Chapter 2 Physical & Data-Link Layers Professor Rick Han University of Colorado at Boulder [email protected]

Chapter 2Physical & Data-Link Layers

Professor Rick HanUniversity of Colorado at Boulder

[email protected]

Prof. Rick Han, University of Colorado at Boulder

Announcements

• Textbooks: Computer Networks, by Peterson and Davie• CU book store’s order has arrived• Read Chapter 1, start Chapter 2, sections 2.1-

2.8, skip Section 2.9 Network Adaptors

• Larger classroom – maybe…• Lectures will be online a few days after

each class• Follow the Lectures link on class Web site

• Homework #1 will be handed out next class

• Next, Chapter 2


The Layered Network Stack

Physical Layer

Application Layer

Transport Layer (TCP/UDP)

Network Layer (IP)

Data Link Layer

Internet Stack

Physical Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

OSI StackPresentation Layer

Application Layer


Layer 1: The Physical Layer

• Host A encodes the bit into an analog signal. Host B decodes the analog signal into a received bit.

Host A Host B

“1” or “0” over wire


Encoding: Mapping Bits to Analog Waveforms

• “1” -> +5 volts, “0” -> -5 volts on copper wire

• light intensity on optical fiber

• Pulse Amplitude Modulation (PAM) encodes bits as diff. levels. NRZ is special case.

• Manchester encodes bits as diff. transitions

• FSK (Frequency Shift Key) encodes bits as diff. frequencies. 1200 bps modems.

Amplitude1 0 0 1 1

time

+5

-5

+5 -5 -5 +5 +5

0

-5


Sending More Bits Per Second

• Square wave is sum of many different frequency components. [analogy: stereo signal]

• High frequencies get attenuated in wire, thereby smearing the waveform• Reducing time slots causes more “smear” from

previous slot to interfere with current slot: InterSymbol Interference (ISI)

• Can reduce the size of time slot• Widely used• Limitation: smearing of the waveforms and

noise limit the maximum bit rate (Shannon’s capacity theorem)


Sending More Bits Per Second (2)

111

110

101

100

011

010

001

000

+7 V

+5

+3

+1

-1

-3

-5

-7

• Can send more bits per time slot using multi-level D/A converter• 8 discrete levels• “111” -> +7 V, …, “000” ->

-7 V

• …100110… -> +1, +5, …

• 3 bits per symbolSymbols



• Transmit a sequence of symbols: S1=+1, S2=+5, S3, …• Baud rate = Symbols/sec

• If baud rate is 1 symbol/sec, then bit rate = 3 bits/sec• Tripled the bit rate!• Limitations: noise

111

110

101

100

011

010

001

000

+7 V

+5

+3

+1

-1

-3

-5

-7• Datam(t)=Sm(t)*SqWv(t-mT)Where Sm(t) is the level of m’th symbol, T=

duration of each symbol, t=time, SqWv is square wave



• Can send more bits over different frequencies, [analogy: AM/FM]• Modulate Sm(t) up to 3000 Hz, demodulate

down• Datam(t) = Sm(t)*cos(2t) + S’m(t)*cos

(2t) [simplified: sq wave not shown]

• Can send more bits over the same frequency!• Datam(t) = Sm(t)*cos(2t) + S’m(t)*sin

(2t)• This is called Quadrature Amplitude

Modulation (QAM). • If Sm(t), and S’m(t) both have 4 levels, then

this is called 16-QAM. 9600 bps modems use this. Baud?


Physical Layer Effects• Goal: maximize the Signal-to-Noise ratio

(SNR) to minimize the probability of bit error, then pass the bit up to Data-Link Layer

• Unreliability:• due to“Smearing”, Interference, … (Wireless:

shadowing, multi-path, doppler, …)• Apply advanced adaptive filtering and digital

signal processing (DSP) to improve SNR

• Propagation Delay• Speed of light c = 3x108 m/s

• Over copper wires, propagation speed is 2/3 of c

• Satellite links have long prop. delays (~120 ms one-way)

• Interactivity requires <400 ms roundtrip


Layer 2: The Data Link Layer

• Next Problem: How do I send a message from Host A to Host B?

• Data Link Layer, also called Layer 2, ensures that host B can decode a digital message from a stream of bits sent by host A

• Examples: PPP (Point-to-Point Protocol), HDLC, LAPB, LAPD, Frame Relay, …

Host A Host B

1011000…?


The Data Link Layer (cont.)

