PUBLIC SWITCHED TELEPHONE NETWORKS.pdf

7/30/2019 PUBLIC SWITCHED TELEPHONE NETWORKS.pdf

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PUBLIC SWITCHED TELEPHONE NETWORKS

PSTN

Synchronous Transfer Mode (STM)

Time-Division-Multiplexing (TDM)

Circuit switching -

RoutingRouting: Connection Oriented

Networking Key word

Asynchronous Transfer Mode (ATM)

Statistical Multiplexing (SM)

Packets Switching

Routing:Routing: Connection/Connectionless

Oriented

Time Division Multiplexing

SCHEDULER

T1 T2 Tm

BROADBAND BUS

Multiplexing with scheduling

Assume that we have m communication terminals, T1, T2, .., Tm

sharing a transmission line, how do we schedule the sharing of

communication bandwidth?

Assume that the bandwidth is shared by the terminals

transmitting at different times.

We also assume that a scheduling mechanism is available

so that the transmissions are conflict free, namely, that no

two terminals attempt to transmit at the same time.

We call this scheduled or arbitrated access communication.

In the absence of an arbitration mechanism, two

communication terminals may transmit at the same time,often resulting in unintelligible transmissions.

Two basic approaches to multiplexing:

1. The first approach assumes a common time reference among the

terminals. We call this t ime reference a frame reference.

The communication bandwidth assigned for each terminal is

termed a circuit. This mode of multiplexing is commonly knownas the Synchronous Transfer Mode (STM).

2. The second approach assumes no frame reference among the

terminals, hence the nameAsynchronous Transfer Mode (ATM).

This mode allows more flexible sharing of bandwidth by avoiding

rigid bandwidth assignments.

Bandwidth is seized on demand, and the information transmitted

(together with a proper label) upon a successful seizure is termed

a packet.

The Asynchronous Transfer Mode

The definition of a frame depends on the bit-rates of the terminals

multiplexed on the transmission link.

The choice of frame structure is difficult since we have little

knowledge of the traffic mix.

An alternative approach abandons the concept of a frame reference

altogether. Instead of choosing a basic terminal bit-rate as in

TDM, ATM achieves more flexible bandwidth sharing

allowing the terminals to seize bandwidth when a sufficient number

of bits are generated.

Without a frame reference, these bits have no implicit ownership,

unlike STM for which each slot is assigned an owner.

Hence a key feature of ATM is that information from each

terminal must be labeled.


2/13


There are many forms of asynchronous multiplexing:

First, we may have fixed length blocks of information from each

terminal.

These blocks are termed cells in ATM terminology.

A cell is labeled block of transmitted information, and usually has

a small information payload (typically from 32 bytes to 128 bytes).

We shall also refer to them as short fixed length packets.


Cell (or Short fixed length packets):

Each cell or packet has a fixed size oflbits. The channel is slotted

into fixed intervals of duration l/C, each transporting a cell.

The terminals are asynchronous in the sense that they have no

common time reference other than the common slot reference.

A label for each time slot must be provided by the terminal which

transmits in that time slot.


The label identifies the terminal generating the bits delivered in the

time slot. A label is included in the header part of a packet. The

header may serve other functions; such as classifying the

information payload (type and priority), and possible error check

sums for protecting the header from transmission error.

t

lBITS SLOTS

INFO

HEADER

PACKET

Multiplexing of Fixed Length Packets


There are two major factors in determining the proper packet size:

First, headers use up part of the communication capacity of the link. This

overhead is inversely proportional to the packet size l, consequently favoring

long packet.

Second, a packetization delay is needed for the terminal to collect the l bits for a

packet. The delay between signal generation and reception is given by , t = l/b

plus the delay taken for the signal to travel in the network.

For some applications, excessive delay results in perceivable degradation of the

quality of communication.

Consequently, minimizing packetization delay requires choosing short packets.

A compromise has to be chosen between two opposing factors.


Variable Length Packets:

Instead of short fixed length packets, it is often convenient

(particularly for data communications) to use long (say 128 bytes

or more) variable length packets.

Besides the label for ownership, the packet header should also

contain the information for packet length to mark the end of the

packet, as well as a flag to mark the beginning of the packet.

t

lBITS SLOTS

INFO

HEADER

PACKET

Multiplexing of Fixed Length Packets


3/13

Local Exchange Carriers (LECs)

LECs provide local telephone service, usually within the boundaries of ametropolitan area, state, or province.

LECs also provide short-haul, long distance service, Centrex, certain enhancedservices such as voice mail, and various data services.

BOCS (Bell Operating Companies), originally were wholly owned by AT&T,dominated the ILECs landscape.

Local Access and Transport Area (LATA)

Effective January 1, 1984, those 22 BOCs were spun off from AT&T as a result ofthe Modified Final Judgement (MFJ).

BOCs were reorganized into seven Regional BOCS (RBOCS).BOCs were limited to providing basic voice and data services within defined

geographical areas, known as Local Access and Transport Areas (LATAs).

