apc_2_dtms_2014

8/10/2019 apc_2_dtms_2014

1/35

Arhitecturi si Protocoale

de Comunicatii (APC)

Data transmission, multiplexing andswitching (overview)

8/10/2019 apc_2_dtms_2014

2/35

Data transmission

Octavian Catrina 2

Data is transmitted encoded in the parameters of an

electromagnetic wave(signal) that propagates from the

transmitter to the receiver on the transmission medium.

10110

Rx Data

10110

Tx Data Tx Signal Rx Signal

Data encoding and signal transmission:bit stringsignal.

Signal propagation through the transmission medium.

Signal reception and data decoding:signal bit string.

NICNIC

NIC = Network

Interface Card

Transmission

medium

8/10/2019 apc_2_dtms_2014

3/35

Octavian Catrina 3

Data and signals

TxC

0101100

t

0 1 0 1 1 0 0 1

Tb

RxC

0101100

t

Tb

RxCTxC 0 1 0 1 1 0 0 1

Tx

Rx

RxC must be synchronized with TxC

Tx: TransmitterTxC: Transmitter clock

Rx: ReceiverRxC: Receiver Clock

Encoding: data signal Decoding: signal data

A simple data encoding (baseband transmission)

A bit string can be encoded as a digital signal with 2 levels.

Try to imagine a data encoding with more levels, based on the same

principle. Example: 4 signal levels, 2 bits/level.

8/10/2019 apc_2_dtms_2014

4/35

Octavian Catrina 4

Modulation techniques

Try to imagine a data encoding with more levels, based on these techniques.

E.g., 4 phase values, or combined amplitude and phase modulation.

Basebandtransmission

1 0 1 1 0 0

t

(truncated signalbandwidth)

f

f

Carrier

signal

Amplitude

modulation

(ASK = AmplitudeShift Keying)

f

f

2ndCarrier

signal

Frequency

modulation(FSK = Frequency

Shift Keying) f

Phase

modulation(PSK = Phase

Shift Keying)f

t

t

t

t

t

Data

8/10/2019 apc_2_dtms_2014

5/35

8/10/2019 apc_2_dtms_2014

6/35

Octavian Catrina 6

Propagation delay

Time to travel from transmitter to receiver Signals travel with finite speed.

Speed depends on medium (and slightly of signal frequency):

vacuum: c = 3108m/s; conductor cable: c = 2.3108m/s, etc.

Distance d, speed c Td= d/c seconds.

Example: d = 100 Km, c = 210

8

m/sTd= 50 s

01011 01011

Tx RxTx: transmitterRx: receiver

Propagation delay Td= d/c

tt0 t0+Td

Distance d

Total packet transfer duration on a link: T = Tp+ Td = N/Rb + d/c

(the packet transmission duration plus the propagation delay)

8/10/2019 apc_2_dtms_2014

7/35 Octavian Catrina 7

Packet transfer - Example 1

Send 1

Receive 1

DATA 1

ACK 1

Send 2

Receive 2

DATA 2

ACK 2

TransmissionTp = L/R

Propagation

Td = D/V

Distance D (cable length)BA

Transfer

T = Tp+Td

R = Data rate (bits/sec)

L = Packet length (bits)

D = Distance

V = Signal propagation

speed

Packet transfer, point-to-point link. Data and acknowledgement.

Stop and go error control: single unacknowledged data packet.

8/10/2019 apc_2_dtms_2014



Packet transfer, point-to-point link. Data and acknowledgements.

Efficient error control: multiple unacknowledged data packets.

Receive1

Send1-4 DATA 1

Distance D (cable length)BA

DATA 2

DATA 3

Receive2ACK 1

DATA 4

ACK 2

ACK 3

ACK 3

Receive3

Receive4

How much time

it takes to deliver

4 data packets?

Compare with

example 1.

8/10/2019 apc_2_dtms_2014


Synchronization

Bit synchronization When does a bit (cell) start/end in the received signal?

Frame synchronization When does a frame start/end in the received bit string?

Physical layer function: Maintain synchronization of

transmitter and receiver clocks.

Short distance: Share a common clock generator (e.g., within

a computer or between a computer and nearby peripherals). Large distance (networking): Include timing information in the

transmitted signal, using appropriate encoding (next slide).

Typically a Data link layer function: Define a frame format that

allows the receiver to detect start/end of frame.

8/10/2019 apc_2_dtms_2014


Bit synchronization

Principle

Receiver electronics use the transitions in the data signal to

adjust the local clock such that it remains synchronized with

the transmitter clock.

