ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
D.D.Udrea
Aims and Objectives
- Explain the concept of networks
- Give examples of LAN (Local Area Network) technologies
- Explain how a LAN can be extended to a WAN (Wide Area Network)
- Give examples of data connections
Networks
There are many different acronyms and technologies associated with the Internet today (e.g. TCP/IP,
Web, Ethernet, DNS, HTTP, etc. explained later), so it is useful to put things in perspective by starting
with a very basic idea: the term network.
One basic definition of a network is two or more electronic devices linked in some way to permit the
exchange of information. In information technology terms, a network is a series of points, or nodes
interconnected by communication paths.
Networks can interconnect with other networks and can contain sub-networks. Often the term network is
in the context of a LAN (Local Area Network) or WAN (Wide Area Network), TCP/IP (data transmission
technology), voice or data, public or private, dial-up or dedicated (type of connections), Internet or
intranet (type of access).
In practical terms, networks are very useful since they allow you to move information from one place to
another in a way that is much faster or more convenient than manually moving the information, say on
disks. Of course there are some drawbacks to using networks. A system that heavily depends on a
network to deliver data can be crippled if there is a problem with one of the nodes or the inter-linking
connection paths. Conversely, by having multiple data paths we can introduce redundancy and therefore
ensure that data can be transferred between nodes via alternative network routes.
Here are several ways of interconnecting devices, based on the number of direct connection, such as one
to one, one to many and many to many.
Illustration of types of interconnecting devices
[Leeson 2001-es154-0001-1]
The physical link or channel between two devices can be a dedicated channel (point to point connection
in which the line transmits only data from the two devices), or a shared channel (switched connections in
which the line transmits multiplexed data from multiple sources). Furthermore, the latter can belong to a
circuit switched (e.g. telephone system) or a packet switched (e.g. the Internet) network.
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ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
D.D.Udrea
In general a network technology is classified into one of three broad categories, depending on the size of
network that can be created: A Local Area network (LAN) can span a single building or campus or can be
extended between a few small size sites. A Wide Area Network (WAN) can span multiple cities,
countries or continents and connect an arbitrarily large number of computers.
LAN topologies
Topology
Star
Ring
Bus
Description
Each computer attaches to a central point called a
hub. The computers send or receive data through the
hub, which is responsible for connecting the sender
with the recipient. Any computer can send data at any
one time.
Computers are connected in a closed loop and each
computer connects directly to two others. Only one
computer has access to the shared ring and can send
data at any one time. Data from one computer has to
pass through all the others and come back to the
original sender before the transmission is complete.
All computers attach to a single, shared cable. Any
computer can send data and all computers receive it.
However, computers must coordinate to ensure that
only one computer sends data at any one time.
Examples
ATM
network,
ARCnet
IBM
Token
ring,
FDDI
Ethernet,
LocalTalk,
Wireless
LANs
Advantages and Disadvantages
(A) Network independent of individual
connections; (A) Computers can send
data simultaneously; (A) Works over
long distances at high speeds; (D) Long
wires;
(A) Easy to coordinate and check that
network operates correctly; (D) Entire
network down if one cable is cut. (D)
Computers can only send data
(A) Fewer wires (A) Very fast (D) Prone
to collision; (D) Works over short
distances;
Illustration of the star topology
[Comer 2001 Chapter8].
Illustration of a ring topology
[Comer 2001 Chapter 8].
Illustration of a bus topology
[Comer 2001 Chapter8].
WAN examples
Many technologies have been created for experimental and production use in WANs. Examples of such
technologies are ARPANET (data), X.25 (voice), Frame Relay (data), SMDS (data), ATM (voice and
data).
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ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
D.D.Udrea
A small WAN formed by interconnecting
packet switches. Connections between packet
switches usually operate at a higher speed than
connections to individual computers
[Comer 2001 Chapter 13].
A bridge using a leased satellite channel to
connect LAN segments at two sites. A satellite
bridge can span arbitrary distance
[Comer 2001 Chapter 11].
Network Hardware
At the lowest level, all computer communication involves encoding data in a form of energy, and sending
the energy across a transmission medium. For example, electric current can be used to transfer data across
a wire, or radio waves can be used to carry data through the air. To connect to a network, a computer
needs dedicated hardware. The hardware available dictates the bandwidth (hence speed) of the data
transfer. Because hardware devices attached to a computer perform the encoding and decoding of data,
programmers and users don’t need to know the details of data transmission. However, because a major
role of the communication software is to handle errors and failures that appear due to the hardware,
understanding such software requires a basic knowledge of data transmission and hardware.
Standards
To ensure that communication hardware built by different vendors will interoperate, the specifications for
communication systems are standardised. A standard is a document which contains a list of
specifications about a specific communication technology, from timing of signals to electrical voltages
and currents, from number of bits to characters and codes. Examples of such standards are RS-232-C
(defines serial, asynchronous communication over short distance), RS-422 (extension of RS232), ASCII
(binary representation of characters).