Are some 170 areas defined by the MFJ Collectively span all BOC territories In general, each Boc territory comprises several LATAs

PSTN PSTN Continue

InterExchange Carriers (IXCs or IECs)

IXCs are responsible for long-haul, long-distance connectionsacross LATA boundaries.

IXC networks are connected to the LECs through a Point ofPresence (POP) which typically is in the form of a tandem

switch.

A POP is a location where IXC interfaces BOC for exchangeaccess to IXC services.

The IXC POP is connected to the LEC access tandem switchvia dedicated trunks leased from the LEC. Alternatively, the

IXC may collocate network termination equipment in the LEC

office, assuming that space is available and that secure

physical separation can be established and maintained.

IXCs provide inter-lata services.

Basic Architecture of a PSTN

Central

end

office

Remote

Terminal

(RT)

Central

Tandem

office

LEC Domain

POP

Tandem

Switch

Tandem

Switch

Tandem

Switch

Access (Local) Network

Feeder

Network

Distribution

Network

Regional Network Long-distance Network

IXC Domain

Switch

POPCustomer

PBX

Direct Access Switched Access

Switch

POP

LEC

End

Office

Customers

Switch

POPLEC

End

Office

LEC

Access

Tandem

Customers

Customer has large enough

volume of traffic accessing

the POP or requiring egressfrom it to pay for the direct

connect facility, bypassing

the LEC switching network.

Customer traffic to/from POP doesnt justifydirect connect.

The IXC purchase access/egress facilities

from the LEC which uses its switched network

to deliver/receive that traffic.

IXC Access Types

Office Park

CAP Fiber Ring

Switch

CAP

ATT POP

Sprint POP

MCI POP

IXC domainEnd user access to an IXC via a

CAP, bypassing the LECAchieving Connectivity

Full Mesh Shared Medium


4/13

Role of Switching

Connectivity, network resource sharing, customer coordination

Sharing Transmission Bandwidth

Dedicated

Line

Time Shared

SynchronousTDM

Time Shared

Packet, Burst

Circuit Switching

Circuit refers to the capability of transmitting one telephone conversation

along one link.

To set up a call, a set of circuits has to be connected, Joining the two telephone

sets. By modifying the connections, the operators can switch the circuits.

Circuit switching occurs at the beginning of a new telephone call. Operators

were later replaced by mechanical switches and, eventually, by electronic

switches.

An electronic interface in the switch converts the analog signal traveling on the

link from the telephone set to the switch into a digital signal, called a bit

stream. The same interface converts the digital signal that travels between the

switches into an analog signal before sending it from the switch to the

telephone.

The switches use a dedicated data communication network Common channel

signaling (CCS) to exchange control information among themselves. ThusCCS separates the functions of call control from the transfer of voice.

Circuit Switching Continue

In current telephone networks, the bit streams in the trunks (linesconnecting switches) and access links (lines connecting subscriber

telephones the switch) are organized in the digital signal (DS) hierarchy.

The DS-1 signal carries 24 DS-0 channels, but its rate is more than 24times 64 kb/s. The additional bits are used to accommodate DS-0

channels with rates that deviate from the nominal 64 because the signals

are generated using clocks that are not perfectly synchronized.

Since the 1980s the transmission links of the telephone network havebeen changing to the SONET or Synchronous Optical Network,

standard.

In circuit switching, the route and bandwidth allocated to the streamremain constant over the lifetime of the stream.

CCiirrccuuiitt SSwwiittcchhiinngg CCoonnttiinnuuee

TThhee ccaappaacciittyy ooffeeaacchh cchhaannnneell iiss ddiivviiddeedd iinnttoo aa nnuummbbeerr ooff ffiixxeedd--rraattee llooggiiccaallcchhaannnneellss,, ccaalllleedd cciirrccuuiittss..TThhee ddiivviissiioonn iiss uussuuaallllyy aaccccoommpplliisshheedd bbyy TTDDMM..

CCiirrccuuiitt sswwiittcchhiinngg iinnvvoollvveess tthhrreeee pphhaasseess::

((11)) TThhee ssoouurrccee mmaakkeess aa ccoonnnneeccttiioonn oorrccaallll rreeqquueessttttoo tthhee nneettwwoorrkk,, tthhee nneettwwoorrkkaassssiiggnnss aa rroouuttee aanndd oonnee iiddllee cciirrccuuiitt ffrroomm eeaacchh lliinnkkaalloonngg tthhee rroouuttee,, aanndd tthhee

ccaallll iiss tthheenn ssaaiidd ttoo bbee aaddmmiitttteedd ((iiff tthhee nneettwwoorrkk iiss uunnaabbllee ttoo mmaakkee tthhiiss

aassssiiggnnmmeenntt,, tthhee ccaallll iiss rreejjeecctteedd)).. TThhiiss pphhaassee iiss ccaalllleedd ccoonnnneeccttiioonn sseettuupp..