How to make sure there are enough transitions, even for long

sequences of 1s or 0s? Use a suitable data encoding:

Example: Manchester encoding

bit 0 = low-to-high signal transition.

bit 1 = high-to-low signal transition.

bit 0 = code bits 01.

bit 1 = code bits 10.

0 1 0 1 1 0 0 1 1 1 1 0 0 0 0 1

Clock

t

tManchester

encoding

NRZ encoding

(Non-Return

to Zero)

8/10/2019 apc_2_dtms_2014


Attenuation and distortion

Medium changes the signal during propagation Attenuation: reduction of signal strength. Distortion, dispersion: change of signal shape.

Attenuation depends on medium properties, distance and

signal frequency. Signal shape is changed by the different

attenuation and propagation delay of the signal's components.

0101

???Tx Rx

Tx: transmitterRx: receiver

tt0 t0+TdDistance d

Effects of attenuation,distortion, dispersion

Medium bandwidth (analog) Range of signal frequencies that the medium can transmit.

(Signal components with frequency outside medium bandwidth are

practically completely attenuated.)

8/10/2019 apc_2_dtms_2014


Noise and other harmful signals

Our signal is not alone in the medium ... Various "noise" signals overlap with the transmitted signal.

EMI/RFI

01011 ???Tx Rx

TxRx

Crosstalk

EMI/RFI noise Electromagnetic Interference, Radio Frequency Interference.

Electromagnetic waves emitted by power lines, engines, radio

transmitters, etc.

Crosstalk signal Induced by radiation of nearby transmission media.

Reflection signal Caused by discontinuities of the medium.

8/10/2019 apc_2_dtms_2014


Data transmission errors

Decoding errors The receiver can fail

to decode the datacorrectly if the signalshape is too muchaltered or the clocksare not synchronizedwell enough.

TxC

0101100

t

0 1 0 1 1 0 0 1

Tb

RxC

0101100

t

Tb

RxCTxC 0 1 0 1 1 0 0 1

Tx Rx

Rx clock synchronized with Tx clock

Encoding: datasignal Decoding: signaldata

Signal attenuation, distortion,noise

t

0 1 0 1 1 0 0 1

Rx

0 1 1 1 0 0 0 1

t

Tx

Errors !

TxC

RxC

8/10/2019 apc_2_dtms_2014

14/35

8/10/2019 apc_2_dtms_2014


Data networks

Challenges Scalability: Large number of computers, any distance.

Efficiency: Cost effective interconnection.

Solutions Efficient resource sharing techniques: multiplexing and

switching. Wide variety of technologies.

Interconnection devicesforward data on the linkstowards the destination:Switching and routing

Many data streams shareeach data link:Multiplexing/demultiplexing

8/10/2019 apc_2_dtms_2014

16/35

Octavian Catrina 16

...

Multiplexing

TDM: Time DivisionMultiplexing

Each source is given certain time

intervalsduring which it can useall the bandwidth.

Multiple sources send at different

points in timeon the same

frequency bandwidth.

Time

Bandwidth (Hz)

1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6

FDM: Frequency DivisionMultiplexing

Each source is given its own

frequency bandand can use itpermanently.

Multiple sources send on

different frequency bandsat the

same time.

Time1

6

45

3

2

Bandwidth (Hz)

Using the same transmission medium for

multiple simultaneous communications

Other techniques (wireless networks): CDM (Code Division

Multiplexing). SDM (Space Division Multiplexing). Etc.

Multiplexed

link...

8/10/2019 apc_2_dtms_2014

17/35

Octavian Catrina 17

FDMmux

frequency (medium bandwidth)

frequency

(signal bandwidth)

Shift signal from each

source in frequency

domain by modulation.

R bits/sR/3 bits/s

R/3 bits/s

R/3 bits/s

FDMdemux

FDM enables wireless (radio-wave) communications (analog/digital) and

the multiplexing of analog signals (e.g. TV broadcast, wireless/wired).

FDM is also used for digital transmissions, often in conjunction with TDM

or CDM, e.g., for wireless digital communications.

WDM (Wavelength Division Multiplexing) uses different light wavelengths

(light colors) to create multiple channels on optical fiber. WDM is FDM

applied to light waves.

Frequency Division Multiplexing

8/10/2019 apc_2_dtms_2014

18/35

Octavian Catrina 18

Asynchronous (statistical) TDM

On demand bandwidth allocation. Much more efficient for variable

bit-rate, bursty streams. Various solutions.

