Your objectives :
be able to discuss:
 the basic tasks of network software
 layering in a theoretical sense
 the functions of the layers in the OSI Reference Model
 the functions of the layers in the TCP/IP Reference
Model
 Modes of communication
B227 Data Communications Lecture 2-1
Peter Cole 2001
Network Software
We have taken a look at network hardware. That is the
physical layout of networks and their topologies and
classification. We will look in finer detail at some of the
aspects of the physical installation of networks in the
next topic.
Network software is required to facilitate the simple use
of networks. Not all the software is actually software as
some of the lower level software has its algorithms
actually incorporated in circuitry and firmware.
When designing complex software we teach (and
hopefully practice) divide-and-conquer techniques such
as the Top-down Design methodology.
The creators of network software have also chosen this
approach to reduce the complexity of designing network
software by breaking the task down into Layers.
B227 Data Communications Lecture 2-2
Peter Cole 2001
Layering
 called Protocol Hierarchies
 names and purpose of layers differ between
networks
 Each entity has a corresponding Peer
 virtual communication through a layer n protocol
 combined called a network architecture
B227 Data Communications Lecture 2-3
Peter Cole 2001
Why layer ?
 Reduce the complexity of the problem
 divide & conquer
 achieve translations therefore communication
between differing network architectures
B227 Data Communications Lecture 2-4
Peter Cole 2001
How it works!
A message, M, is produced by an application process
running in layer 5 and given to layer 4 for transmission.
Layer 4 puts a header in front of the message to identify
the message and passes the result to layer 3.
B227 Data Communications Lecture 2-5
Peter Cole 2001
The header includes control information, such as
sequence numbers, to allow layer 4 on the destination
machine to deliver messages in the right order if the
lower layers do not maintain sequence. In some layers,
headers also contain sizes, times, and other control
fields.
In many networks, there is no limit to the size of
messages transmitted in the layer 4 protocol, but there is
nearly always a limit imposed by the layer 3 protocol.
Consequently, layer 3 must break up the incoming
messages into smaller units, packets, pre-pending a layer
3 header to each packet. In this example, M is split into
two parts, M1 and M2.
Layer 3 decides which of the outgoing lines to use and
passes the packets to layer 2. Layer 2 adds not only a
header to each piece, but also a trailer, and gives the
resulting unit to layer 1 for physical transmission.
At the receiving machine the message moves upward,
from layer to layer, with headers being stripped off as it
progresses. None of the headers for layers below n are
passed up to layer n.
B227 Data Communications Lecture 2-6
Peter Cole 2001
The important thing to understand is the relation
between the virtual and actual communication and the
difference between protocols and interfaces. The peer
processes in layer 4, for example, conceptually think of
their communication as being "horizontal," using the
layer 4 protocol. Each one is likely to have a procedure
called something like SendToOtherSide and
GetFromOtherSide, even though these procedures
actually communicate with lower layers across the 3/4
interface, not with the other side.
The peer process abstraction is crucial to all network
design. Using it, the unmanageable task of designing
the complete network can be broken into several
smaller, manageable, design problems, namely the
design of the individual layers.
B227 Data Communications Lecture 2-7
Peter Cole 2001
Communication Modes
Simplex
half-duplex
XOR
full-duplex
B227 Data Communications Lecture 2-8
Peter Cole 2001
OSI
ISO (International Standards Office) developed a seven-layer
model known as the OSI (Open Systems Interconnection)
Reference Model.
B227 Data Communications Lecture 2-9
Peter Cole 2001
We will discuss each layer of the model in turn, starting
at the bottom layer. Note that the OSI model itself is not
a network architecture because it does not specify the
exact services and protocols to be used in each layer. It
just tells what each layer should do. However, ISO has
also produced standards for all the layers, although these
are not part of the reference model itself. Each one has
been published as a separate international standard.
The physical layer is concerned with transmitting raw
bits over a communication channel. The design issues
have to do with making sure that when one side sends a
1 bit, it is received by the other side as a 1 bit, not as a 0
bit. Typical questions here are:
how many volts should be used to represent a 1 and how
many for a 0
how many microseconds a bit lasts
full-duplex or not
how connection is made
how many pins the network connector has and what
each pin is used for
B227 Data Communications Lecture 2-10
Peter Cole 2001
The design issues here largely deal with mechanical,
electrical, and procedural interfaces, and the physical
transmission medium, which lies below the physical
layer.
