptor. • Bits are exchanged between adaptors, but correct frames are exchanged between nodes. • Adaptors are controlled by software running on the node – the device driver.

4. Hardware Building Block • Networks are constructed from nodes and links. • Nodes are often general-purpose computers. • Network links are implemented on a variety of different physical media. • Twisted pair • Coaxial cable • Optical fiber • Space (radio waves, microwaves, infrared beams)

5. Hardware Building Block • Media is used to propagate signals. These signals are actually electromagnetic waves traveling at the speed of light. • Wavelength = speed / Frequency (Figure 2.2) • A link is said to be full-duplex if two bit streams can be simultaneously transmitted over the link at the same time. A link that supports only one direction is called half-duplex.

6. Hardware Building Block • Cables (Table 2.1) • Category 5 twisted pair: 10 – 100 Mbps, 100m • Thin-net coax: 10 – 100 Mbps, 200m • Thick-net coax: 10 – 100 Mbps, 500m • Multimode fiber: 100 Mbps, 2m • Single-mode fiber: 100 – 240 Mbps, 40km

7. Hardware Building Block • Leased Lines • DS1/T1: Digital Signal/Transmission Level 1.544 Mbps • DS3/T3: 44.736 Mbps • STS-N/OC-N: Synchronous Transport Signal/Optical Carrier: 51.84 Mbps ~ 2.48832 Gbps

8. Hardware Building Block • Last-Mile Links • POTS (Plain Old Telephone System): 28.8-56 Kbps • ISDN (Integrated Services Digital Network): 64-128 Kbps • xDSL (Digital Scriber Line): 16 Kbps – 55.2 Mbps • CATV (Cable TV): 20 – 40 Mbps

9. Hardware Building Block • Advanced Mobile Phone System (AMPS) has been the standard for celluar phones. • PCS (Personal Communication Services) – USA, Canada • GMS (Global Mobile System) – Rest of the world • Wireless Links (Table 2.4) • ICO • Globalstar • Iridum • Teledesic • Bluetooth is being designed to operate at 2.45 GHz within 10 meters.

10. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Encoding • Signals propagate over a physical medium • modulate (add the signal to an electronic or optical signal carrier) electromagnetic waves • Modulation can be applied to electric current or voltage (mainly by turning it on and off, e.g., vary voltage) • Encode binary data onto signals • e.g., 0 as low signal and 1 as high signal with with no neutral or rest condition • known as Non-Return to zero (NRZ) (compare to RZ)

11. Problem: Consecutive 1s or 0s • Low signal (0) may be interpreted as no signal • High signal (1) leads to baseline wander: The baseline distinguishing between low and high signals wanders. • Unable to recover clock: A long period of time without such a transition leads to clock drift.

12. 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 • Solve the baseline wander and clock drift but only 50% efficient.

13. Encodings (cont) • 4B/5B (Table 2.5) • 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 (NRZI solves the problem of consecutive 1s. So reduce 0 in the 5-bit coding.) • achieves 80% efficiency

14. Bit Rates and Baud Rates • Data Transfer Rate - The time, T, required to transmit one character depends on: • the encoding method • the baud rate (signalling speed) which is the number of times per second that the signal changes its value. • Note: baud rate is not the same as bit rate (Page 80).

15. Bits 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 NRZ Clock Manchester NRZI Encodings (cont)

16. Bits Node A Adaptor Adaptor Node B Frames Framing • Break sequence of bits into a frame • Typically implemented by network adaptor

17. Framing • Byte-oriented protocols • BISYNC (BInary SYNchronous Communication) Protocol: sentinel approach • PPP (Point-to-Point Protocol): sentinel approach • DDCMP (Digital Data Communication Message Control): byte-counting approach • Bit-oriented protocols - HDLC (High-Level Data Link Control) • Clock-Based Framing – Synchronous Optical Network (SONET)

18. Byte-Oriented Protocols 8 8 8 8 8 16 BISYNC SYN SYN SOH Header STX Body ETX CRC • BISYNC (IBM 1960) (DataLink Protocol) • SYN = synchronization character (start of frame) • SOH = "Start of Header" character • STX/ETX = Start/End-of-Text characters. (What if ETX occurs in Body? char stuff =prefix DataLink Esc) • CRC (Cyclic Redun Chk) field to detect trans errors. • Header: for link-level reliable delivery algorithm.

