DATA TRANSMISSION IN PSTN

The transmission medium is the physical foundation for all the data
communications. The amount of data carried across the networks crossed
the voice trafc level. The data is growing at a rate of 30 percent per year. It
will take 12 years to double the amount of voice carried on the network at
the current growth rate whereas the data is doubling appro!imately every
"0 days. In the beginning data transmission was organi#ed using telegraph
or tele! networks as they could carry digital signals directly. $ut teletype
machines were slow noisy and consumed large amounts of power. The
speed was limited to 110 bauds. %$aud rate is a measure of the rate at which
binary data are transmitted and received&. $ut data rates for transmission
have been on the rise. 'ith public switched telephone network there is a
possibility of carrying signals at higher speeds. (ublic switched telephone
networks and electronic ()$*+s are designed to carry analog voice signals.
They can be used for data transmission by employing suitable interfaces.
Data Rates in PSTN
Baud rate. The ma!imum rate of signal transitions that can be supported by a
channel is known as baud rate. $aud rate is a close measure of information
throughput or the e,ective information data transfer rate from sender to
receiver. Thus baud rate is one that can be supported in a noiseless channel.
'e know a voice channel in a (-T. is band limited with a nominal
bandwidth of 3.1 k/#. ) ma!imum data rate that a noiseless or ideal voice
channel can support can be obtained from the .y0uist theorem
1 2 2 $ log2 3 bps ...%1.1&
where 1 2 4a!imum data rate %in $aud or bps&
$ 2 $andwidth of the channel
3 2 .umber of discrete levels in the signals.
5or a 3 k/# channel and a binary signal the ma!imum data rate is 6000
bps if the signal level is two.
5or higher data rates we translate information rate into symbols per
second. ) symbol is any element of an electrical signal that can be used to
represent one or more binary data bits. The rate at which symbols are
transmitted is the symbol rate. This rate may be represented as a systems
baud rate. 5ig. 1.1 illustrates the pulse representation of the binary numbers
used to code the samples
In 5ig. 1.1 shown each three digit binary number that speci7es a 0uanti#ed
sample value is called a word. 8g is called guard time between pulses.
Bit rate. In the noisy channel there is an absolute ma!imum limit for the bit
rate. This limit arises because the di,erence between two ad9acent signal
levels become comparable to the noise level when the number of signal level
is increased. 5or noisy channel data rate is calculated by
1b 2 $ log2 %1 : -;.& ...%1.2&
'here 1b 2 1ata rate in noisy channel %in bps&
$ 2 $andwidth of the channel
-;. 2 -ignal to noise ratio.
5or -;. of 30 d$ and 3 k/# $andwidth noisy channel 1b is 30000 bps.
Relation between baud rate (or symbol rate) and bps : The baud rate
and bit rate are related as
1b 2 1 < n ...%11.3&
where n 2 number of bits re0uired to represent signal levels.
In the e!ample considered for baud rate e!planation n is assumed as one.
/ence baud rate is e0ual to bps. 5ig. 1.2 illustrates the relation between
baud and bit rates.
)bove %a& shows the baud rate e0ual to bit rate. 5ig. 11.2 %b& and %c& shows
the baud rate e0ual to one=half and one=fourth of bit rate respectively. It is
proved that up to 2>00 bauds may be transmitted reliably through a (-T.
voice channel. $y increasing the signal levels the e,ective bit rate
increases.
5or low=speed applications the di,erence between baud and bit rate are
insigni7cant.
Thus 300 and 1200 bps modems orginally used with personal computers
were fre0uently referred
to as 300 or 1200 baud modems.
Data Communications Link
In order to communicate from a terminal computer or any e0uipment
the following si! parts have to be put together in proper order.
1. The transmission medium that carries the trafc between source and
destination.
2. 1ata communication e0uipment or data circuit terminating e0uipment
%18?&.
3. 1ata terminal e0uipment %1T?&.
>. 8ommunication protocols and software.
@. Terminal devices.
6. Interface.
below shows the typical arrangement of the communication link for the data
communication. 1ata link refers to the process of connecting or linking two
stations together.
