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

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 CableTypical BandwidthsDistance Category 5 twisted pair (Cat-5) 10 – 100 Mbps100m Thin-net Coax10 – 100 Mbps200m Thick-net coax10 – 100 Mbps500m Multimode fiber100 Mbps2km Single-mode fiber Mbps40km

6 Common bandwidths available from the carriers ServicesBandwidth DS Mbps DS Mbps STS Mbps STS Mbps STS-NN * Mbps

7 Common services available to connect your home ServicesBandwidth POTS28.8 – 56 Kbps ISDN64 – 128 Kbps ADSL16Kbps – Mbps VDSL Mbps CATV20 – 40 Mbps Last-Mile Links

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

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

10 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) Bits 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 Encodings (cont) Bits NRZ Clock Manchester NRZI

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

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 SYN HeaderBody SYN SOH STXETX CRC

17 Bit-Oriented Protocols –delineate frame with special pattern: –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 HeaderBody CRC Beginning sequence Ending sequence

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-  s long –STS-n (STS-1 = 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

Parity bits Parity byte Data Two-Dimensional Parity

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= 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 2 + 1

24 CRC (cont) 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) = 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 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 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 Problem: keeping the pipe full Example –1.5Mbps link x 45ms RTT = 67.5Kb (8KB) –1KB frames implies 1/8th link utilization SenderReceiver

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

30 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  SWS LARLFS ……

31 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 LAF discarded Send cumulative ACKs  RWS LFR LAF ……

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