Synchronous TDM

Fixed bandwidth allocation. Good for constant bit-rate streams.Inefficient for variable bit-rate, bursty streams. Simple, cost effective.

Time Division Multiplexing

Variable slot (or fixed). Variable cycle.R1bits/s

R2bits/s

R3bits/s

R1+R2+R3 R

R bits/s

E.g.: First In

First Served

Header Data

Fixed size slot. Fixed cycle: N slots (3)

One slot for each sourceper cycle: blue, red, green;

empty slot if no data.

R bits/s

R1 R/3bits/s

R2 R/3 bits/s

R3 R/3bits/s

Multiplexer Demultiplexer

8/10/2019 apc_2_dtms_2014

19/35

Octavian Catrina 19

TDM example (historical)

This is a simple example. Actually, we need flexible techniquesable to multiplex a much larger number of data streams or/and

data streams with much higher bit-rates.

We need a digital hierarchy.

E.g., multiplex 4 E1 streams in a 8 Mbps stream, and so on.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 3131 0

E1frame: 125 s; 32 time slots; 8 bits/slot

32 channels 64 Kbps(8bit/125s). Total bit rate: 2.048 Mbps(3264Kbps).

Frame synchronization Signaling channel

E1 multiplex (ITU-T standard)

Originally designed to multiplex 64 Kbpsdigital voice channels.

Also used for WAN data links (2 Mbps

digital channel or a fraction of it).

8/10/2019 apc_2_dtms_2014

20/35

Octavian Catrina 20

TDM: Digital hierarchies (I)

Plesiochronous Digital Hierarchy (PDH) Developed in 1960s-1970s. The digital signals are generated from

independent reference clocks, with (inherent) slight differences. So

they are "almost" synchronous, i.e., plesyochronous. The differences

accumulate and must be compensated by the multiplexing technique.

Disadvantage: Needs complex multi-stage multiplexers/demultiplexers.

A low level stream cannot be easily extracted from a higher level

stream (complete demultiplexing followed by re-multiplexing!).

2Mbps

E3

E0 E1 E2

32Mbps8Mbps64Kbps 2Mbps

E3

E0E1E2

32Mbps 8Mbps 64Kbps

ANSI: North America, etc.

Signal Bit-rate Channels

DS0 64 Kbps 1 DS0

DS1 (T1) 1.54 Mbps 24 DS0

DS2 (T2) 6.3 Mbps 4 DS1 (96 DS0)

DS3 (T3) 44.8 Mbps 7 DS2 (28 DS1)

- - -

ITU-T: Europe, etc.

Signal Bit-rate Channels

E0 64 Kbps 1 E0

E1 2.048 Mbps 32 E0

E2 8.45 Mbps 4 E1 (128 E0)

E3 34 Mbps 4 E2 (16 E1)

E4 140 Mbps 4 E3 (64 E1)

Obsolete

E1/E3, T1/T3 still used.

8/10/2019 apc_2_dtms_2014

21/35

Octavian Catrina 21

TDM: Digital hierarchies (II)

Synchronous Digital Hierarchy Developed at the end of 1980s. Widely deployed in 1990s. The digital

signals are generated from a common and extremely accurate reference

clock (e.g., cesium atomic clock).

SONET: Synchronous Optical Network (ANSI standard).

SDH: Synchronous Digital Hierarchy (ITU-T standard, similar).

Features Lower level streams can be easily added to or dropped from a higher

level stream with a single stage multiplexer/demultiplexer (ADM).

High reliability (automatic path reconfiguration in case of faults) and

comprehensive means to control the network (enable/disable circuits),

and monitor network operation and performance, etc.

SONET (ANSI) Bit-rate SDH (ITU-T)

STS-1, OC-1 51.84 Mbps (50 Mbps) -

STS-3, OC-3 155.52 Mbps (150 Mbps) STM-1

STS-12, OC-12 622.08 Mbps (600 Mbps) STM-4

STS-24, OC-24 1244.16 Mbps (1.25 Gbps) -

STS-48, OC-48 2488.32 Mbps (2.5 Gbps) STM-16

STS-192, OC-1929953.28 Mbps (

10 Gbps)STM-64

ADMSTS-n link

Add/Drop lower level streams

(DS-m, STS-k, k

8/10/2019 apc_2_dtms_2014

22/35

Octavian Catrina 22

Circuit switching

Synchronous TDM

multiplexors

Synchronous TDM

demultiplexors

S1S2

S3

A

B

C

D

E

F

G

H

Circuits

Fixed capacity communication channels. Phases: circuit setup, communication, circuit release.

