Xiaowei Yang xwy@cs.duke

CompSci 356: Computer Network ArchitecturesLecture 4: Link layer: Encoding, Framing, and Error Detection Xiaowei Yang xwy@cs.duke.edu

Overview • Review • Physical links • Link/network performance metrics • Bandwidth / throughput • Latency / delay • Bandwidth * delay product • Link layer functions • Encoding • Framing • Error detection • Reliable transmission (if have time)

The simplest network is one link plus two nodes Hi Alice… ?

Each node (e.g. a PC) connects to a network via a network adaptor. The adaptor delivers data between a node’s memory and the network. A device driver is the program running inside the node that manages the above task. Recap: Put bits on the wire • At one end, a network adaptor encodes and modulates a bit into signals on a physical link. • At the other end, a network adaptor reads the signals on a physical link and converts it back to a bit.

Different types of physical links • Wired links • Copper • Fiber optics • Wireless links • Wifi, WiMax, Bluetooth, ZigBee, …

Commonly Used Physical Links • Different links have different transmission ranges • Signal attenuation • Cables • Connect computers in the same building • Leased lines • Lease a dedicated line to connect far-away nodes from telephone companies

Cables • CAT-5: twisted pair • Coaxial: thick and thin • Fiber CAT-5 10BASE2 cable, thin-net 200m 10Base4, thick-net 500m

Leased lines • Tx series speed: multiple of 64Kpbs • Copper-based transmission • DS-1 (T1): 1,544, 24*64kpbs • DS-2 (T2): 6,312, 96*64kps • DS-3 (T3): 44,736, 672*64kps • OC-N series speed: multiple of OC-1 • Optical fiber based transmission • OC-1: 51.840 Mbps • OC-3: 155.250 Mbps • OC-12: 622.080 Mbps

Last mile links • Wired links • POTS: 28.8-56Kbps (Plain old telephone service) • ISDN: 64-128Kbps (Integrated Services Digital Network) • xDSL: 128Kbps-100Mbps (over telephone lines) • Digital Subscriber Line • CATV: 1-40Mpbs (shared, over TV cables) • Wireless links • Wifi, WiMax, Bluetooth, …

xDSL wiring 1.5-8.4Mpbs Central Office Subscriber premises 16-640Kpbs Local loop Runs on existing copper 18,000 feet at 1.544Mbps 9,000 at 8.448 Mbps ADSL 13-55Mpbs OC links Central office Nbrhood optical Network unit Subscriber premises 1000-4500 feet of copper Must install VDSL transmission hardware VDSL (Very high) Symmetric

Wireless links • Wireless links transmit electromagnetic signals through space • Used also by cellular networks, TV networks, satellite networks etc. • Shared media • Divided by frequency and space • FCC determines who can use a spectrum in a geographic area, ie, “licensing” • Auction is used to determine the allocation • Expensive to become a cellular carrier • Unlicensed spectrum • WiFi, Bluetooth, Infrared

Link Performance Metrics Propagation delay How long it takes for one bit to travel from one end of a link to the other end Bandwidth How many bits a link can transmit in a unit time Each bit is a pulse on the wire Must have certain width for the receiver to decode it

Latency and Throughput • Latency of a message = Propagation + Transmit + Queue • Propagation = Distance/SpeedOfWave • Transmit = Size/Bandwidth • Throughput = Size / Latency

Example • 1Mbps, 100ms, 1MB data • Latency = 1MB/1Mbps + 100ms = 8.1s • Throughput = 1MB/8.1s ≈ 1Mbps • 1Gbps, 100ms, 1MB data • Latency = 1MB/1Gbps + 100ms = 108ms • Throughput = 1MB/108ms = 74.1Mbps < 1Gbps • Why?

Delay ×Bandwidth Product • Measures the volume of a pipe • The maximum number of bits can be in transit through the pipe at any given instant • To achieve high throughput, one should keep the pipe full

High speed versus low speed links

Link-layer functions • Most functions are completed by adapters • Encoding • Framing • Error detection • Reliable transmission (if have time)

Overview • Review • Physical links • Link/network performance metrics • Bandwidth / throughput • Latency / delay • Bandwidth * delay product • Link layer functions • Encoding • Framing • Error detection • Reliable transmission (if have time)

Encoding • Encoding is the process to turn binary data (bits, 0s and 1s) into physical signals sent over a physical link • Done by a piece of hardware on a network adaptor • High and low signals, ignore modulation • Simplest one: 1 to high, 0 to low

Non-return to zero • 1 to high, 0 to low • Not good for decoding • Baseline wander • Consecutive 1s or 0s cause the average signal level to drift • Receiver uses it to distinguish high/low signals • Clock recovery • Receiver uses transition to 1s or 0s as clock boundaries to synchronize clock

Solution 1: Nonreturn to zero inverted (NRZI) • A transition from current signal encodes 1 • No transition encodes 0 • Does it solve all problems? • Not for consecutive 0s NRZI

Solution 2: Manchester encoding • Clock XOR NRZ • 1: high  low; 0: low  high • Drawback: doubles the rate at which signals are sent • Baud rate: signal change rate • Bit rate = half of baud rate. 50% efficient

Final solution: 4B/5B • Key idea: insert extra bits to break up long sequences of 0s or 1s • 4-bit of data are encoded in a 5-bit code word • 16 data symbols, 32 code words  choose the good codes that do not have long sequence of 0s • At most one leading 0, two trailing 0s • For every pair of codes, no more than three consecutive 0s • 5-bit codes are sent using NRZI

Exercise: • 00101101 • What’s the high/low signal sequence?