((22)) DDaattaa ttrraannssffeerr nnooww ooccccuurrss--tthhee dduurraattiioonn ooff tthhee ttrraannssffeerr iiss ccaalllleedd tthhee ccaallllhhoollddiinngg ttiimmee..

((33)) WWhheenn tthhee ttrraannssffeerr iiss ccoommpplleettee,, tthhee rroouuttee aanndd tthhee cciirrccuuiittss aarree ddeeaallllooccaatteedd..TThhaatt pphhaassee iiss ccaalllleedd ccoonnnneeccttiioonn tteeaarrddoowwnn..

Rate in Mb/s

Meium Signal No. of Voice

Circuits

North America Europe

T-1 paired

Cable

DS-1 24 1.5 2.0

T-1C paired

cable

DS-1C 48 3.1

T-2 paired

cable

DS-2 96 6.3 8.4

T-3 coax, radio,

fiber

DS-3 672 45.0 32.0

Coax,

waveguide,

radio, fiber

DS-4 4032 274.0

Digital Signal Hierarchy

Note that the bit rate of a DS-1 signal is greater than 24

times the rate of voice signal (64 Kb/s) because of the

additional framing bit required.


5/13

Circuits / Time Slots

TTDDMM iiss iiddeeaall ffoorrccoonnssttaanntt bbiitt rraattee ttrraaffffiicc.. TThhee ccaappaacciittyy oofftthhee oouuttggooiinngg cchhaannnneell iiss ddiivviiddeedd iinnttoo NN llooggiiccaall cchhaannnneellss.. TTiimmee oonn tthhee oouuttggooiinngg cchhaannnneell iiss ddiivviiddeedd iinnttoo ffiixxeedd--lleennggtthh iinntteerrvvaallss ccaalllleedd ffrraammeess.. FFrraammeess aarree ddeelliimmiitteedd bbyy aa ssppeecciiaall bbiitt sseeqquueennccee ccaalllleedd aa ffrraammiinngg ppaatttteerrnn.. TTiimmee iinn eeaacchh ffrraammee iiss ffuurrtthheerr ssuubbddiivviiddeedd iinnttooNN ffiixxeedd--lleennggtthh iinntteerrvvaallss ccaalllleedd

sslloottss//cciirrccuuiittss..

EEaacchh ffrraammee ccoonnssiissttss ooffaa sseeqquueennccee ooffsslloottss:: sslloott 11,, sslloott 22,,....,, sslloottNN..((AA sslloott iiss uussuuaallllyy11 bbiitt oorr11 bbyyttee wwiiddee))..

AA llooggiiccaall cchhaannnneell ooccccuuppiieess eevveerryy NNtthh sslloott.. TThheerree aarree tthhuussNNllooggiiccaall cchhaannnneellss.. TThheeffiirrsstt llooggiiccaall cchhaannnneell ooccccuuppiieess sslloottss 11,, NN ++ 11,, 22NN ++ 11,,....;; tthhee sseeccoonndd ooccccuuppiieess sslloottss 22,,

NN++22,, 22NN++22,,......;; aanndd ssoo oonn..

Time Division Multiplexing

...

... ...

Channel 1

Channel 2

ChannelN

1 12 2N N

Frame 1 Frame 2

Synchronous Transfer Mode

PBX

Workstation

Router

STM

Multiplexer

STM Multiplexing is also known as Time Division Multiplexing (TDM)

13 23 12

The T1 Frame (or the OSI term, PDU) consists of 24 8-bits slots.The TDM multiplexer operates as follows:

The data bits in each incoming channe1 are read into a separate FIFO (first in,first out) buffer.

The multiplexer reads this buffer in sequence for an amount of time equal tothe corresponding slot time: buffer 1 is read into slot 1, buffer 2 is read into slot

2, etc.

If there are not enough bits in a buffer, the corresponding slot remains partiallyempty.

The bit stream of the outgoing channel is easily demultiplexed: thedemultiplexer detects the framing pattern from which it determines the begi-

nning of each frame, and then each slot.

TDM Continues

...

Channel 1

Channel 2

ChannelN

1 N 21

Statistical Multiplexing (SM)

Most effective in the case of bursty input data.

As in TDM, the data bits in each incoming channel are read into separate FIFOs.

The multiplexer reads each buffer in turn until the buffer empties.

The data read in one turn is called a data packet.

Asynchronous Transfer

Mode

Workstation

PBX

Router

ATM

Multiplexer

C

B

A

Z

Y

YZ Y Z Z Z

SM Continues

In TDM each FIFO is read for a fixed amount of time-one slot-andso each incoming channel is allocated a fixed fraction of theoutgoing channel capacity, independent of the data rate on thatchannel.

By contrast, in SM, the capacity allocated to each incomingchannel varies with time, depending on the instantaneous datarate: the higher the rate, the larger the capacity allocated to it at

that time.

