University of Delaware CPEG 419 1

Data Encoding (Chap. 5)

Transforming original signal just before transmission.

Both analog and digital data can be encoded into either analog or digital signals.

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Digital Transmission Terminology

Data element: bit.Signaling element: encoding of data

element for transmission.Unipolar signaling: signaling

elements have same polarization (all + or all -).

Polar signaling: different polarization for different elements.

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More Terminology

Data rate: rate in bps at which data is transmitted; for data rate of R, bit duration (time to emit 1 bit) is 1/R sec.

Modulation rate = baud rate (rate at which signal levels change).

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Approach 1: NRZ

Switch when a 1 occurs

But how do you know when to sample?Phase-locked-loop (PLL) – measures the difference when transitions occur on the wire and when they occur on a local adjustable oscillator, and then make adjustments accordingly.YOU MUST HAVE TRANSISTIONS TO LOCK ON TO.

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Multilevel Binary

Pros:No DC component.Can be used to force transitions (to help PLL).

Cons:We are using 3 levels and could send ?? bits instead of 1

opposite direction

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Scrambling – to help the PLL

If there are not enough transitions, the PLL may have problems.

So we force extra transitions when there are not enough.

Approach 1 – Use special coding so that long strings of zeros (or ones) don’t occur.

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Scrambling – to help the PLL Approach 2 – Use multilevel binary and set illegal

transitions to long strings of zeros. Here, if an octet of zeros occurs, send a special illegal

sequence. The receiver must be able to interpret this special

sequence.

used in long-distance transmission

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Biphase – Differential Manchester(Self-Clocking)

A transition always occurs in the middle of the period.A zero is represented by a transition occurring at the beginning of the period.A one is represented by no transition at the beginning of the period.

0 0 1 1

always a transition in the middle

Used in CD players and Ethernet

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Methods to Encode Digital Signals

NRZMultilevel binaryManchester

Issues: DC? Self Clocking? How big is the spectrum?

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Sending Digital Signals over Analog (e.g. Modem)

Amplitude shift keying (ASK) (Amplitude Modulation)

Frequency shift keying (FSK) (Frequency modulation)

Phase shift keying (PK) (Phase Modulation)

Modems use phase and amplitude.

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Modulation Techniques

ASK

FSK

PSK

0bit when 0

1bit when2cos tfAts c

0bit when 2cos

1bit when 2cos

1

1

tfA

tfAts

0bit hen w2cos

1bit when2cos

1tfA

tfAts c

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Fixed mistakes on last slide?

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Phase-shift Keying

Quadrature phase-shift keying (QPSK) - send 2 bits.

3 sending when4/72cos

2 sending when4/52cos

1 sending when4/32cos

0 sending when4/2cos

tfA

tfA

tfA

tfA

ts

c

c

c

c

0

90

180

270

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QAM - Quadrature Amplitude Modulation

0

90

180

270

QAM-16(16 levels, how many bits)

constellation diagrams

0

90

180

270QAM - 64

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V32

128 bits: 6 data and 1 parity (error correction)

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How fast is V32?The phone system transmits over the 300 to 3400 Hz band.

So what data rate can we use? How fast can we send symbols?

Use 2400 sample each way - duplex

Definition: a duplex connection means that we can send data in both directions at the same time.A simplex or half-duplex connection only sends data in one direction at a time.

So 2400 * 6 = 14400 bps

What is the baud rate?

V.34 2400 baud - with 12 data bits/symbolV.34 bis 2400 baud – with 14 data bits/symbolThat’s the fastest there is!

To get 56K you send at 4000 baud (if the phone system can handle it)

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Digital Subscriber Lines (DSL)

ADSL – A for asymmetric, faster down load speed than up.

The 56kbps or 33kbps modem speed is due to a filter installed at the end office.

If this filter is removed, then the full spectrum of the twisted pair is available (which is?)

But, if you are far from the office, then you can’t get a very high data rate because…?

The DSL standard goes up to 8 Mbps down and 1 Mbps up.

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DSL

VoicePOTS (stands for ?)(channel 0)

empty

A total of 256 4kHz channels

25kHz (channel 6)

Upstream

downstream

250 parallel channels: Each data channel uses QAM 16 (with 1 parity bit).

What is the maximum data rate?

The quality of each channel is monitored and adjusted. So channels may transmit at different speeds

channel 6+32

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ADSL configuration

ADSL Modem

voiceswitch

DSLAMdigital subscriber line access multiplexer

TO ISP

splittersplittertelephone line

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Cable - History

Starting in the late 1940s, mountain people put a big antenna on a hill top and ran a cable to them and their friends.

1970, 1000s of independent cable systems existed.