B227 Data Communications Lecture 2-11
Peter Cole 2001
The Data Link Layer
The main task of the data link layer is to take a raw
transmission facility and transform it into a line that
appears free of undetected transmission errors to the
network layer.
accomplished by having the sender break the input data
up into data frames (typically a few hundred or a few
thousand bytes)
transmit the frames sequentially
process the acknowledgment frames sent back by the,
receiver
Therefore, the Data-link layer is responsible for data
integrity
It is split into two sublayers
at the top
The Logical Link Control Sub-layer (LLC)
And below the Medium-Access Control Sub-layer (MAC)
B227 Data Communications Lecture 2-12
Peter Cole 2001
B227 Data Communications Lecture 2-13
Peter Cole 2001
The Network Layer
The network layer is concerned with controlling the operation
of the subnet
A key design issue is determining how packets are
routed from source to destination
Routes can be based on static tables that are "wired into"
the network and rarely changed
They can also be determined at the start of each
conversation, for example a terminal session
they can be highly dynamic, being 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 get in each other's way, forming
bottlenecks. The control of such congestion also
belongs to the network layer.
there is often some accounting function built into the
network layer
B227 Data Communications Lecture 2-14
Peter Cole 2001
The Transport Layer
The transport layer is a true end-to-end layer, from source to
destination. In other words, a program on the source machine
carries on a conversation with a similar program on the
destination machine, using the message headers and control
messages
 The basic function of the transport layer is to accept data
from the session layer, 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
 Furthermore, all this must be done efficiently, and in a way
that isolates the upper layers from the inevitable changes in
the hardware technology
 Under normal conditions, the transport layer creates a distinct
network connection for each transport connection required by
the session layer.
 If the transport connection requires a high throughput,
however, the transport layer might create multiple network
connections, dividing the data among the network
connections to improve throughput.
 In all cases, the transport layer is required to make the
multiplexing transparent to the session layer.
B227 Data Communications Lecture 2-15
Peter Cole 2001
The Session Layer
 The session layer allows users on different machines to
establish sessions between them.
 A session allows ordinary data transport, as does the transport
layer, but it also provides enhanced services useful in some
applications. A session might be used to allow a user to log
into a remote time-sharing system or to transfer a file between
two machines.
 One of the services of the session layer is to manage dialogue
control. Sessions can allow traffic to go in both directions at
the same time, or in only one direction at a time. If traffic can
only go one way at a time (analogous to a single railroad
track), the session layer can help keep track of whose turn it
is.
 Another session service is synchronisation. Consider the
problems that might occur when trying to do a 2-hour file
transfer between two machines with a 1-hour mean time
between crashes. After each transfer was aborted, the whole
transfer would have to start over again and would probably
fail again the next time as well. To eliminate this problem,
the session layer provides a way to insert checkpoints into the
data stream, so that after a crash, only the data transferred
after the last checkpoint have to be repeated.
B227 Data Communications Lecture 2-16
Peter Cole 2001
The Presentation Layer
The presentation layer performs certain functions that are
requested sufficiently often to warrant finding a general
solution for them.
A typical example of a presentation service is encoding data
in a standard agreed upon way. Most user programs do not
exchange random binary bit strings. They exchange things
such as people's names, dates, amounts of money, and invoices.
These items are represented as character strings, integers,
floating-point numbers, and data structures composed of several
simpler items.
Different computers have different codes for representing
character strings (eg., ASCII and Unicode), integers (eg., one's
complement and two's complement), and so on. In order to
make it possible for computers with different representations to
communicate, the data structures to be exchanged can be
defined in an abstract way, along with a standard encoding to
be used "on the wire." The presentation layer manages these
abstract data structures and converts from the representation
used inside the computer to the network standard representation
and back
B227 Data Communications Lecture 2-17
Peter Cole 2001
The Application Layer
 The application layer contains a variety of protocols that are
commonly needed.
