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Chapter 2/2 (Physical Layer)

• Public Switched Telephone System (2) • The Mobile Telephone System• Cable Television

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Public Switched Telephone System (2)

• Trunks and Multiplexing

• Switching

• The Mobile Telephone System PLMN

• Cable Television

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Frequency Division Multiplexing

Group hierarchy

1 group = 12 voice channels (48 kHz)

1 supergroup = 5 groups = 60 voice channels (240 kHz)

1 mastergroup = 5 supergoups = 300 voice channels (1200 kHz)

Multiplexed channel

Original bandwidths

The bandwidths raised in frequency

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Wavelength Division Multiplexing

Each channel carries 2.5 Gbps and they multiplex 40 channels going to 200 Channels.Because of slow electric to optical conversion bandwidth is less than 5 GHz or 10 Gbps.Since fiber bandwidth is 25000 GHz there is theoretical room for 5000 channels.

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Time Division Multiplexing (CCITT)

T1 carrier (1.544 Mbps)

• Voice: 7 bits for data 1 for signaling.

• Data: 23 data channels and 24-th channel for signaling.

E1 carrier 32*64 kbps = 2.048 Mbps

• 30 used for voice/data

• 31 and 32 used for signaling

8-th bit every 6-th frame.

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Time Division Multiplexing hierarchy

Multiplexing T1 streams into higher carriers.

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Sonet time Division Multiplexing

First two bytes contain frame synchronization bit pattern.SONET frame: 810 bytes/125 mks = 51.84 Mbps.SPE (Synchronous Payload Envelope – user data)

First row in line overhead points to the first payload byte.

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SONET and SDH* multiplex rates

*SDH – Synchronous Digital Hierarchy

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Circuit Switching

(a) Circuit switching. (b) Packet switching.

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Message Switching

(a) Circuit switching (b) Message switching (c) Packet switching

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Circuit switched vs. packet-switched networks

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The Mobile Telephone System

• First-Generation Mobile Phones: Analog Voice AMPS 800 MHz band

• Second-Generation Mobile Phones: Digital Voice 1900 MHz IS-136, IS-54, IS-95

• 2.5 generation introduces data over voice network: GSM -> GPRS.

• Third-Generation Mobile Phones:Digital Voice and Data: UMTS/W-CDMA, CDMA-2000.

• Fourth-Generation Mobile Phones:WiFi and WiMax

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Advanced Mobile Phone System

(a) Frequencies are not reused in adjacent cells: 832 full duplex channels,

21 for control hardwired into phone. Typical 45 channels/cell.

(b) To add more users, smaller cells can be used.

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TDM Channel Categories

800 MHz + 1900 MHz band 30 KHz channels are divided into four categories:

• Control (base to mobile) to manage the system. 21 channels hardwired into phone ROM.

• Paging (base to mobile) to alert users for calls for them

• Access (bidirectional) for call setup and channel assignment

• Data (bidirectional) for voice, fax, or data. Voice is compressed to 8 kbps for 3 slots per frame or 4 kbps for 6 slots per frame.

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D-AMPS (Digital Advanced Mobile Phone System)||TDMA

(a) D-AMPS channel with three users.

(b) D-AMPS channel with six users.

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GSM: Global System for Mobile Communications

GSM uses 124 frequency channels 200 kHz each of which uses an eight-slot (992 slots) TDM system.

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GSM framing structure

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CDMA concept

1 1 1 1

-1 -1 -1 -1

1 1 1 1

-1 -1 -1 -1

d1o=1

d11=-1Data bits

Chips

Senders

1 1 1 1 1 1

-1 -1

d2o=1d2

1=1

1 1 1 1 1 1

-1 -1

Data bits

Chips

Zi,1

m = di1cm

1

Zi,2

m = di2cm

2

2 2 2 2 2 2

-2 -2

1 1 1 1

-1 -1 -1 -1

1 1 1 1

Channel Zi,*m

d1o=1

d11=-1

d1i = (m Zi,

*mc1

m)/M -1 -1 -1 -1

1 1 1 1 1 1

-1 -1

1 1 1 1 1 1

-1 -1

d2i = (m Zi,

*mc2

m)/M

d2o=1d2

1=1

Chip rate Spreading factor = chip_rate/data_rate.dB = 10 log( spreading rate/data rate )has the same effect as dB (signal/noise).

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Third-Generation Mobile Phones:Digital Voice and Data

Basic services an IMT-2000 network should provide

• High-quality voice transmission• Messaging (e-mail, fax, SMS, chat, etc.)• Multimedia (music, videos, films, TV, etc.)• Internet access (web surfing, w/multimedia.)

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Cable Television

• Community Antenna Television

• Internet over Cable

• Spectrum Allocation

• Cable Modems

• ADSL versus Cable

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An early cable television system

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Phone and Internet over Cable

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TV over Subscriber Loop

The fixed telephone system.

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TV Spectrum Allocation for Internet access

Upstream; QPSK is used due to large noise.Downstream: QAM-64 and QAM-256 is used. For 6 MHz bandwidth it is 36 Mbps.

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Cable Modems

Upstream channels are divided into minislots. Headend assigns upstream minislot and downstream channels (184 byte payloadlength) to the powered-up modem. Typical minislot is 8 bytes. Collision is Slotted Aloha with random backoff doubled each successive collision. IP address is dynamically allocated to modem using DHCP (Dynamic Host Configuration Protocol).

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The Data Link Layer

Chapter 3

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Functions of the Data Link Layer

Error free service provided to the Network Layer• Framing• Error Control: dealing with transmission errors• Flow Control: regulating data flow (through

acknowledgments) so that slow receivers are not swamped by fast senders.