The size of packets read from each FIFO can vary across channelsand over time within each channel.

The demultiplexer cannot sort the packets belonging to differentchannels merely from their positions within a frame.


6/13

SM Continues

Additional bits, which delimit each packet and identify the correspondingincoming channel or source, must be added to each packet.

The resulting overhead is significantly larger than under TDM.Multiplexer and demultiplexer implementations are more difficult;Multiplexer must now add the packet delimiter and channel or source

identifier.

Demultiplexer must locate and decode those bit patterns.These increases in complexity and overhead must be balanced against high

utilization in the face of bursty data to determine whether SM or TDM is moreefficient.

DATA COMMUNICATIONSDATA COMMUNICATIONS

Data BBiinnaarryy CCooddeess

BBeettwweeeenn mmaacchhiinneess,, iinnffoorrmmaattiioonn iiss eexxcchhaannggeedd bbyy bbiinnaarryy ddiiggiittss ((bbiittss))..TTwwoo sseettss aarree iinn ccoommmmoonn uussee ttooddaayy::

AASSCCIIII:: tthhee AAmmeerriiccaann SSttaannddaarrdd CCooddee ffoorr IInnffoorrmmaattiioonn IInntteerrcchhaannggee

eemmppllooyyss aa sseeqquueennccee ooffsseevveenn bbiittss.. SSiinnccee eeaacchh bbiitt mmaayy bbee 00 oorr 11,, AASSCCIIII

ccoonnttaaiinnss 112288 uunniiqquuee ppaatttteerrnnss..

EEBBCCDDIICC::tthhee EExxtteennddeedd BBiinnaarryy CCooddeedd DDeecciimmaall IInntteerrcchhaannggee CCooddee

eemmppllooyyss aa sseeqquueennccee ooffeeiigghhtt bbiittss.. IItt ccoonnttaaiinnss 225566 uunniiqquuee ppaatttteerrnnss..

TThheerree aarree ttwwoo bbaassiicc mmeetthhooddss ooff ddaattaa ttrraannssmmiissssiioonn AAssyynncchhrroonnoouuss aannddSSyynncchhrroonnoouuss..

AAssyynncchhrroonnoouuss ((CChhaarraacctteerr FFrraammeedd)) TTrraannssmmiissssiioonn;;

CChhaarraacctteerrss aarree ggeenneerraatteedd aanndd ttrraannssmmiitttteedd ssiinnggllyy,, oonnee aafftteerr tthhee ootthheerr..IInn ssoommee tteerrmmiinnaallss,, tthhee cchhaarraacctteerrss aarree ccoolllleecctteedd uunnttiill aa ccoommpplleettee lliinnee ooff

tteexxtt iiss ccrreeaatteedd,, oorr tthhee rreettuurrnn kkeeyy iiss pprreesssseedd,, ccaauussiinngg tthhee lliinnee ttoo bbee sseenntt aass aa

bbuurrsstt ooffccoonnttiinnuuoouuss cchhaarraacctteerrss..

Data Continues

WWhheetthheerrsseenntt oonnee--bbyy--oonnee aass tthheeyy aarree ggeenneerraatteedd,, oorrsseenntt lliinnee--bbyy--lliinnee aass eeaacchh lliinneeiiss ccoommpplleetteedd,, eeaacchh cchhaarraacctteerr iiss ffrraammeedd bbyyaassttaarrtt bbiitt ((00))aanndd aassttoopp bbiitt ((11))

SSyynncchhrroonnoouuss ((MMeessssaaggee FFrraammeedd )) TTrraannssmmiissssiioonn::

Such transmission is message framed and overcome the inefficiencies ofasynchronous, start-stop transmission for high speed data transmission.

Rather than surrounding each character with start and stop bits, arelatively large set data is framed, or blocked with one or more

synchronization bits or bit patterns used to synchronize the receiving

terminal on the rate of transmission of the data.

The start sequence is called the header it contains synchronizing,address, and control information. The stop sequence is called the trailer

it contains error checking and terminating information.The entire data entity is called a Frame

Stop Bit (1) Star t B it (0)

Character

Framed characters sent as they are created -- a data

stream typical of keyboard input to a terminal or

communications controller.

Framed characters that are concatenated and sent when aFramed characters that are concatenated and sent when a

string is completedstring is completed ---- a datastream typical of a terminala datastream typical of a terminal

sending keyboard input linesending keyboard input line--byby--line to a communicationsline to a communications

controllercontroller

Trailer Header

CharacterFrame

Data Block

Asynchrono

usTransmissionFormat

In asynchronous transmission,

each character is framed by one

start bit and one or two stop bits.

Characters are assembledCharacters are assembled

into a datablock that isinto a datablock that is

framed by a header and aframed by a header and a

trailer to produce a frame.trailer to produce a frame.

The frame is sent when aThe frame is sent when a

command is received fromcommand is received from

the controlling unit in thethe controlling unit in the

communication system.communication system.