 For example, there are hundreds of incompatible terminal
types in the world. Consider the plight of a full screen editor
that is supposed to work over a network with many different
terminal types, each with different screen layouts, escape
sequences for inserting and deleting text, moving the cursor,
etc.
 One way to solve this problem is to define an abstract
network virtual terminal that editors and other programs
can be written to deal with.
 To handle each terminal type, a piece of software must be
written to map the functions of the network virtual terminal
onto the real terminal. For example, when the editor moves
the virtual terminal's cursor to the upper left-hand corner of
the screen, this software must issue the proper command
sequence to the real terminal to get its cursor there too. All
the virtual terminal software is in the application layer.
 Another application layer function is file transfer, Email, and
users applications
B227 Data Communications Lecture 2-18
Peter Cole 2001
B227 Data Communications Lecture 2-19
Peter Cole 2001
B227 Data Communications Lecture 2-20
Peter Cole 2001
TCP/IP
Probably the single most important network architecture
Unlike OSI, some of the TCP/IP protocols were in use
before the reference model was laid down
The TCP/IP model has only 4 layers as opposed to the
OSI model
B227 Data Communications Lecture 2-21
Peter Cole 2001
Layer 1, the application
layer
exists in both models.
In TCP/IP it will hold, among others,
the following Telnet : Virtual Terminal (VT100)
FTP
: File transfer Protocol
SMTP : Simple Mail Transfer Protocol
DNS
: Domain Name Service
NNTP : Network New Transfer Protocol
HTTP : HyperText Transmission Protocol
The Session and application layers of the OSI are
missing in TCP/IP
Not really required and consequently became redundant
for this model
B227 Data Communications Lecture 2-22
Peter Cole 2001
Layer 2, the transport layer
holds two protocols TCP (Transmission Control Protocol)
 provides reliable data delivery service with end-toend error detection and correction (connection
oriented service)
 handshake required before data is sent
UDP(User Datagram Protocol)
 provides low-overhead unreliable (connectionless)
datagram delivery service
 no method in UDP to determine if datagram
was delivered correctly
both TCP and UDP deliver data to both the Application
layer and the Internet layer
B227 Data Communications Lecture 2-23
Peter Cole 2001
Layer 3 The Internet Layer
This layer holds the Internet protocol (IP) and defines
the IP datagram format (packet), the basic building
block of The Interenet. It  defines the data packet (unit of transmission in the
Internet)
 defines the Internet addressing scheme
 routes datagrams to remote hosts
 fragments and reassembles datagrams
 moves data between the transport layer and the
Host-to-network layer
B227 Data Communications Lecture 2-24
Peter Cole 2001
Layer 4, The Host-Tonetwork layer
Tanebaum states that there is not much anyone,
including the TCP/IP model, has to say about what
happens in the Host-to-network layer. He refers to it as a
“.. great void”
However, Hunt (see Overview of TCP/IP), who refers to
the Host-to-network layer as the “Network Access
Layer” states that this layer
 provides the means for the system to deliver data to
the other devices on a directly connected network
 defines how to use the network to transmit an IP
datagram
Basically, this layer holds all the protocols necessary
for the source host machine to connect to the
destination host.
There will be one protocol for each physical network
standard eg.
RFC 826 -Address Resolution Protocol (Maps IP
address to Ethernet addresses)
RFC 894 - A standard for the Transmission of IP
Datagrams over Ethernet Networks (how to
encapsulate a datagram for transmission on Ethernet)
B227 Data Communications Lecture 2-25
Peter Cole 2001
Networks
Many networks exist in either government or private
hands
Novell NetWare
 predates OSI
 LAN (most popular ? Tanenbaum)
Very similar architecture to TCP/IP (Or the structure
of the text)
B227 Data Communications Lecture 2-26
Peter Cole 2001
The Internet
Grew out of the US DoD ARPANET
ARPANET was available to Universities with research
grants from DARPA.