1974 Time Inc started HBO

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Early CableCommunity Antenna Television

Head end

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Internet over Cable

Headend

very high bandwidth

(fiber)

fiber

coaxial

A group of homes share the cable. (As oppose to DSL.) The number of homes per group is around 500-2000.

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Cable Spectrum

Upstream5 – 42 MHZ

TVFM

54 MHz88MHz

108MHz

TVHBOshoppingDownstream

data

550MHz 750MHz

Each TV channel gets its own frequency.This is called frequency multiplexing

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Multiplexing

One high bandwidth link is shared by many.But how?

It cost the same to install a high bandwidth connection as it does to install a low bandwidth one. So the approach is to install a few high bandwidth connections and make it seem like many low bandwidth connections.

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Frequency Division Multiplexing

FDMUseful bandwidth of medium exceeds

required bandwidth of channelEach signal is modulated to a different

carrier frequencyCarrier frequencies separated so signals do

not overlap (guard bands)e.g. broadcast radioChannel allocated even if no dataADSL uses FDM

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FDM

Channel 1 Channel 2 Channel n

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Analog Carrier SystemsAT&T (USA)Hierarchy of FDM schemesGroup

12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz

Supergroup 60 channel FDM of 5 group signals on carriers between 420kHz

and 612 kHzMastergroup

10 supergroups

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Back to Cable

Why put downstream data at highend of spectrum? When they made the system they put in amplifiers that

could work at these high frequencies (why?) So the downstream was all set to go. But upstream

amplifiers had to be installed.

Downstream data also uses FDM: 6MHz or 8MHz channel width with QAM-64 (what is the data rate?)

But with overhead you only get 27Mbps. The upstream also uses FDM, but with QPSK (2 bits

per symbol).

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Cable modems

On modem boot-up, the headend tells the modem which channel to use for upstream and downstream transmission.

Many users may share the same channel.

To share the same channel, cable uses time division multiplexing (TDM).

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Time Division Multiplexing (TDM)

channel 1

channel 2 channel n

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TDM

Data rate of medium exceeds data rate of digital signal to be transmitted

Multiple digital signals interleaved in timeMay be at bit level of blocksTime slots preassigned to sources and fixedTime slots allocated even if no dataTime slots do not have to be evenly

distributed amongst sources

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TDM vs FDM

TDM FDM

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TDM

Must provide synchronizing mechanismAdded digit framing

One control bit added to each TDM frameLooks like another channel - “control channel”

Identifiable bit pattern used on control channel e.g. alternating 01010101…unlikely on a data

channel Can compare incoming bit patterns on each

channel with sync pattern

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Back to Cable

Downstream is easy. The headend just transmits packets as it wants. Each packet has a label, so the modem can detect that the packet belongs to it.

Upstream is hard. The upstream channel is shared. TDM is used, but still each TDM slot is shared

by many users. What happens if two users try to send

upstream data on the same channel?

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Cable: Upstream Media Access (link layer) Time is divided into minislot. It is possible to transmit 8 bytes in one

minislot. The modem asks the headend if it can transmit a packet. The headend responds with an acknowledgment and tells the modem

which minislots it can use. Problem: How can the modem sent the request without permission to use

minislots? Solution: on boot-up, the headend tells the modem which minislots it can

use for requesting minislots and the headend never allocates these minislots for upstream data.

Problem: These special control minislots are shared by many users (why?), so what happens if two users make a request at the same time?

Solution: If two users transmit at the same time, the signal cannot be understood by the headend and is ignored. Thus, no acknowledgement is made. The competing modems then wait a random amount of time and try again. It if fails again, then they wait a random amount of time again, but the maximum time they might wait is doubled.

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Cable MAC (media access control )modem headend

modem sends a request for upstream minislots

Headend gets request.Thinks about it.And sends acknowledgement with which minislots to use.

modem sends data

upstream request for bandwidth minislots

data does not overlap with request minislot

time

} minislot

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Cable MAC: ContentionTwo modems send request for bandwidth at the same time. The headend can figure out what

was transmitted and ignores itThe modems wait for the acknowledgement that will never arrive. The amount of time they wait is random.

This time the red modem gets through.

The blue modem tries again, but the green modem also sends a request.

Green gets throughNow the blue doubles the maximum random amount of time it waits before sending another request.

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Cable vs. DSL Cable could give higher bandwidth, but it might give

less. It depends on the number of users. If there are too many users in a group, the cable

operator has to put in a fiber and headend. That cost money, so they try not to do it.

DSL can promise 1Mbps down and 256kbps up, and you will likely get it.

Note that sharing bandwidth is much more efficient than assigning each user a fixed chunk.

The telephone system is very reliable. When was the last time you picked up the phone and there was no dial tone (major earthquake or huge storm). Cable will go down when the power goes out.

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Mobile Phone

FE

GA

D

CB

F

GE

FD

E

C

A