Transmission medium. The transmission medium include communication
channels path links trunks and circuits. The transmission medium may be a
telephone lines coa!ial cable twisted pair 5iber cable radio waves %free
space& microwave link or satellite link.
Terminal devices. These are the end points in a communication link.
Terminal devices are also called as nodes. 5or the two point network the
node points are the primary station and the remote or secondary station. )
primary station is responsible for establishing and maintaining the data link
between it and a secondary station. The terminal devices includes main
frame computer personal computer peripherals such as printers keyboards
5)* machines and data display terminals.
Data terminal equipment (DTF). The terminal devices communication
station A)BT and line control unit %38A& grouped together and named as
1T?. 5ig. 11.> shows typical arrangement of 1T?.
Connection-Oriented and Connectionless Services
Layers can offer two different types of service to the layers above them: connection-oriented and
connectionless. In this section we will look at these two types and examine the differences
between them.
Connection-oriented service is modeled after the telephone system. To talk to someone, you pick
up the phone, dial the number, talk, and then han up. !imilarly, to use a connection-oriented
network service, the service user first establishes a connection, uses the connection, and then
releases the connection. The essential aspect of a connection is that it acts like a tube: the sender
pushes ob"ects #bits$ in at one end, and the receiver takes them out at the other end. In most cases
the order is preserved so that the bits arrive in the order they were sent.
In some cases when a connection is established, the sender, receiver, and subnet conduct a
neotiation about parameters to be used, such as maximum messae si%e, &uality of service
re&uired, and other issues. Typically, one side makes a proposal and the other side can accept it,
re"ect it, or make a counterproposal.
In contrast, connectionless service is modeled after the postal system. 'ach messae #letter$
carries the full destination address, and each one is routed throuh the system independent of all
the others. (ormally, when two messaes are sent to the same destination, the first one sent will
be the first one to arrive. )owever, it is possible that the first one sent can be delayed so that the
second one arrives first.
'ach service can be characteri%ed by a &uality of service. !ome services are reliable in the sense
that they never lose data. *sually, a reliable service is implemented by havin the receiver
acknowlede the receipt of each messae so the sender is sure that it arrived. The
acknowledement process introduces overhead and delays, which are often worth it but are
sometimes undesirable.
+ typical situation in which a reliable connection-oriented service is appropriate is file transfer.
The owner of the file wants to be sure that all the bits arrive correctly and in the same order they
were sent. ,ery few file transfer customers would prefer a service that occasionally scrambles or
loses a few bits, even if it is much faster.
-eliable connection-oriented service has two minor variations: messae se&uences and byte
streams. In the former variant, the messae boundaries are preserved. .hen two /012-byte
messaes are sent, they arrive as two distinct /012-byte messaes, never as one 1023-byte
messae. In the latter, the connection is simply a stream of bytes, with no messae boundaries.
.hen 1023 bytes arrive at the receiver, there is no way to tell if they were sent as one 1023-byte
messae, two /012-byte messaes, or 1023 /-byte messaes. If the paes of a book are sent over
a network to a phototypesetter as separate messaes, it miht be important to preserve the
messae boundaries. 4n the other hand, when a user los into a remote server, a byte stream
from the user5s computer to the server is all that is needed. 6essae boundaries are not relevant.
+s mentioned above, for some applications, the transit delays introduced by acknowledements
are unacceptable. 4ne such application is diiti%ed voice traffic. It is preferable for telephone
users to hear a bit of noise on the line from time to time than to experience a delay waitin for
acknowledements. !imilarly, when transmittin a video conference, havin a few pixels wron
is no problem, but havin the imae "erk alon as the flow stops to correct errors is irritatin.
(ot all applications re&uire connections. 7or example, as electronic mail becomes more
common, electronic "unk is becomin more common too. The electronic "unk-mail sender
probably does not want to o to the trouble of settin up and later tearin down a connection "ust
to send one item. (or is /00 percent reliable delivery essential, especially if it costs more. +ll
that is needed is a way to send a sinle messae that has a hih probability of arrival, but no
uarantee. *nreliable #meanin not acknowleded$ connectionless service is often called
dataram service, in analoy with teleram service, which also does not return an
acknowledement to the sender.
