Hardware Building Blocks Five Issues Encoding Framing Error Detection Reliable delivery

1. Direct Link Networks • Hardware Building Blocks • Five Issues • Encoding • Framing • Error Detection • Reliable delivery • Access Mediation

2. Building Blocks • Nodes: PC, special-purpose hardware… • hosts • switches • Links (Transmission Media): • Guided: twisted pair, coax cable, optical fiber • Unguided: radio, microwaves, infrared, lasers

3. The Electromagnetic Spectrum

4. Properties of A Link • Frequency • Encoding • Lower layers • modulation • Upper layers • “high” signal • “low” signal • Multiple-access links • How many bit streams can be encoded on it at a given time. • Point-to-point links • Full-duplex • Half-duplex • How can I get one?

5. Common types of cables and fibers available for local links

6. Common bandwidths available from the carriers

7. Common services available to connect your home Last-Mile Links

8. 1.554 – 8.448 Mbps 16 – 640 Kbps Central Subscriber office premises Local loop ADSL (asymmetric digital subscriber line) STS- N VDSL at 12.96 – 55.2 Mbps Central Neighborhood optical Subscriber office network unit premises over fiber over 1000 – 4500 feet of copper VDSL

9. ADSL versus Cable • Bandwidth • The number of subscriber • Availability • Security • Reliability

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 electromagnetic waves • e.g., vary voltage • Encode binary data onto signals • e.g., 0 as low signal and 1 as high signal • known as Non-Return to zero (NRZ)

11. Problem: Consecutive 1s or 0s • Long strings of 1s or 0s cause: • baseline wander • Unable to recover clock (clock drift) • Low signal (0) may be interpreted as no signal

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 • 0 low-to-high transition • 1 high-to-low transition • transmit XOR of the NRZ encoded data and the clock • only 50% efficient (the bit rate = the half baud rate)

13. Encodings (cont) • 4B/5B (FDDI) • 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

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

15. Bits Node A Adaptor Adaptor Node B Frames Framing • Break sequence of bits into a frame • Typically implemented by network adaptor • Determining where the frame begins and ends

16. Sentinel Based Approach • Byte-Oriented Protocols • e.g. BISYNC, PPP • problem: special character appears in the data portion • solution: character stuffing • Preceding the ETX with a DLE (data-link-escape) character 8 8 8 8 8 16 STX ETX SYN SYN SOH Header Body CRC

17. 8 16 16 8 Beginning Ending Header Body CRC sequence sequence • Bit-Oriented Protocols • delineate frame with special pattern: 01111110 • e.g., HDLC, SDLC • 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

18. Counter-based Approaches • Counter-based • include payload length in header • e.g., DDCMP • problem: count field corrupted • solution: catch when CRC fails

19. Clock-based Approaches • SONET • e.g., SONET: Synchronous Optical Network • each frame is 125-ms long • STS-n (STS-1 = 51.84 Mbps)

20. Encoding in SONET • NRZ • sender: XOR of the data by the use of a well- known bit pattern (127 bits long). • Receiver: ???

21. Error Detection • Two-Dimensional Parity • Cyclic-Redundancy Check • Internet Checksum Algorithm

22. Two-Dimensional Parity Parity bits 0101001 1 1101001 0 1011110 1 Data 0001110 1 0110100 1 1011111 0 Parity 1111011 0 byte

23. Cyclic Redundancy Check • 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

24. CRC (cont) • Transmit polynomial P(x) that is evenly divisible by C(x) • shift left k bits, i.e., M(x)xk • subtract remainder of M(x)xk / C(x) from M(x)xk • Receiver polynomial P(x) + E(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)

25. 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)

26. Internet Checksum Algorithm • 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); }

27. Acknowledgements & Timeouts

28. Stop-and-Wait Sender Receiver • Problem: keeping the pipe full • Example • 1.5Mbps link x 45ms RTT = 67.5Kb (8KB) • 1KB frames implies 1/8th link utilization

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

30. £ 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

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

32. 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

33. Concurrent Logical Channels • Multiplex 8 logical channels over a single link • Run stop-and-wait on each logical channel • Maintain three state bits per channel • channel busy • current sequence number out • next sequence number in • Header: 3-bit channel num, 1-bit sequence num • 4-bits total • same as sliding window protocol • Separates reliability from order