Circuit switchesSwitch tiny, fixed-size data units between time slots on

synchronous TDM links, using mapping stored at circuit setup.

Guaranteed bandwidth. Low, constant transfer delay.

Ideal for real-time, constant bit-rate traffic (audio, video).

Inefficient for variable bit-rate traffic.

Examples: telephone network, ISDN, SONET/SDH.

8/10/2019 apc_2_dtms_2014

23/35

Octavian Catrina 23

SONET/SDH networks

OC-n

DCC

ADM

ADM

Survivable SONET ring

OC-N

OC-nOC-n

DCC

TM

TM

TM

TM

ADM DCC

ADM

TM

OC-n

OC-n

DS--n

OC-n

TM - Terminal Multiplexer.ADM - Add/Drop Multiplexer.DCS - Digital Cross-Connect.

Hub

OC-n

DS-n

STS-n

(ATM,

IP)

SONET & SDH allow the creation of high speed circuit-switched

networks, that can provide an arbitrary mesh of high-capacity

digital circuits (STS-n, DS-n).

Used to interconnect switches in the core (backbone) of PSTN

and packet switched networks.

Typical topologies: Ring (dual, survivable). With hub (star)

extensions.

8/10/2019 apc_2_dtms_2014

24/35

Octavian Catrina 24

Packet switching

Packets

Encapsulated data units with routing information in header. Packet switches

Switch variable-size packets between asynchronous TDM links

based on information in the header and forwarding tables.

Dynamic bandwidth allocation. Variable transfer delay. More

efficient for variable bit-rate traffic.

8/10/2019 apc_2_dtms_2014

25/35

Octavian Catrina 25

Packet switching: connection-oriented

Virtual circuit (VC)

Logical path set up between network nodes across a packet-switched network. Identified on each link by a VC identifier (VCI).

Phases: VC setup, communication, VC release.

Switching table

Indicates how to forward packetson VCs: maps VCI on each input

link to output link and next VCI.

QoS supportOrdered packet delivery.

Can guarantee QoSbyreserving resources on VC

(bandwidth, delay).

Examples: MPLS, Frame

Relay, ATM, X.25 (obsolete).

Packet switch 1Input Output

Link VCI Link VCI

2 16 3 24

... ... ... ...


Link VCI Link VCI

1 24 2 43

... ... ... ...


Link VCI Link VCI

3 43 2 19

... ... ... ...

Switching tables

VCI data

S2

S1

S3

S4

S5

S6

2 1

31 2

1 2

34

16

24 43

19

8/10/2019 apc_2_dtms_2014

26/35

Octavian Catrina 26

Packet switching: connectionless

Routing table

Gives next hop on the path

to each known destination.

Best effort service

Packet delivery and ordered

delivery not guaranteed.

Can add some QoS support(traffic classes, priorities, and

resources allocated per class).

Datagrams

Standalone packets, forwarded independently of each other,based on source and destination addresses in the header.

Examples: Internet Protocol

(IP), Novell IPX (obsolete).

DA,SA data

R1R3

R4

R6

R2 R5

yxyx yxyx

yx yx

yx

Router R1DA Next hop

y R3, R4

... ...

Routing tables


y R6

... ...


y -

... ...

8/10/2019 apc_2_dtms_2014

27/35

Octavian Catrina 27


Send

Receive

DATA

ACK

Receive

Packet transfer. Store and forward packet switching. Data and acknowledgement.

DATA

ACK

Distance D Distance D Distance D

DATA

ACK

BA

How much time

it takes to deliver a

data packet?

Compare with

example 1.

This example assumes

that the packet queues in

all the switches are empty

(no queuing delay ).

How much is the transferdelay if the packet queues

are not empty?

8/10/2019 apc_2_dtms_2014

28/35

8/10/2019 apc_2_dtms_2014

29/35

Octavian Catrina 29

Example (historical): Frame Relay (I)

Light-weight packet-switching, connection-oriented Layer 2 packet (frame) switching. Successor of X.25.

Developed after 1988 in the framework of ISDN (ITU-T).

Frame Relay virtual circuits (VC) Called Data Link Connections (DLC).

Distinguished by DLC identifiers (DLCI).

DLCI have local (link) or global (network) significance.

Frame Relay

WANPhysical

Data Link

Link bandwidth: up to 45 Mbps.

DLCI=1

DLCI=1DLCI=9

DLCI=2

DLCI=5

DLCI=4

DLCI=3

DLCI=7DLCI=4

Frame Relayswitch

Frames

Layer-2 connection-oriented packet

switching.