In other situations, the convenience of not havin to establish a connection to send one short
messae is desired, but reliability is essential. The acknowleded dataram service can be
provided for these applications. It is like sendin a reistered letter and re&uestin a return
receipt. .hen the receipt comes back, the sender is absolutely sure that the letter was delivered
to the intended party and not lost alon the way.
!till another service is the re&uest-reply service. In this service the sender transmits a sinle
dataram containin a re&uest8 the reply contains the answer. 7or example, a &uery to the local
library askin where *ihur is spoken falls into this cateory. -e&uest-reply is commonly used
to implement communication in the client-server model: the client issues a re&uest and the server
responds to it. 9elow fi summari%es the types of services discussed above.
The concept of usin unreliable communication may be confusin at first. +fter all, why would
anyone actually prefer unreliable communication to reliable communication: 7irst of all, reliable
communication #in our sense, that is, acknowleded$ may not be available. 7or example,
'thernet does not provide reliable communication. ;ackets can occasionally be damaed in
transit. It is up to hiher protocol levels to deal with this problem. !econd, the delays inherent in
providin a reliable service may be unacceptable, especially in real-time applications such as
multimedia. 7or these reasons, both reliable and unreliable communication coexist.
The OSI Reference Model
The 4!I model #minus the physical medium$ is shown in fi below. This model is based on a
proposal developed by the International !tandards 4rani%ation #I!4$ as a first step toward
international standardi%ation of the protocols used in the various layers #<ay and =immermann,
/>3?$. It was revised in />>@ #<ay, />>@$. The model is called the I!4 4!I #4pen !ystems
Interconnection$ -eference 6odel because it deals with connectin open systems-that is, systems
that are open for communication with other systems. .e will "ust call it the 4!I model for short.
Figure for . The OSI reference model.
The 4!I model has seven layers. The principles that were applied to arrive at the seven layers
can be briefly summari%ed as follows:
/. + layer should be created where a different abstraction is needed.
1. 'ach layer should perform a well-defined function.
?. The function of each layer should be chosen with an eye toward definin internationally
standardi%ed protocols.
2. The layer boundaries should be chosen to minimi%e the information flow across the
interfaces.
@. The number of layers should be lare enouh that distinct functions need not be thrown
toether in the same layer out of necessity and small enouh that the architecture does not
become unwieldy.
9elow we will discuss each layer of the model in turn, startin at the bottom layer. (ote that the
4!I model itself is not a network architecture because it does not specify the exact services and
protocols to be used in each layer. It "ust tells what each layer should do. )owever, I!4 has also
produced standards for all the layers, althouh these are not part of the reference model itself.
'ach one has been published as a separate international standard.
The Physical Layer
The physical layer is concerned with transmittin raw bits over a communication channel. The
desin issues have to do with makin sure that when one side sends a / bit, it is received by the
other side as a / bit, not as a 0 bit. Typical &uestions here are how many volts should be used to
represent a / and how many for a 0, how many nanoseconds a bit lasts, whether transmission
may proceed simultaneously in both directions, how the initial connection is established and how
it is torn down when both sides are finished, and how many pins the network connector has and
what each pin is used for. The desin issues here larely deal with mechanical, electrical, and
timin interfaces, and the physical transmission medium, which lies below the physical layer.
The ata Lin! Layer
The main task of the data link layer is to transform a raw transmission facility into a line that
appears free of undetected transmission errors to the network layer. It accomplishes this task by
havin the sender break up the input data into data frames #typically a few hundred or a few
thousand bytes$ and transmit the frames se&uentially. If the service is reliable, the receiver
confirms correct receipt of each frame by sendin back an acknowledement frame.
+nother issue that arises in the data link layer #and most of the hiher layers as well$ is how to
keep a fast transmitter from drownin a slow receiver in data. !ome traffic reulation mechanism
is often needed to let the transmitter know how much buffer space the receiver has at the
moment. 7re&uently, this flow reulation and the error handlin are interated.
9roadcast networks have an additional issue in the data link layer: how to control access to the
shared channel. + special sublayer of the data link layer, the medium access control sublayer,
deals with this problem.