Synchronous Transmission Format

Sender Receiver

Message Message

Datastream that includes redundant bits and the

result of the senders calculations

Sender adds redundant bits and

performs calculations to assist the

receiver in error detection

Receiver checks redundant bits and

repeats calculations looking for

agreement with senders results

Because each character is assigned a unique code, i t is extremely

important to be sent without error. For instance, the ASCII code for p

is 11100001110000. An error in bit # 1produces 1110001110001 which is the

code for q.

Error detection is a cooperative activity between the sender and the

receiver in which a sender adds information to the character or frame

to assist the receiver in determining whether an error has occurred in

transmission or reception.

Error Control/DetectionError Control/Detection


7/13

Sender performs calculation...

MK

Gn+1= integer + Fn

Receiver performs same calculation...

MK

Gn+1= integer + Fn

If Fn = Fn transmission is without error

If Fn Fn transmission is without error

Sender adds

Frame Check

Sequence

(Fn) to frame

Receiver

re-calculates

Fn

Cyclic Redundancy Check

MK MKMKFn

Gn+1Gn+1

Generating

Function

Generating

Function

Error CorrectionError Correction

Once detected,an error must be corrected. Two basic approaches to

error correction:

1. Automatic-Repeat-Request (ARQ):

Requires the transmitter to re-send the portions of the exchange in

which errors have been detected. ARQ techniques include:

Stop-and-Wait: The sender sends a frame and waits for

acknowledgement from the receiver. This technique is slow.

Go-back-n:

2. Forward Error Correction (FEC): FEC techniques employ special

codes that allow the receiver to detect and correct a limited number of

errors without referring to the transmitter. This convenience is bought

at the expense of adding more bits (more overhead)

DTE

DTE

EIA232 DCE

EIA232 DSU/CSU

Analog (Voice

Grade) Line

Data Circuit

Terminating

EquipmentDigital Signals

MODEM

Data Terminal

Equipment

Digital

Line

The data equivalent of Customer Premise Equipment (CPE) in the

voice world, Data Terminal Equipment (DTE) comprises the computer

transmit and receive equipment; are digital devices that send or receivedata messages.

Internally, their signals are simple, unipolar pulses; externally, they

may use one the more sophisticated digital signaling schemes.

Data Communication

Data Circuit Terminating Equipment (DCE): is the equipment that interfaces the DTEto the network; maps the incoming bits into signals appropriate for the channel, and at

the receiving end, maps the signals back to bits.

DCEs includes mmooddeemmss, digital service units ((DDSSUUss)),, and channel service units((CCSSUUss)).

If the transmission channel is an analog line (voice-grade), the DCE is called amodem. When sending, DCE convert the ddiiggiittaall ssiiggnnaall received by the DTE to

aannaalloogg ssiiggnnaallss to match the bandwidth of the channel.

If the connections are digital connections, the DCE consists of two parts:DDSSUU-- receives uunniippoollaarrddiiggiittaall ssiiggnnaallss from the DTE and converts them to bbiippoollaarr

ssiiggnnaallss.

CCSSUU: provides loopback (for testing), limited diagnostic capabilities. Whensending, it converts bipolar signals to AMI.

Data Communication Continues

EEIIAA223322 iinntteerrffaaccee

A DET is connected to a DCE by a cable that conforms to EIA232 standard.EIA232 describes a multi-wire cable that terminates in 25-pin connectors.The cable supports asynchronous or synchronous operation at speed up to

19.2 kb/s. At 19.2 kb/s, the cable length is limited to 50 feet.

The EIA232 circuits linking DTE and DCE carry signals that initiate,maintain, and terminate communication between the two.

HHiigghheerr SSppeeeedd IInntteerrccoonnnneeccttiioonnssEEIIAA444499:: It permits operation up to 2 Mb/s at distances up to 4000 feet.

EEnntteerrpprriissee SSyysstteemmss CCoonnnneeccttiioonn ((EESSCCOONN))::an optical fiber connection operating up to 40 kilometers at 17 Mb/s.

FFiibbeerr CChhaannnneell SSttaannddaarrdd ((FFCCSS)):: Operates up to 10 kilometers at speeds up

to 800 Mb/s. FCS includes error control and switching.

ProtocolsProtocols

Data Link Control (DLC) Protocol

A set of rules that governs the exchange of messages over a data link.

DLC protocols are divided into two classes:

Asynchronous Operation: Start-Stop DLC protocol

synchronous Operation:

Bit-oriented DLC protocol (e.g., SDLC): Introduced in 1972, SDLC

was modified and standardized by ITU-T and ISO as:

HDLC (High Level Data Link Control Protocol)

LAP-B (Link Access-Procedure Balanced), for X.25 Standard

LAP-D ((Link Access-Procedure Channel), for ISDN-D Channel

LAP-F ((Link Access-Procedure Frame Relay), a version of LAP-D

used in Frame Relay applications.