8/10/2019 apc_2_dtms_2014

30/35

Octavian Catrina 30

Frame Relay (II)

7

6

9

9

9 8 12

5 6 5

Input OutputPort DLCI Port DLCI

1 9 4 8

1 5 4 6

2 7 3 11


2 6 3 9

Input Output

Port DLCI Port DLCI

1 9 2 9


1 8 4 12

1 6 4 5

Return paths not shown

in the switching tables!

8/10/2019 apc_2_dtms_2014

31/35

Example: ATM

Octavian Catrina 31

PBX

VoiceVideo

/Audio

Data

Cells:

5 octets

header,

48 octets

payload.

ATM: Asynchronous Transfer Mode

First universal digital carrier: voice, video, data.Foundation of Broadband Integrated Services

Digital Network (B-ISDN): first attempt to unify

data, telephone, and video/audio networks.

Widely deployed in the 1990s, being phased out.

Cell switching: convergence of technologies As in circuit switching, ATM is connection-oriented

and uses small, fixed-size data units (known as cells).

However, ATM uses asynchronous TDM and packet

switching on virtual circuits.

Small, fixed-size cell reduces the end-to-end delay

and jitter (required by telephony) and simplifies the

design and implementation of high-speed switches.

Asynchronous TDM improves the efficiency for a

broad range of QoS requirements (see next slide).

8/10/2019 apc_2_dtms_2014

32/35

ATM service categories (ATM Forum)

Octavian Catrina 32

Service category Description AAL

CBR

(Constant Bit Rate)

Fixed data rate, low delay and delay variationspecified in service contract and guaranteed.

Non-compressed real-time video, audio (tele-

conferencing, telephony, video-on-demand).

Type 1

VBR-RT (real-time

Variable Bit Rate)

Variable data rate, low delay and delay variation

specified in service contract and guaranteed.

Compressed real-time video, audio.

Type 2

VBR-NRT

(non real-time VBR)

Similar with VBR-RT, but delay constraints not

guaranteed. Other real-time applications.Type 2

UBR

(Unspecified Bit Rate)

Best effort service. No traffic and QoS

commitment. E-mail, ftp.Type 3/4

ABR

(Available Bit Rate)

Variable data rate specified in service contract,

minimum rate guaranteed, higher rate provided

whenever resources are available. Bursty traffic.

Type 5

8/10/2019 apc_2_dtms_2014

33/35

8/10/2019 apc_2_dtms_2014

34/35

Octavian Catrina 34

i110.4.3.0/24

i1R1

i2 i3i2 i1R2

i2

R4

i1R3 10.5.0.0/16

IP packet forwarding without MPLS

DA=10.5.1.1 DA=10.5.1.1 DA=10.5.1.1

R1: Routing table

Destination Out IF NH

10.4.3.0/24 i2 R2

10.5.0.0/16 i2 R2

R2: Routing table


10.5.0.0/16 i3 R3

10.4.3.0/24 i1 R4

R3: Routing table


10.4.3.0/24 i1 R2

10.5.0.0/16 i2 Direct

DA=10.5.1.1

A routing protocol determines the routes.

Each router on the path to the IP packet's destination makes a

longest match routing table lookup to find the route to be taken by

the packet (destination-based FEC determined at each hop).

Let's add now MPLS and destination-based label switched paths.

8/10/2019 apc_2_dtms_2014

35/35

i1R1 R2

i2 i3

i1R5

i1R4i3

i1 i2

R3i2i1

Packet forwarding using MPLS

Ingress router classifies the packets, determines the appropriate LSP(destination, service class), then labels and forwards them on the LSP.

Internal routers forward the packets along the path according to the label

and switching tables (instead of destination address and routing table).

LSP egress router removes the labels and then forwards the packets.

LSPs are set up based on IP routing tables using label distribution protocols.

Label switching table

In IF In L Out IF Out L

i2 L1 i3 L2

Edge LSRLSR

LSR

LSREdge LSR

MPLS domainLSR = Label Switching Router

Label switching table

In IF In L Out IF Out L

i1 L2 i2 L3

LSP

LSP = Label Switched Path

IP IP L1 IP L2 IP L3 IP

At Edge:Ingress LSRClassifies IP packets& Adds labels (MPLSheader)

At Edge:Egress LSRRemoves labels(MPLS header) &Forwards IP packets

In Core:LSRs forward packets based on labels.

Label switching (aka label swapping)

apc_2_dtms_2014

Documents