19. Byte-Oriented Protocols 8 8 8 16 16 8 PPP Flag Adr Ctrl Prot Payload Chksm Flag • PPP (typically run over dialup networks) (DataLink Protocol) • Flag = 01111110 (Sentinel character) • Adr/Ctrl usually default values (unused) • Protocol identifies high-level protocol (IP, IPX) • Payload (default=1500B or negotiated by LCP) • Checksum field is 2 or 4 bytes (2 default) • LCP (Link Control Protocol): Sends control message encapsulated • PPP uses character stuffing when sentinel occurs in Payload also.

20. Byte-Oriented Protocols • Couter-based • include payload length in header • e.g., DDCMP (Digital Data Communication Message Control) • problem: count field corrupted • solution: receiver accumulates as many bytes as Count indicates then uses error detection field (e.g., CRC) to determine if it is correct (framing error).

21. 8 16 16 8 Beginning Ending Header Body CRC sequence sequence Bit-Oriented Protocols • It views the frame as a collection of bits. • Sentinel-based • delineate frame with special pattern: 01111110 • e.g., Synchronous Data Link Control (SDLC), HDLC • problem: special pattern appears in the payload • solution: bit stuffing • sender: insert 0 after five consecutive 1s • receiver: delete 0 that follows five consecutive 1s

22. Clock-based Framing • Synchronous Optical Network (SONET) • The full specification is larger than this book. • It addresses both the framing and encoding problems. • It multiplexes several low-speed links onto one high-speed link.

23. Clock-based Framing • SONET Frame Structure • 9 x 90 = 810 bytes • First two bytes of the frame contain a special pattern. • Overhead has multiple functions: across different links, specify voice channel, concatenation frames. • Overhead bytes are encoded in NRZ. The payload bytes are scrambled by exclusive-ORing (XOR) a 127 bit pattern. • STS-N frame can be thought of as consisting of N STS-1 frames.

24. Clock-Based Framing • Clock-based • each frame is 125us long • e.g., SONET: Synchronous Optical Network • STS-n (STS-1 = 51.84 Mbps)

25. Error Detection • Two approaches can be taken: • Notify the sender to retransmit • Error-correcting code • Techniques for detecting transmission errors • Two-dimensional parity • Checksums • Cyclic redundancy check (CRC): used in nearly all link-level protocol, for example, HDLC, DDCMP, CSMA

26. Error Detection • The basic idea is to add redundant information that can be used to determine if there is any error. • We can provide quite strong detection capability while sending only k redundant bits for an n-bit message, where k << n. • On a Ethernet, a frame carrying 12,000 bits requires only a 32-bit CRC code. • When the error-detecting algorithm to create the code is based on addition, they may be called a checksum.

27. Two-Dimensional Parity • 1-D parity adds 1 bit to 7-bit code to balance # of 1s • Odd parity adds a bit so the # of 1-bits is odd. • Even parity adds a bit so the # of 1-bits is even. • 2-D parity does 1-D parity and then the same across each bit of all bytes. • 2-D (even) parity for a 6 byte frame (above) • catches all 1,2,3 bit and most 4-bit errors.