The "et#or! Layer
The network layer controls the operation of the subnet. + key desin issue is determinin how
packets are routed from source to destination. -outes can be based on static tables that are 55wired
into55 the network and rarely chaned. They can also be determined at the start of each
conversation, for example, a terminal session #e.., a loin to a remote machine$. 7inally, they
can be hihly dynamic, bein determined anew for each packet, to reflect the current network
load.
If too many packets are present in the subnet at the same time, they will et in one another5s way,
formin bottlenecks. The control of such conestion also belons to the network layer. 6ore
enerally, the &uality of service provided #delay, transit time, "itter, etc.$ is also a network layer
issue.
.hen a packet has to travel from one network to another to et to its destination, many problems
can arise. The addressin used by the second network may be different from the first one. The
second one may not accept the packet at all because it is too lare. The protocols may differ, and
so on. It is up to the network layer to overcome all these problems to allow heteroeneous
networks to be interconnected.
In broadcast networks, the routin problem is simple, so the network layer is often thin or even
nonexistent.
The Trans$ort Layer
The basic function of the transport layer is to accept data from above, split it up into smaller
units if need be, pass these to the network layer, and ensure that the pieces all arrive correctly at
the other end. 7urthermore, all this must be done efficiently and in a way that isolates the upper
layers from the inevitable chanes in the hardware technoloy.
The transport layer also determines what type of service to provide to the session layer, and,
ultimately, to the users of the network. The most popular type of transport connection is an error-
free point-to-point channel that delivers messaes or bytes in the order in which they were sent.
)owever, other possible kinds of transport service are the transportin of isolated messaes, with
no uarantee about the order of delivery, and the broadcastin of messaes to multiple
destinations. The type of service is determined when the connection is established. #+s an aside,
an error-free channel is impossible to achieve8 what people really mean by this term is that the
error rate is low enouh to inore in practice.$
The transport layer is a true end-to-end layer, all the way from the source to the destination. In
other words, a proram on the source machine carries on a conversation with a similar proram
on the destination machine, usin the messae headers and control messaes. In the lower layers,
the protocols are between each machine and its immediate neihbors, and not between the
ultimate source and destination machines, which may be separated by many routers. The
difference between layers / throuh ?, which are chained, and layers 2 throuh A, which are end-
to-end, is illustrated in 7i. /-10.
The Session Layer
The session layer allows users on different machines to establish sessions between them.
!essions offer various services, includin dialo control #keepin track of whose turn it is to
transmit$, token manaement #preventin two parties from attemptin the same critical operation
at the same time$, and synchroni%ation #checkpointin lon transmissions to allow them to
continue from where they were after a crash$.
The Presentation Layer
*nlike lower layers, which are mostly concerned with movin bits around, the presentation layer
is concerned with the syntax and semantics of the information transmitted. In order to make it
possible for computers with different data representations to communicate, the data structures to
be exchaned can be defined in an abstract way, alon with a standard encodin to be used 55on
the wire.55 The presentation layer manaes these abstract data structures and allows hiher-level
data structures #e.., bankin records$, to be defined and exchaned.
The %$$lication Layer
The application layer contains a variety of protocols that are commonly needed by users. 4ne
widely-used application protocol is )TT; #)yperText Transfer ;rotocol$, which is the basis for
the .orld .ide .eb. .hen a browser wants a .eb pae, it sends the name of the pae it wants
to the server usin )TT;. The server then sends the pae back. 4ther application protocols are
used for file transfer, electronic mail, and network news.
The TCP&IP Reference Model
Let us now turn from the 4!I reference model to the reference model used in the randparent of
all wide area computer networks, the +-;+('T, and its successor, the worldwide Internet.
+lthouh we will ive a brief history of the +-;+('T later, it is useful to mention a few key
aspects of it now. The +-;+('T was a research network sponsored by the <o< #*.!.