In an attempt to bring universal networking capabilities
to academics the NFS formed its own network
(CSNET) and later still commissioned the NFSNet
backbone
subnet
Fuzzballs
(TCP/IP)
supercomputers
When ARPANET and the NFSNet were interconnected
the speed of additional networks becoming
interconnected grew exponentially
year
nwrks
hosts
1990
3000
200,000
1992
1,000,000
B227 Data Communications Lecture 2-27
Peter Cole 2001
B227 Data Communications Lecture 2-28
Peter Cole 2001
B227 Data Communications Lecture 2-29
Peter Cole 2001
B227 Data Communications Lecture 2-30
Peter Cole 2001
Data Comm. Services
Switched Multimegabit Data Service - SMDS
 intended to connect multiple LANS in same company
scenario
 uses local lines to nearest SMDS router
 alternative leased lines
 connectionless packet delivery service
 can broadcast to several telephone numbers
B227 Data Communications Lecture 2-31
Peter Cole 2001
X.25
 Connection oriented service
 telephone standard - analogue
 X.21 - physical layer protocol requires digital input
therefore not highly supported
 Switched and permanent virtual circuits
Frame Relay
 cheaper & faster than X.25
 can use actual leased lines or permanent virtual circuits
actual leased lines - go full speed all of the time
permanent virtual circuit is bursty but cheaper
B227 Data Communications Lecture 2-32
Peter Cole 2001
N- ISDN & B-ISDN
 ISDN = Integrated Services Digital network
 N = Narrowband
 B = Broadband
 This system offers high speed multimedia data
exchange
 currently being installed in most Australian capital
cities
 relies on Asynchronous Transfer Mode - ATM
 uses small fixed sized “cells” to transmit data
containing a 5byte header and 48 bytes of payload
ATM promises to be the service of the future.
We will look at ATM a little later in the course in
greater detail
B227 Data Communications Lecture 2-33
Peter Cole 2001
B227 Data Communications Lecture 2-34
Peter Cole 2001
Topic 2 The Physical layer
The Physical layer, the lowest level of the OSI,
physically dispatches and receives the bit streams
between two or more machines
We will start off this section by looking at the
physical limitations of differing transmission
media taking note of the theoretical basis which
supports it.
There are three different types of media
 Wire ( copper or other highly conductive material)
 Glass Fiber
 air or atmosphere
these require respectively power in terms of
 electricity
 light
 radio waves
B227 Data Communications Lecture 2-35
Peter Cole 2001
The Theories
Fourier Analysis :
Jean-Baptise Fourier - early 19th century
tells us that we can determine the amplitude of a
harmonic in a known frequency an
With each term in a Fourier series or expansion we
can calculate an and with each new term in the
series an becomes smaller
This phenomenon is called Attenuation
Attenuation is the reduction of signal amplitude due
to the physical properties of the media
B227 Data Communications Lecture 2-36
Peter Cole 2001
differing media have differing attenuation properties
CURE :
amplify the diminished signal using a REPEATER of
some sort
Distortion results if the attenuation is not uniform
across the frequency spectrum - signal does not distort
uniformly
Amplification may cause a loss of data because the
distortion is also amplified
Bandwidth limitation :
a transmission medium carries signals lying within a
spectrum or range of frequencies; the absolute width
of the spectrum is called the bandwidth
therefore if a signal has components that lay outside
the bandwidth they are totally attenuated, that is,
chopped off
CURE:
filter and reconstruct the signal before re-transmission
or use a transmission medium with qualities more
suited to the job
B227 Data Communications Lecture 2-37
Peter Cole 2001
B227 Data Communications Lecture 2-38
Peter Cole 2001
Maximum data Rate of a
transmission channel
Nyquist
assists in the calculation of the Maximum data Rate of
a transmission channel If you pass a signal through a low-pass filter of
bandwidth H then the signal can be completely
recovered using only 2H samples per second
Therefore no point to looking at (sampling) faster than
2H per second as the higher frequency signals have
been filtered out
Shannon
Add noise to the equation and the maximum data rate
of a channel is reduced further
B227 Data Communications Lecture 2-39
Peter Cole 2001
Transmission Media
B227 Data Communications Lecture 2-40
Peter Cole 2001
B227 Data Communications Lecture 2-41
Peter Cole 2001
B227 Data Communications Lecture 2-42
Peter Cole 2001