28. Internet Checksum Algorithm • Add up all the words and then transmit the result of that sum. • View message as a sequence of 16-bit integers; sum using 16-bit ones-complement arithmetic; take ones-complement of the result. u_short cksum(u_short *buf, int count) { register u_long sum = 0; while (count--) { sum += *buf++; if (sum & 0xFFFF0000) { /* carry occurred, so wrap around */ sum &= 0xFFFF; sum++; } } return ~(sum & 0xFFFF); }

29. Cyclic Redundancy Check • A major goal in designing error detection algorithms is to maximize the probability of detecting errors using only a small number of redundant bits. • In general, correcting is more expensive than detecting and re-transmitting. • Add k bits of redundant data to an n-bit message • want k << n • e.g., k = 32 and n = 12,000 (1500 bytes) • Represent n-bit message as n-1 degree polynomial • e.g., MSG=10011010 as M(x) = x7 + x4 + x3 + x1 • Let k be the degree of some divisor polynomial • e.g., C(x) = x3 + x2 + 1

30. CRC (cont) • Transmit polynomial P(x) that is evenly divisible by C(x) • shift left k bits, i.e., M(x)xk, append k zero bits to low order end of the frame • subtract remainder of M(x)xk / C(x) from M(x)xk • Suppose that a transmission error E(x) has occurred. Receiver polynomial P(x) + E(x) arrives instead of P(x). P(x) + E(x) / C(x) = E(x)/C(x) • 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)

31. CRC Example 11111001 ----------- 1101 /10011010000 <- Message 1101 ----- 1001 1101 ----- 1000 1101 ----- 1011 1101 ---- 1100 1101 ----- 1000 1101 ---- 101 <- Remainder • M(x)=10011010 • C(x)=1101 • k=3 • P(x) = 10011010 101

32. CRC Example 1100001010 ------------ 10011 /11010110110000 10011 ----- 10011 10011 ----- 10110 10011 ----- 10100 10011 ----- 1110 • M(x)=1101011011 • C(x)=10011 • k=4 • P(x) = 1101011011 1110

33. Selecting C(x) • All single-bit errors, as long as the xk and x0 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 • See Table 2.6 on page 102 for common C(x): • CRC-16 = x16+x15+x2+1 => 16 bit check sum. => catches all single,double,odd errors. => catches all burst errors of length <=16

34. Error Detection or Error Correction • Error correction requires a greater number of redundant bits and is used when: • Errors are quite probable. Wireless. • High cost of the retransmission. A satellite link. • The use of error-correcting codes is sometimes referred as forward error correction (FEC).

35. Reliable Transmission • The state of the art in error-correcting codes is not advanced enough to handle the range of bit and burst errors without excessive overhead. • Corrupt frames generally must be discarded. • A link-level protocol that delivers frames reliably must recover from these discarded frames. • This is accomplished by acknowledges and timeouts and called automatic repeat request (ARQ).

36. Reliable Transmission • An acknowledge (ACK) is a small control frame sent back to its peer confirming an earlier frame has been received. A control frame means a header without data. A protocol can piggyback an ACK which is sent back with the data frame. • A timeout is the action of waiting a reasonable amount of time to retransmit a frame. • Three ARQ algorithms are introduced: • Stop-and-Wait • Sliding Window • Concurrent logical channels

37. Timeline for the stop-and-wait • A timeline is a common way to depict a protocol’s behavior.‘lost’ means the frame was corrupted while in transit.

38. Stop-and-Wait Sender Receiver • To prevent the duplicate, add 1-bit sequence number in the header. • Shortcoming: have only one outstanding frame. Far below the link’s capacity. • Principle: keeping the pipe full. • Example • 1.5Mbps link x 45ms RTT = 67.5Kb (8KB) • 1KB frames imples 1/8th link utilization

39. Sender Receiver … ime T … Sliding Window • Allow multiple outstanding (un-ACKed) frames • Upper bound on un-ACKed frames, called window

40. £ SWS … … LAR LFS SW: Sender • Assign sequence number to each frame (SeqNum) • Maintain three state variables: • send window size (SWS) • last acknowledgment received (LAR) • last frame sent (LFS) • Maintain invariant: LFS - LAR <= SWS • Advance LAR when ACK arrives • Buffer up to SWS frames