Different in the detailed meaning of specific control field bits, all of

these protocols share a common structure. In the order that they are

transmitted, they consist of the following fields: FlagFlag, AddressAddress,

ControControl, TextText, Frame Check SequenceFrame Check Sequence, and FlagFlag.


8/13

Start

Bit

Line

Idle

State

0 11 0 0 0 0 1

Timing Mark

CHARACTER

ASCII a

1

Stop

Bit

Line

Idle

State

Timebetween characters

10 1 0 0 0 10

Start

Bit

Stop

Bit

1

CHARACTER

ASCII b

Line

Idle

State

Timing Mark

Transmission Format for StartTransmission Format for Start-- stop (Asynchronous)stop (Asynchronous)

Signaling. In idle state, the line is maintained at the 1Signaling. In idle state, the line is maintained at the 1

level. The start bit (0) reduces the level to zerolevel. The start bit (0) reduces the level to zero

signaling the commencement of activity.signaling the commencement of activity.

F

L

A

G

Address

F

L

A

GControl F

C

S

TEXT

usually 1024 bits

(not Supervisory Frames)

Header Trailer

SDLC FRAME

8bits

24 8 816N x 8

0111111001111110

0 FNS NR

Information Format

1 PMode NR

Supervisory Format

0

NO TEXT

NR Receive Sequence Number

Number (in sequence 000

through 110) of frame

expected. 111

acknowledges sequence of

seven frames.

NS Send Sequence Number

Number (in sequence 000

through 110) of this

frame.

Mode 00 = Ready to Receive

10 = Not ready to Receive

01 = Reject

P = 0 = not polled

1 = poll

F = 0 = more frames to come.

Information transfer is not

complete.

1 = last Frame

SDLC Frame Format

PACKET SWITCHING

Packet Switching

The data stream originating at the source is divided into packets of fixed orvariable size.

The time interval between consecutive packets may vary, depending on theburstiness of the stream.

As the bits in a packet arrive at a switch or router; they are read into abuffer when the entire packet is stored, the switch routes the packet over

one of its outgoing links.

The packet remains queued in its buffer until the outgoing link becomesidle. This store-and-forward technique thereby introduces a randomqueuing delay at each link;

The delay depends on the other traffic sharing the same link. Packets fromdifferent sources sharing the same link are statistically multiplexed.

Packet Switching Continues

In datagrampacket networks, each packet within a stream is independently routed.A routing table stored in the router (switch) specifies the outgoing link for each

destination. The table may be static, or it may be periodically updated.

Each packet must contain bits denoting the address of the source and destination.In virtual circuitpacket networks, a fixed route is selected before any data is transmittedin a call setup phase similar to circuit-switched networks.

However; there is no notion of a fixed-rate circuit or logical channel. All packetsbelonging to the same data stream follow this fixed route, called a virtual circuit.

Packets must now contain a virtual circuit identifier; this bit string is usually shorter thanthe source and destination address identifiers needed for datagrams. However; the call

setup phase takes time and creates a delay not present in datagram packet networks.

The routing decision

Connectionless (datagraConnectionless (datagram) Connection Oriented (virtual circuit)

Connection-Oriented vs Connectionless

Transport

Could changeMaintainedMaintainedPacket Sequence

Share PainShare PainBusyOverload

SharedSharedGuaranteedBandwidth

VariableVariableConstantDelay

NoYesYesConnection State

Shared

Resource

Guaranteed

Resource

Connectionless

Connection Oriented

Circuits and Virtual

Circuits


9/13

Connection Oriented Packet Transport

Connection Request

Resource Check

Route Selection

Destination Acceptance

Connection begins

Connectionless Transport

Lower Level Protocol (IP)

Send and Pray

Upper Level Protocol

Guaranteed delivery

Relay

Techniques

Direct

Connection

Store &

forward

Hold &

forward

Hold &

forward

Hold &

forward

Media

Copper,

wireless

Copper,

wireless

Copper,

wireless,

optical

Copper,

wireless,

optical

Copper,

wireless,

optical

Sizeof

PDU No such

thing

Variable,

large to

small

Variable,

large to

small

Variable,

large to

small

Fixed, very

small

Delay

Very Fast Slow Fast Faster Very Fast

CircuitSwitching

MessageSwitching

PacketSwitching

Frame

Relay

(Switching)

Cell Relay(Switching)

Switching Technologies

Fast Relay

Frame Relay

(Variable size

PDUs--frames)

Cell Relay

(Fixed size PDUs-

-cells)

PVC

(LAPD)

SVC

(Q.931)

802.6 Based

(For SMDS)

ATM Based

(For B-ISDN)

PVC SVC

(Q.2931)

Types of relay systems

User User

X.25

X.75 (NNI)

X.25

= Packet switches

Typical X.25 Topology

X.25 is not a packet switching specification. Its a packet netwX.25 is not a packet switching specification. Its a packet networkork

interface specification. X.25 says nothing about operations withinterface specification. X.25 says nothing about operations withinin

the network.the network.