<epartment of <efense$. It eventually connected hundreds of universities and overnment
installations, usin leased telephone lines. .hen satellite and radio networks were added later,
the existin protocols had trouble interworkin with them, so a new reference architecture was
needed. Thus, the ability to connect multiple networks in a seamless way was one of the ma"or
desin oals from the very beinnin. This architecture later became known as the TC;BI;
-eference 6odel, after its two primary protocols. It was first defined in #Cerf and Cahn, />A2$.
+ later perspective is iven in #Leiner et al., />3@$. The desin philosophy behind the model is
discussed in #Clark, />33$.
Diven the <o<5s worry that some of its precious hosts, routers, and internetwork ateways miht
et blown to pieces at a moment5s notice, another ma"or oal was that the network be able to
survive loss of subnet hardware, with existin conversations not bein broken off. In other
words, <o< wanted connections to remain intact as lon as the source and destination machines
were functionin, even if some of the machines or transmission lines in between were suddenly
put out of operation. 7urthermore, a flexible architecture was needed since applications with
diverent re&uirements were envisioned, ranin from transferrin files to real-time speech
transmission.
The Internet Layer
+ll these re&uirements led to the choice of a packet-switchin network based on a connectionless
internetwork layer. This layer, called the internet layer, is the linchpin that holds the whole
architecture toether. Its "ob is to permit hosts to in"ect packets into any network and have them
travel independently to the destination #potentially on a different network$. They may even arrive
in a different order than they were sent, in which case it is the "ob of hiher layers to rearrane
them, if in-order delivery is desired. (ote that 55internet55 is used here in a eneric sense, even
thouh this layer is present in the Internet.
The analoy here is with the #snail$ mail system. + person can drop a se&uence of international
letters into a mail box in one country, and with a little luck, most of them will be delivered to the
correct address in the destination country. ;robably the letters will travel throuh one or more
international mail ateways alon the way, but this is transparent to the users. 7urthermore, that
each country #i.e., each network$ has its own stamps, preferred envelope si%es, and delivery rules
is hidden from the users.
The internet layer defines an official packet format and protocol called I; #Internet ;rotocol$.
The "ob of the internet layer is to deliver I; packets where they are supposed to o. ;acket
routin is clearly the ma"or issue here, as is avoidin conestion. 7or these reasons, it is
reasonable to say that the TC;BI; internet layer is similar in functionality to the 4!I network
layer. 7iure /-1/ shows this correspondence.
Figure '-('. The TCP&IP reference model.
The Trans$ort Layer
The layer above the internet layer in the TC;BI; model is now usually called the transport layer.
It is desined to allow peer entities on the source and destination hosts to carry on a
conversation, "ust as in the 4!I transport layer. Two end-to-end transport protocols have been
defined here. The first one, TC; #Transmission Control ;rotocol$, is a reliable connection-
oriented protocol that allows a byte stream oriinatin on one machine to be delivered without
error on any other machine in the internet. It framents the incomin byte stream into discrete
messaes and passes each one on to the internet layer. +t the destination, the receivin TC;
process reassembles the received messaes into the output stream. TC; also handles flow control
to make sure a fast sender cannot swamp a slow receiver with more messaes than it can handle.
The second protocol in this layer, *<; #*ser <ataram ;rotocol$, is an unreliable,
connectionless protocol for applications that do not want TC;5s se&uencin or flow control and
wish to provide their own. It is also widely used for one-shot, client-server-type re&uest-reply
&ueries and applications in which prompt delivery is more important than accurate delivery, such
as transmittin speech or video. The relation of I;, TC;, and *<; is shown in 7i. /-11. !ince
the model was developed, I; has been implemented on many other networks.
Figure '-((. Protocols and net#or!s in the TCP&IP model initially.
The %$$lication Layer
The TC;BI; model does not have session or presentation layers. (o need for them was perceived,
so they were not included. 'xperience with the 4!I model has proven this view correct: they are
of little use to most applications.
4n top of the transport layer is the application layer. It contains all the hiher-level protocols.