41. £ RWS … … NFE LFA SW: Receiver • Maintain three state variables • receive window size (RWS) • largest frame acceptable (LFA) • last frame received (LFR)/next frame expected (NFE) • Maintain invariant: LFA - LFR <= RWS • Frame SeqNum arrives: • if LFR < SeqNum < = LFA accept • if SeqNum < = LFR or SeqNum > LFA discarded • Send cumulative ACKs

42. Example of Sliding Window • Example • LFR=5 (i.e. the last ack the receiver sent was for seq no 5) • RWS=4; thus, LAF=9 • If R gets frames 7 and, they are buffered because they are within the window. • Since frame 6 is yet to arrive, Frames 7 and 8 are said to be arriving out of order • If Frame 6 arrived, R sends Ack for 8, sets LFR to 8, and sets LAF to 12.

43. Example of Sliding Window • Other variations that will improve performance at the cost of complexity: • Ack 5 when Frames 7 and 8 arrived; • NAK for Frame 6 when Frame 7 and 8 arrived • Selective acknowledgement vs. cumulative acks: ack 7 and 8 • Window size: commensurate with bandwidth * delay product. RWS=1 or RWS=SWS. Does make sense to have RWS >SWS?

44. Sequence Number Space • SeqNum field is finite; sequence numbers wrap around • Sequence number space must be larger then number of outstanding frames • SWS <= MaxSeqNum-1 is not sufficient • suppose 3-bit SeqNum field (0..7) • SWS=RWS=7 • sender transmit frames 0..6 • arrive successfully, but ACKs lost • sender retransmits 0..6 • receiver expecting 7, 0..5, but receives second incarnation of 0..5 • SWS < (MaxSeqNum+1)/2 is correct rule • Intuitively, SeqNum “slides” between two halves of sequence number space • Implementation: Page 109-114

45. Frame Order and Flow Control • The sliding window protocol is perhaps the best known algorithm in computer networking. • The first role is the one we have been concentrating on in this section - to reliably deliver frames across an unreliable link. • The second role is to preserve the order in which frames are transmitted. • The third role that the sliding window algorithm sometimes plays is to support flow control - a feedback mechanism by which the receiver is able to throttle the sender.

46. ARPANET - Concurrent Logical Channels • Multiplex 8 logical channels over a each ground link and 16 over each satellite link • Run stop-and-wait on each logical channel • Maintain three state bits per channel • channel busy • current sequence number for the frame to be sent out • next sequence number for the frame coming in • Ground link Header: 3-bit channel number, 1-bit sequence number • 4-bits total • same as sliding window protocol: RWS=SWS

47. Shared Access Networks Outline Bus (Ethernet 802.3) Token ring (FDDI) Wireless (802.11)

48. Ethernet Overview • History • developed by Xerox PARC in mid-1970s • roots in Aloha packet-radio network • standardized by Xerox, DEC, and Intel in 1978 • similar to (subset of) IEEE 802.3 standard • CSMA/CD • carrier sense – all nodes can distinguish between an idle and a busy link • multiple access – a set of nodes share a link • collision detection – a node can detect if there is a collision

49. Ethernet – Physical Properties • Fast Ethernet: 100-Mbps, Gigabit Ethernet: 1 Gbps • 50-ohm coaxial cable (10Base5, thick-net): • A transceiver is a small device tapping to the Ethernet cable. • Multiple Ethernet segments can be joined by repeaters. A repeater is a device that forwards digital signal. • An Ethernet limits 4 repeaters and supports 1024 hosts. • Terminators attached to the end of each segment absorb the signal.

50. Ethernet – Physical Properties • 10Base2, thin-net • 10 means 10 Mbps. • Base refers to the cable is used in a baseband system. • 2 means a segment not longer than 200m. • T-joint is used. • 10BaseT, Twisted pair • Under 100 m. • Several point-point segments coming out of a multiway repeater, called hub.