Overview • Link layer functions • Encoding • Framing • Error detection

Framing • Now we’ve seen how to transmit bitstreams • But nodes send blocks of data (frames) • A’s memory  adaptor  adaptor  B’s memory • An adaptor must determine the boundary of frames Block of data

Variety of Framing Protocols • Framing: determining where the frame begins and ends • Why is it an important task of an adaptor? • Frames may belong to different apps • Need to decide when to deliver them • Design choices • Byte-oriented protocols: the smallest unit of data is a byte • Sentinel approach • Byte-counting approach • Bit-oriented protocols • Clock-based framing

Byte-oriented protocols: the sentinel approach • View each frame as a collection of bytes (characters) • Use special characters SYN, ETX to detect frame start and end • What if special characters appear in a data stream? • Insert data link escape (DLE) characters • Character stuffing • Transmitted from the leftmost bit • Binary Synchronous Communication (BISYNC) by IBM in late 60s

Point-to-Point Protocol (PPP) • A data link protocol used to establish a direction connection between two nodes • Internet dialup access • More recent, RFC 1661, 1994 • Flag: 01111110; Address & Control: default • Protocol: demultiplexing • IP, Link Control Protocol, …, • Checksum: two or four bytes • Link Control Protocol • Set up and terminate the link • Negotiate other parameters such as maximum receive unit

Byte-oriented protocols: the byte counting approach • Use a byte count field to detect the end of a frame. • The corruption of the count field may cause back-to-back frame losses • A similar problem may occur in the sentinel approach. • Corrupted ETX • DDCMP by DECNET

Bit-oriented protocols • View a frame as a collection of bits • 01111110 is the beginning and ending sequence • The sequence is also transmitted when the links are idle • Bit-stuffing for data • Sender: inserts a 0 after every five consecutive 1’s • Receiver: after five consecutive 1’s, • If the next bit is 0, removes it • If the next bit is 1 • If the next bit is 0 (i.e. the last 8 bits are 01111110), then frame ends • Else error; discard frame, wait for next 01111110 to receive • Frames are of variable length, dependent on the data • Mainly because of stuffing • High-level data link control (HDLC) protocol

An exercise • Suppose a receiver receives the following bit sequence • 011010111110101001111111011001111110 • What’s the resulting frame after removing stuffed bits? Indicate any error.

Clock-based Framing • STS-1/OC-1 frame • 51.840Mbps • The slowest SONET link • Synchronous Optical Network (SONET) • A complex protocol • Each frame is 125 us long, 810bytes = 125 us * 51.84Mbps • 9 rows of 90 bytes each • First 3 bytes are overhead • First two bytes of each frame has a special pattern marking the start of a frame • When the special pattern turns up in the right place enough times (every 810B), a receive concludes it’s in sync.

Synchronized timeslots as placeholder • Real frame data may float inside

Overview • Link layer functions • Encoding • Framing • Error detection

Error detection • Error detection code adds redundant information to detect errors • Analogy: sending two copies of the same message • Parity • Checksum • CRC • Error correcting code: more sophisticated code that can correct errors

Two-dimensional parity • Even parity bit • Make an even number of 1s in each row and column • Detect all 1,2,3-bit errors, and most 4-bit errors A sample frame of six bytes

Internet checksum algorithm • Basic idea • Add all the words transmitted and then send the sum. • Receiver does the same computation and compares the sums • IP checksum • Adding 16-bit short integers using 1’s complement arithmetic • Take 1’s complement of the result • Used by lab 1 and lab 2 to detect bit errors

1’s complement • -x is each bit of x inverted • If there is a carry bit, add 1 to the sum • Example: 4-bit integer • -3: 1100 (invert of 0011) • -4: 1011 (invert of 0100) • -3 + -4 = 0111 + 1 = 1000 (invert of 0111 (8))

IP checksum implementation • uint16_t cksum (const void *_data, int len) { const uint8_t *data = _data; uint32_t sum; for (sum = 0;len >= 2; data += 2, len -= 2) sum += data[0] << 8 | data[1]; if (len > 0) sum += data[0] << 8; while (sum > 0xffff) sum = (sum >> 16) + (sum & 0xffff); sum = htons (~sum); return sum ? sum : 0xffff;}

Remarks • Can detect 1 bit error • But not all two-bits • One increases the sum, and one decreases it • Efficient for software implementation • Needs to be done for every packet inside a router!

Cyclic Redundancy Check A branch of finite fields High-level idea: Represent an n+1-bit message with an n degree polynomial M(x) Divide the polynomial by a k-bit divisor C(x) k-bit CRC: remainder after divided by a degree-k divisor polynomial Send Message + CRC that is dividable by C(x)

Polynomial arithmetic modulo 2 • B(x) can be divided by C(x) if B(x) has higher degree • B(x) can be divided once by C(x) if of same degree • Remainder of B(x)/C(x) = B(x) – C(x) • Substraction is done by XOR each pair of matching coefficients

CRC algorithm • Multiply M(x) by x^k. Add k zeros to Message. Call it T(x) • Divide T(x) by C(x) and find the remainder • Send P(x) = T(x) – remainder • Append remainder to T(x) • P(x) dividable by C(x)

An example 8-bit msg 10011010 Divisor (3bit CRC) 1101

How to choose a divisor Complicated Intuition: unlikely to be divided evenly by an error Corrupted msg is P(x) + E(x) If E(x) is single bit, then E(x) = xi If C(x) has the first and last term nonzero, then detects all single bit errors Find C(x) by looking it up in a book

Hardware implementation Very efficient: XOR operations 0 to k-1 registers (k-bit shift registers) If nth (n < k) term is not zero, places an XOR gate x3 + x2 + 1

Summary • Link performance metrics reviewed • Link layer functions • Encoding • Framing • Error detection • Parity, Checksum, CRC • Next lecture • Reliable transmission • Multi-access link

Xiaowei Yang xwy@cs.duke