The early ones included virtual terminal #T'L('T$, file transfer #7T;$, and electronic mail
#!6T;$, as shown in 7i. /-11. The virtual terminal protocol allows a user on one machine to
lo onto a distant machine and work there. The file transfer protocol provides a way to move
data efficiently from one machine to another. 'lectronic mail was oriinally "ust a kind of file
transfer, but later a speciali%ed protocol #!6T;$ was developed for it. 6any other protocols have
been added to these over the years: the <omain (ame !ystem #<(!$ for mappin host names
onto their network addresses, ((T;, the protocol for movin *!'('T news articles around, and
)TT;, the protocol for fetchin paes on the .orld .ide .eb, and many others.
The )ost-to-"et#or! Layer
9elow the internet layer is a reat void. The TC;BI; reference model does not really say much
about what happens here, except to point out that the host has to connect to the network usin
some protocol so it can send I; packets to it. This protocol is not defined and varies from host to
host and network to network. 9ooks and papers about the TC;BI; model rarely discuss it.
%synchronous Transfer Mode
Eet another, and far more important, connection-oriented network is +T6 #+synchronous
Transfer 6ode$. The reason for the somewhat strane name is that in the telephone system, most
transmission is synchronous #closely tied to a clock$, and +T6 is not.
+T6 was desined in the early />>0s and launched amid truly incredible hype #Dinsbur, />>F8
Doralski, />>@8 Ibe, />>A8 Cim et al., />>28 and !tallins, 1000$. +T6 was oin to solve all the
world5s networkin and telecommunications problems by merin voice, data, cable television,
telex, teleraph, carrier pieon, tin cans connected by strins, tom-toms, smoke sinals, and
everythin else into a sinle interated system that could do everythin for everyone. It did not
happen. In lare part, the problems were similar to those we described earlier concernin 4!I,
that is, bad timin, technoloy, implementation, and politics. )avin "ust beaten back the
telephone companies in round /, many in the Internet community saw +T6 as Internet versus the
Telcos: the !e&uel. 9ut it really was not, and this time around even diehard dataram fanatics
were aware that the Internet5s &uality of service left a lot to be desired. To make a lon story
short, +T6 was much more successful than 4!I, and it is now widely used deep within the
telephone system, often for movin I; packets. 9ecause it is now mostly used by carriers for
internal transport, users are often unaware of its existence, but it is definitely alive and well.
%TM *irtual Circuits
!ince +T6 networks are connection-oriented, sendin data re&uires first sendin a packet to set
up the connection. +s the setup packet wends its way throuh the subnet, all the routers on the
path make an entry in their internal tables notin the existence of the connection and reservin
whatever resources are needed for it. Connections are often called virtual circuits, in analoy
with the physical circuits used within the telephone system. 6ost +T6 networks also support
permanent virtual circuits, which are permanent connections between two #distant$ hosts. They
are similar to leased lines in the telephone world. 'ach connection, temporary or permanent, has
a uni&ue connection identifier. + virtual circuit is illustrated in 7i. /-?0.
Figure '-+,. % virtual circuit.
4nce a connection has been established, either side can bein transmittin data. The basic idea
behind +T6 is to transmit all information in small, fixed-si%e packets called cells. The cells are
@? bytes lon, of which @ bytes are header and 23 bytes are payload, as shown in 7i. /-?/. ;art
of the header is the connection identifier, so the sendin and receivin hosts and all the
intermediate routers can tell which cells belon to which connections. This information allows
each router to know how to route each incomin cell. Cell routin is done in hardware, at hih
speed. In fact, the main arument for havin fixed-si%e cells is that it is easy to build hardware
routers to handle short, fixed-lenth cells. ,ariable-lenth I; packets have to be routed by
software, which is a slower process. +nother plus of +T6 is that the hardware can be set up to
copy one incomin cell to multiple output lines, a property that is re&uired for handlin a
television proram that is bein broadcast to many receivers. 7inally, small cells do not block
any line for very lon, which makes uaranteein &uality of service easier.
Figure '-+'. %n %TM cell.
+ll cells follow the same route to the destination. Cell delivery is not uaranteed, but their order
is. If cells / and 1 are sent in that order, then if both arrive, they will arrive in that order, never
first 1 then /. 9ut either or both of them can be lost alon the way. It is up to hiher protocol
levels to recover from lost cells. (ote that althouh this uarantee is not perfect, it is better than
what the Internet provides. There packets can not only be lost, but delivered out of order as well.
