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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.
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The Asynchronous Transfer Mode
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.
The Asynchronous Transfer Mode
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 Asynchronous Transfer Mode
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
The Asynchronous Transfer Mode
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.
The Asynchronous Transfer Mode
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
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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
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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.
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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.
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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
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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.
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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
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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.
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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_
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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.
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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
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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.