It Provides for an interface between an end-user device (DTE) and a

network (DCE). Its formal title is Interface between DTE and DCE

for terminals operating in the packet node on public data networks

In X.25, the DCE is the agent for the packet network to the DTE.

X.25 ContinueX.25 Continue

X.25 encompasses the lower three layers of the OSI modelX.25 encompasses the lower three layers of the OSI model

X.25X.25--3 layer (network layer)3 layer (network layer)Packets are created at the network layer that Establishes, manage,

and teardown the connections between the user and the network.

X.25X.25--2 layer (data link layer)2 layer (data link layer)

The packet is encapsulated within the Link Access Procedure, Balanced

(LAPB) protocol as the information field. The LAPB protocol is a sub-

set of HDLC (High Level Data Link Control).

X.25X.25--1 layer (physical layer)1 layer (physical layer)

The physical layer is the physical interface between the DTE and the

DCE.


10/13

X.25 Continue

X.25 uses logical channel numbers (LCNs) to identify the DTE connections to thenetwork. An LCN is really nothing more than a virtual circuit identifier (VCI).

Octets #1 and Octet #2 of the packet header provide a 12-bit identifier. If all-zerospossibility is excluded, as many as 4095 logical channels (i.e., user sessions) can be

assigned to a physical channel.

The LCN serves as an identifier (a label) for each user's packets that are transmittedthrough the physical circuit to and from the network.Typically, the virtual circuit is identified with two different LCNs-one for the user at the

local side of the network and one for the user at the remote side of the network.

X.25 provides two mechanisms to establish and maintain communications between theuser devices and the network (and ATM has borrowed these concepts): Permanent

Virtual Circuit (PVC) and Switched Virtual Circuit (SVC).

X.25 Continue

PPVVCCssmmaayy ssuuppppoorrtt llaarrggee uusseerrss.. AAllll ppaacckkeettss ttrraavveell tthhee ssaammee ppaatthh bbeettwweeeenn ttwwoo ccoommppuutteerrss;;wwhhiicchh ppaatthh iiss eessttaabblliisshheedd bbyy rroouuttiinngg iinnssttrruuccttiioonnss pprrooggrraammmmeedd iinn tthhee iinnvvoollvveedd nnooddeess..

TThhee cciirrccuuiittss iinnvvoollvveedd iinn tthhee rroouuttee aarree ddeeffiinneedd oonn aa ppeerrmmaanneennttbbaassiiss,, uunnttiill ssuucchh ttiimmee aasstthheeyy aarree ppeerrmmaanneennttllyy rreeddeeffiinneedd,, ppeerrhhaappss aass tthhee sseerrvviiccee

AAlltteerrnnaattiivveellyy,, tthhee nneettwwoorrkkmmaayy sseelleecctt tthhee mmoosstt aavvaaiillaabbllee aanndd aapppprroopprriiaattee ppaatthh oonn aa ccaallll--bbyy--ccaallll bbaassiiss uussiinngg SSwwiittcchheedd VViirrttuuaall CCiirrccuuiittss ((SSVVCCss));;

AAggaaiinn,, aallll ppaacckkeettss iinn aa ggiivveenn sseessssiioonn ttrraavveell tthhee ssaammee ppaatthh..SSVVCCss ddeemmaanndd aa ggrreeaatteerr lleevveell ooffnneettwwoorrkkiinntteelllliiggeennccee tthhaatt aaddddss ttoo ttoottaall nneettwwoorrkkccoosstt;; tthhiiss

ttrraannssllaatteess iinnttoo hhiigghheerr ccoosstt ttoo tthhee eenndd--uusseerr oorrggaanniizzaattiioonn..

TThhee eessttaabblliisshhmmeenntt ooffaa SSVVCC aallssoo iinnvvoollvveess ssoommee lleevveell ooffddeellaayy ssiinnccee tthhee nneettwwoorrkknnooddeessmmuusstt eexxaammiinnee mmuullttiippllee ppaatthhss iinn oorrddeerr ttoo mmaakkee aa pprrooppeerr sseelleeccttiioonn..

Transport

Packet

X.25-3

LAPB

X.25-2

X.21

X.25-1

DTE

LAPB

X.21

Data Link

Physical

Network

Packet Header

Packet

Data

LAPB

Header

LAPB

Trailer

Data

DCE

Users DataUser Stack

USER-NETWORK INTERFACE

X.25PACKET NETWORK

USERS INFORMATION

I.e. message data and/or headers from upper layers

Users Data

Segment

F

L

A

G

A

d

d

r

e

s

s

C

o

n

t

r

o

l

FCS

F

L

A

G

Packet

Headers

Users Data

Segment 1024

bits

1 D QLogical Grp #

Logical Channel Number

0 P(S) M P(R)

Users Data

Segment

Users Data

Segment

Users Data

Segment

PacketHeader Trailer

HDLC FRAME

X.25 Packet and Frame Format

0

COMPUTER NETWORKS

The RS-232-C standard for the serial linespecifies the transfer of one 8-bit character at atime, separated by time intervals. The speedand distance of the serial line are limited.