+T6, in contrast, uarantees never to deliver cells out of order.
+T6 networks are orani%ed like traditional .+(s, with lines and switches #routers$. The most
common speeds for +T6 networks are /@@ 6bps and F11 6bps, althouh hiher speeds are also
supported. The /@@-6bps speed was chosen because this is about what is needed to transmit hih
definition television. The exact choice of /@@.@1 6bps was made for compatibility with +TGT5s
!4('T transmission system.The F11 6bps speed was chosen so that four /@@-6bps channels
could be sent over it.
The %TM Reference Model
+T6 has its own reference model, different from the 4!I model and also different from the
TC;BI; model. This model is shown in 7i. /-?1. It consists of three layers, the physical, +T6,
and +T6 adaptation layers, plus whatever users want to put on top of that.
Figure '-+(. The %TM reference model.
The physical layer deals with the physical medium: voltaes, bit timin, and various other issues.
+T6 does not prescribe a particular set of rules but instead says that +T6 cells can be sent on a
wire or fiber by themselves, but they can also be packaed inside the payload of other carrier
systems. In other words, +T6 has been desined to be independent of the transmission medium.
The +T6 layer deals with cells and cell transport. It defines the layout of a cell and tells what the
header fields mean. It also deals with establishment and release of virtual circuits. Conestion
control is also located here.
9ecause most applications do not want to work directly with cells #althouh some may$, a layer
above the +T6 layer has been defined to allow users to send packets larer than a cell. The +T6
interface sements these packets, transmits the cells individually, and reassembles them at the
other end. This layer is the ++L #+T6 +daptation Layer$.
*nlike the earlier two-dimensional reference models, the +T6 model is defined as bein three-
dimensional, as shown in 7i. /-?1. The user plane deals with data transport, flow control, error
correction, and other user functions. In contrast, the control plane is concerned with connection
manaement. The layer and plane manaement functions relate to resource manaement and
interlayer coordination.
The physical and ++L layers are each divided into two sublayers, one at the bottom that does the
work and a converence sublayer on top that provides the proper interface to the layer above it.
The functions of the layers and sublayers are iven in 7i. /-??.
Figure '-++. The %TM layers and su-layers. and their functions.
The ;6< #;hysical 6edium <ependent$ sublayer interfaces to the actual cable. It moves the bits
on and off and handles the bit timin. 7or different carriers and cables, this layer will be
different.
The other sublayer of the physical layer is the TC #Transmission Converence$ sublayer. .hen
cells are transmitted, the TC layer sends them as a strin of bits to the ;6< layer. <oin this is
easy. +t the other end, the TC sublayer ets a pure incomin bit stream from the ;6< sublayer.
Its "ob is to convert this bit stream into a cell stream for the +T6 layer. It handles all the issues
related to tellin where cells bein and end in the bit stream. In the +T6 model, this
functionality is in the physical layer. In the 4!I model and in pretty much all other networks, the
"ob of framin, that is, turnin a raw bit stream into a se&uence of frames or cells, is the data link
layer5s task.
+s we mentioned earlier, the +T6 layer manaes cells, includin their eneration and transport.
6ost of the interestin aspects of +T6 are located here. It is a mixture of the 4!I data link and
network layers8 it is not split into sublayers.
The ++L layer is split into a !+- #!ementation +nd -eassembly$ sublayer and a C!
#Converence !ublayer$. The lower sublayer breaks up packets into cells on the transmission
side and puts them back toether aain at the destination. The upper sublayer makes it possible to
have +T6 systems offer different kinds of services to different applications #e.., file transfer
and video on demand have different re&uirements concernin error handlin, timin, etc.$.
+s it is probably mostly downhill for +T6 from now on, we will not discuss it further in this
book. (evertheless, since it has a substantial installed base, it will probably be around for at least
a few more years. 7or more information about +T6, see #<obrowski and Drise, 100/8 and
Dadecki and )eckart, />>A$.