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Relationship between packets and frames

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Services Provided to Network Layer

(a) Virtual communication.(b) Actual communication.

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DLL within a router

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Framing with character count

A character stream:

(a) Without errors: first byte contains the message length (in bytes).

(b) With one error: receiver looses synchronization and cannot recover.

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Framing with ‘esc’ character: byte stuffing (PPP)

(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after stuffing.

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Framing with zero insertion (HDLC)

(a) The original data.

(b) The data as they appear on the line.

(c) The data as they are stored in receiver’s memory after destuffing.

Flag bit pattern: 0 1 1 1 1 1 1 0Zero insertion eliminates flag bit pattern to appear in the output

text

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Error Detection and Correction(using additional bits)

• Error-Correcting Codes (used in voice communication)

• Error-Detecting Codes (used in data communication)Sender receives a positive acknowledgment if data received correctly and negative ack to ask for retransmission.

• What to do if sent frame is completely lost?Solution is to use a timer slightly larger than round-trip delay. If sender doesn’t receive ack it retransmits.

There is a possibility that receiver receives the same frame twice. To avoid that the frames are numbered in increasing order and each frame carries this (sequence) number.

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Hamming DistanceHamming distance d: the number of bits in which two words differ. To mistaken one word with the other at least d bits must be in error.

Hamming distance of the code: the minimum Hamming distancebetween any two code words.

For error detection with d bit error the code must have distance d+1.

S1 S2

d

d + 1

d

For error correction with d bit error the code must have distance 2d+1.

R S1 S2 are two valid code words. R isthe word received. Since not within theset of valid words it is an error. We have no way to say whether R is S1 or S2.Hence error detection.

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Error Correction

S1 S2

d

2d + 1

d

Code with hamming distance 2d + 1 is capable of error correction within d or less erroneous bits.If R is received we say that the sending code word is S2.

R

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Hamming code corrects single bit errors

000

101

011

110

100

010

001

111 m - word bitsr - check bitsn = m + r code bits

Example:m = 1n = 3r = 2

For single bit error correction: each valid word has n bit pattern dedicated to it (obtained by one bit change in a valid word).Therefore

(n+1)*2m <= 2n.

(m + r + 1)*2m <= 2m+r.

m+r+1 <= 2r

m r1 2 complete2 33 34 3 complete7 4

1000 10

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Hamming (11,7) Code

p1 p2 d1 p3 d2 d3 d4 p4 d5 d6 d7

Data word (without parity) 1 0 0 1 0 0 0

p1 0 1 0 1 0 0

p2 0 1 0 1 0 0

p3 1 0 0 1

p4 0 0 0 0

Data word (with even parity) 0 0 1 1 0 0 1 0 0 0 0

p1 p2 d1 p3 d2 d3 d4 p4 d5 d6 d7Parity check Parity

bit

Received word 0 0 1 1 0 0 1 0 0 0 1

p10 1 0 1 0 1 fail 1

p20 1 0 1 0 1 fail 1

p31 0 0 1 pass 0

p40 0 0 1 fail 1

Coding

Decoding: the position of error bit = 8*p4 + 4*p3 + 2*p2 + p1 -> 8 + 2 + 1 = 11.

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Use of a Hamming code to correct burst errors

Words are sent leftmost column first then the next column etc. If the burst error (burst <= 12) occurs it can be recovered by Hamming correction code.

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Cyclic Redundancy Check Error Detecting Codes

Only words (represented as a single number) that are divisible by given

divisor are valid. In modulo arithmetic there is finite number of these

words. They are evenly spaced (equidistant from each other). Say all

number up to 100 divisible by 7 are legal.

Coding: transform message M word into code word T using generator

polynomial G.

Assume we want to send M = 50.

1. Multiply M*10 -> 500 (10 is the order of G)

2. Divide M*10 : G -> 500 : 7 = 71 + 3/7 -> 500 = 71*7 + 3 (remainder)

3. Subtract remainder from M*10 -> T = 500 – 3 = 497 (certainly divisible by 7).

Decoding: T/G -> 497/7 = 71 (good word divisible by 7)

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CRC codes

G(x) is generating polynomial of degree r.

M(x) is a message word of degree m > r.

Coding

1. Append r 0’s to the M(x) -> xr M(x).

2. Divide modulo 2: xr M(x) : G(x) = Q(x) + R(x)/G(x)

3. T(x) = xr M(x) - R(x). (Certainly divisible by G(x))

On the next slide G(x) = x4 + x + 1 = 10011 -> r = 4.

M(x) = x9 + x8 + x6 + x4 + x3 + x + 1 = 1101011011 -> m = 9.

1. x4M(x) = 11010110110000

2. R(x) = x4M(x) : G(x) = 1110 = x3 + x2 + x < G(x)

3. T(x) = x4M(x) + R(x) = 11010110111110 (certainly divisible by G(x)).

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Calculation of the polynomial code checksum.

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Analysis of CRC coding: choice of G(x)

Decoding:Receiver calculates the remainder: (T(x) + E(x))/G(x) = E(x)/G(x).1. Single bit error: E(x) = xi . If G(x) contains 2 or more terms xi will never be divisible by

G(x) => single error is always detected.2. Double error: E(x) = xi + xj = xj(xi-j + 1) -> xk + 1 (0 < k < frame length)3. G(x) = x15 + x14 + 1 detects double errors up to 32768 frame length.….4. Polynomial coding with r check bits will detect all burst errors with the burst error length

<= r.

CRC calculations are performed on a fly by shift registers with feedback that is determined by G(x).