RS-232-C (1969)

2.4 38 Kbps

01101011_11011010_


11/13

The Synchronous Data Link Control and relatedThe Synchronous Data Link Control and related

standards transmit long packets of bits. The header (H)standards transmit long packets of bits. The header (H)

contains the preamble that starts the receiver clock,contains the preamble that starts the receiver clock,

which is kept in phase by the selfwhich is kept in phase by the self-- synchronizingsynchronizing

encoding of the bits. The receiver uses the cyclicencoding of the bits. The receiver uses the cyclic

redundancy check (CRC) bits to verify that the packets isredundancy check (CRC) bits to verify that the packets is

correctly received.correctly received.

A B

C

D

E

StoreStore--andand--forward transmissions proceed by sending the packetforward transmissions proceed by sending the packet

successively along links from the source to the destination. Thesuccessively along links from the source to the destination. The

packet header specifies the source and destination addresses (Apacket header specifies the source and destination addresses (A

and E, for example) of the packet. When it receives a packet, aand E, for example) of the packet. When it receives a packet, a

computer checks a routing table to find out on which link itcomputer checks a routing table to find out on which link it

should next send the packet.should next send the packet.

Ethernet. In this network, computers are attached to aEthernet. In this network, computers are attached to acommon coaxial cable. The computers read every transmittedcommon coaxial cable. The computers read every transmitted

packet and discard those not addressed to them.packet and discard those not addressed to them.

B

C

D

E

AA B

C DE

Token ring. The computers share a ring. Access is regulatedToken ring. The computers share a ring. Access is regulated

by a tokenby a token-- passing protocol.passing protocol.

4 or 16 Mbps

A B

C DE

Fiber Distributed Data Interface (FDDI). A tokenFiber Distributed Data Interface (FDDI). A token--passingpassing

protocol is used to share the ring. The computers time theirprotocol is used to share the ring. The computers time their

holding of the token. This network guarantees that everyholding of the token. This network guarantees that every

computer gets to transmit within an agreedcomputer gets to transmit within an agreed--on time.on time.

100 Mbps

155-622 Mbps

A B

CD

E

Asynchronous Transfer Mode (ATM) network. The networkAsynchronous Transfer Mode (ATM) network. The network

transports information in 53transports information in 53--byte cells. Total throughput ofbyte cells. Total throughput of

this network is much larger than that of FDDI or of a 100this network is much larger than that of FDDI or of a 100--MbpsMbps

Ethernet.Ethernet.


12/13

LAYERING APPROACHLAYERING APPROACH RAM

VRAM

DISK

CPU

CACHE

Display

NIC

Keyboard,

mouse, etc.

Computer

CPU

RAM

NIC

User

System

Message TransfersThe left panel gives a simple architecture of a host computer and its

connection to the network. The right panel shows the four copies

that may be involved across the CPU bus to run an application,

reducing the host throughput.

OSI Hierarchy

Physical

SONET, T1, T3

Link

Ethernet, FDDI

Circuit, ATM, FR

switches

Network Routing, Call control

IP internetworking

Physical

Transport

Network

Link

Application

Presentation

Session

1

4

3

2

7

6

5

OSI Hierarchy

Transport

Error and congestion

control

TCP, UDP

Session, Presentation,

Application

Data, voice encodings

Authentication

web/http, ftp, telnet

Physical

Transport

Network

Link

Application

Presentation

Session

1

4

3

2

7

6

5

Data Transfer Over Frame-based

Networks

File

TCP

IP

Frame

(Ethernet,

FR, PPP)

Data Transfer Over Cell-based

Networks

File

TCP

IP

Adaptation

ATM Cells


13/13

Internet Protocol Architecture

RTPRTP

LANsLANs PPPPPPATMATM FRFR

TCPTCP UDPUDP OSPFOSPF

BGPBGP

SNMPSNMPDNSDNSTELNETTELNETFTPFTP

SMTPSMTP

HTTPHTTPPingPing

ICMP

IP

RIPRIP

10/100BaseT10/100BaseT Dedicated B/W:

DSx, SONET, ...

Dedicated B/W:

DSx, SONET, ...Circuit-Switched B/W:

POTS, SDS, ISDN, ...

Circuit-Switched B/W:

POTS, SDS, ISDN, ...

CDPDCDPD

WirelessWireless

Why a Synchronous Network

Visibility of each byte at the line rate

Simplification of the multiplexing and

switching process

Simple access to overhead bytes

Stuffing Bits

OH OH

AsynchronousAsynchronous

SynchronousSynchronous

Overhead functions framing, monitoring,

fault location, protection switching,

management communications.

Documents

PUBLIC SWITCHED TELEPHONE NETWORKS.pdf