Pac!et S#itching
.ith messae switchin, there is no limit at all on block si%e, which means that routers #in a
modern system$ must have disks to buffer lon blocks. It also means that a sinle block can tie
up a router-router line for minutes, renderin messae switchin useless for interactive traffic. To
et around these problems, packet switchin was invented, as described in Chap. /. ;acket-
switchin networks place a tiht upper limit on block si%e, allowin packets to be buffered in
router main memory instead of on disk. 9y makin sure that no user can monopoli%e any
transmission line very lon #milliseconds$, packet-switchin networks are well suited for
handlin interactive traffic. + further advantae of packet switchin over messae switchin is
shown in 7i. 1-?>#b$ and #c$: the first packet of a multipacket messae can be forwarded before
the second one has fully arrived, reducin delay and improvin throuhput. 7or these reasons,
computer networks are usually packet switched, occasionally circuit switched, but never messae
switched.
Circuit switchin and packet switchin differ in many respects. To start with, circuit switchin
re&uires that a circuit be set up end to end before communication beins. ;acket switchin does
not re&uire any advance setup. The first packet can "ust be sent as soon as it is available.
The result of the connection setup with circuit switchin is the reservation of bandwidth all the
way from the sender to the receiver. +ll packets follow this path. +mon other properties, havin
all packets follow the same path means that they cannot arrive out of order. .ith packet
switchin there is no path, so different packets can follow different paths, dependin on network
conditions at the time they are sent. They may arrive out of order.
;acket switchin is more fault tolerant than circuit switchin. In fact, that is why it was invented.
If a switch oes down, all of the circuits usin it are terminated and no more traffic can be sent
on any of them. .ith packet switchin, packets can be routed around dead switches.
!ettin up a path in advance also opens up the possibility of reservin bandwidth in advance. If
bandwidth is reserved, then when a packet arrives, it can be sent out immediately over the
reserved bandwidth. .ith packet switchin, no bandwidth is reserved, so packets may have to
wait their turn to be forwarded.
)avin bandwidth reserved in advance means that no conestion can occur when a packet shows
up #unless more packets show up than expected$. 4n the other hand, when an attempt is made to
establish a circuit, the attempt can fail due to conestion. Thus, conestion can occur at different
times with circuit switchin #at setup time$ and packet switchin #when packets are sent$.
If a circuit has been reserved for a particular user and there is no traffic to send, the bandwidth of
that circuit is wasted. It cannot be used for other traffic. ;acket switchin does not waste
bandwidth and thus is more efficient from a system-wide perspective. *nderstandin this trade-
off is crucial for comprehendin the difference between circuit switchin and packet switchin.
The trade-off is between uaranteed service and wastin resources versus not uaranteein
service and not wastin resources.
;acket switchin uses store-and-forward transmission. + packet is accumulated in a router5s
memory, then sent on to the next router. .ith circuit switchin, the bits "ust flow throuh the
wire continuously. The store-and-forward techni&ue adds delay.
+nother difference is that circuit switchin is completely transparent. The sender and receiver
can use any bit rate, format, or framin method they want to. The carrier does not know or care.
.ith packet switchin, the carrier determines the basic parameters. + rouh analoy is a road
versus a railroad. In the former, the user determines the si%e, speed, and nature of the vehicle8 in
the latter, the carrier does. It is this transparency that allows voice, data, and fax to coexist within
the phone system.
+ final difference between circuit and packet switchin is the charin alorithm. .ith circuit
switchin, charin has historically been based on distance and time. 7or mobile phones,
distance usually does not play a role, except for international calls, and time plays only a minor
role #e.., a callin plan with 1000 free minutes costs more than one with /000 free minutes and
sometimes niht or weekend calls are cheaper than normal$. .ith packet switchin, connect time
is not an issue, but the volume of traffic sometimes is. 7or home users, I!;s usually chare a flat
monthly rate because it is less work for them and their customers can understand this model
easily, but backbone carriers chare reional networks based on the volume of their traffic. The
differences are summari%ed in 7i. 1-20.
Figure (-/,. % com$arison of circuit-s#itched and $ac!et-s#itched net#or!s.
9oth circuit switchin and packet switchin are important enouh that we will come back to
them shortly and describe the various technoloies used in detail