• A Data Link Layer Protocol implements:• Delimiting/framing of a message• Fragmenting of a long message• Retransmission of a lost message• …

Host A Host B

1011000


Defining a Protocol

• A protocol is an agreement between two parties or endpoints as to how information is to be transmitted

• A protocol implements this agreement via:• A Header• How each endpoint responds to control

info in the header (& external input)

Host A Host B

1011000


Framing

• Where is the start of the data?• Where is the end of the data?

…0110011011011000100110100001110001001…

• Use a special flag to signal the start (and end) of packet data, e.g. flag = “1001101”• “Sentinel” approach – flags act as sentinels

• What if this flag appears in the data?• Character stuffing for byte-oriented

protocols• Bit stuffing for bit-oriented protocols

Receiver’s A/D:


Character/Byte Stuffing• BiSync and PPP (common over modems)• Data is divided into 8-bit bytes• Byte boundaries synchronized btwn sender

and receiver• Define a special start-of-packet N-byte

flag, e.g. let flag be X (one byte flag)• Stuff X in data: replace X with escape

character “E” (DLE in textbook) and X, i.e X -> (E, X)

• Stuff E in data: replace E with (E, E)• At rcvr, first X is start of packet (variants exist)

AKLWXIKKEZLXKDKLEXYBEData:

XAKLWEXIKKEEZLEXKDKLEEEXYBEEXSend:


Bit Stuffing

• HDLC uses this• Similar to byte stuffing, except bit stuffing

is not confined to byte boundaries• HDLC denotes beginning and end of a

packet/frame with “01111110” flag• Since “01111110” may occur anywhere

(across byte boundaries) in data, stuff it:• At sender, after 5 consecutive ones, insert a

“0”• At receiver, “0111110” => stuffing, so destuff,

“01111110” => end of frame, “01111111” => error

…01001000001111110000001110001001…


Byte Counting

• Deduce End-of-Packet with a length field in the header, rather than explicit sentinel flag• …010010000000000100000011100010010…

Start-of-Packet length data

• Length = 2 bytes

• Limitation: the length field could be corrupted by a transmission bit error, due to noise, etc.• Bit corruption in header and start-of-packet

usually catastrophic• Bit corruption can also occur in data

• How do we handle errors caused by bit corruption?


Error Detection

• Simple technique: Send two copies of data in addition to the data, and then do majority logic decoding at the receiver

• Inefficient• Two errors in the same bit => cannot detect

error,• 0100 0110 0110 -> 0110 [Error]

• so each technique has some probability of error

Original data: 0100

Send: 0100 0100 0100 Receiver: 0100 0110 0100Decode: 0100

CorruptBit


Error Detection (2)• General technique: Add redundancy to

the data and use this redundancy to detect bit errors• …01001000000000010011010100000011100010010…

Start-of-Packet length err. det. data

• The error detection field can be computed over the data only, the header only (IP) or both (TCP)

• Can be located in header or at end of packet• The error detection field is also called a Cyclic Redundancy Check (CRC)• Most link-layer protocols use CRC’s for error

detection, e.g. HDLC, PPP• A checksum is a special case of a CRC where

the CRC is computed using binary addition


Error Detection (3)• The Internet checksum:

• Divide your data into 16-bit segments• Add each of the 16-bit segments, using one’s

complement binary addition• One’s complement of the final sum is your 16-

bit IP checksum• Send IP checksum with the IP packet

• An alternative checksum: XOR instead of one’s complement

• Checksum’s are relatively weak compared to CRC’s but are easy to compute• A bit error twice in the same offset position

within the 16-bit segment will go undetected


Probability of Packet Error• Suppose N bits are sent, and the link is

independently corrupting each bit with some probability of bit error pb. What is the probability of packet error?• Prob[packet error] = Prob[at least 1 bit is

corrupt] = 1 – Prob[every bit is

clean]

= 1-(1- pb)N

• If pb =10-6, and N=10000 bits, then Prob[packet error] = 9.95*10-3 ~= 1%

• Optical links have much lower probabilities of bit error: 10–12

• Wireless links have much higher probabilities of bit error: 10-3 in fades [send small packets]


Error Correction• Forward Error Correction (FEC):

• Rather than just detect a bit error, correct a bit error

• Add K bits of redundancy to N bits, to form a (N+K)-bit long packet, or vector

• N dimensions -> N+K dimensions• 2N patterns or vectors mapped into 2N+K

possibilities• Spread out these vectors as far away from

neighbors as possible in (N+K)-dimensional space• N=2, K=3, N+K=5• Receive 01111 - closest to 11111, so decode “11” and correct one bit error

00 0000001 1010110 0101011 11111

Documents

Chapter 2 Physical & Data-Link Layers Professor Rick Han University of Colorado at Boulder [email protected]