Network media
Cables
Unshielded
twisted pair
(UTP)
Shielded
twisted pair
(STP)
Coaxial
cable
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Description
4 pairs of copper wires inside the
same plastic jacket; twists minimise
interference with other cables.
RJ-45 connector
2 pairs of copper wires, individually
shielded in foil, surrounded by a
braided metal shielding.
IBM data connector
Single copper wire surrounded by a
braided metal shielding.
BNC connector
Advantages
Low cost;
Easy to repair;
Disadvantages
Signal suffers
from
interference;
Application
Short length
LAN and
Telephone;
BW
1-200
MHz
Low cost;
Easy to repair;
Less
interference
but bulky;
Short length
high speed
LAN;
0.1-1
GHz
Short length
LANs;
Ethernet and
ARCnet
0.3-10
GHz
Low cost;
Easy to repair;
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ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
Fibre optic
cable
A bunch of optic fibres in a plastic
sleeve. Each fibre is made of glass
and cladding and surrounded by
reinforcing layers of plastic.
ST fibre connector
Wireless
Infrared
Description
Infrared light
through air.
Radio
Radio waves
through air or
vacuum
Microwaves
through air
Microwave
No interference;
Low attenuation;
One fibre only;
Huge bandwidth
Expensive;
Difficult to
pinpoint and
repair
D.D.Udrea
Long distance
and high
traffic LANs
and WANs;
THz
Advantages
Mobile,
Inexpensive,
Huge bandwidth
Flexible, mobile
Disadvantages
Short distance
Line of sight
Application
Wireless LAN; Short distance
portable computers
BW
THz
Large antennas
Interference,
Wireless LAN and WAN Ground
stations, satellites, cellular
MHZ
Flexible, mobile,
directional
Line of sight
Wireless LAN and WAN point to
point, ground stations, satellite,
cellular
2-30
GHz
Network interfaces
Interface
Network Card
or LAN adapter or NIC
(Network Interface Card)
Modem
MOdulator/DEModulator)
DSU/CSU (Data Service
Unit/Channel service unit)
Description
A hardware device that plugs into a computer internal bus and connects the
computer to a network. It is built for a specific network technology.
A device that encodes/decodes digital information in a carrier wave for
transmission across (from) copper wire or a dialup telephone connection (typically
analogue).
A device that connects a leased digital data circuit to computer equipment. It
translates between the digital format used by the telephone companies and the
format used by the computer industry.
Illustration of dialup modems that use
the voice telephone system to
communicate. To the telephone
system, a dialup modem appears to
be a telephone
[Comer 2001 Chapter 6].
Illustration of a digital circuit with
a DSU/CSU on each end. The
DSU/CSU converts between the
digital standards used in the
telephone system and those used by
computer vendors
[Comer 2001 Chapter 12].
Organization of the hardware in a
computer attached to a LAN. Because
it is powerful and independent, the
network interface hardware does not
use the CPU when transmitting or
receiving data
[Comer 2001 Chapter 9].
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ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
D.D.Udrea
Network hardware components
Hardware
Transceiver
Hub
Repeater
Bridge
Router
Route Server
Switch
Description
A device that connects a NIC in a computer to a physical medium. Are used with Ethernet.
An electronic device that implements a network. Computers connected to a hub can communicate as if
they attach to a network.
A device (typically analogue) used to extend a LAN. It connects directly two cable segments,
continuously monitors the signals on each segment and re-transmits an amplified version of the signals
tot he other segment.
A more advanced type of repeater which re-transmits signals only as far as necessary and can isolate
problems which appear on the cable.
The basic block of an Internet. A computer that attaches to two or more networks and forwards packets
in more than one direction according to information in its routing table. It runs IP. A router can
interconnect networks that use different technologies (ie media, addressing scheme, frame format)
A server that contains complete routing information for the global Internet. All route servers together
form the routing arbiter system.
The basic block of a switched network (e.g. star network). It uses the destination address of a data
block to determine which computer should receive the data.
A bridge connecting LAN segments in two buildings.
For example, an optical fibre is used to connect the
bridge to a remote LAN segment [Comer 2001
Chapter 11].
A repeater R connecting two
Ethernets. The repeater connects
directly to the cable; it does not
use a transceiver.
[Comer 2001 Chapter 11].
The concept underlying a switched LAN. Electronic
circuits in the switch provide each computer with the
illusion of a separate LAN segment connected to
other segments by bridge [Comer 2001 Chapter 11].
Network software
Communication protocols
Basic communication hardware consists of mechanisms that can transfer bits from one point to another.
However, using raw hardware to communicate is cumbersome and inconvenient. To aid programmers,
computers on a network use complex software that provides convenient, high-level interface for
applications. Thus, most applications rely on network software to communicate and they do not handle
hardware directly.
All parties involved in a communication must agree on a set of rules to be used when exchanging
messages, which specifies the format of messages and the appropriate actions required to handle each
message. This is called a communication protocol.
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ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
D.D.Udrea
Instead of having a single, giant protocol that specifies complete details for all possible forms of
communication, designers have chosen to divide the problem into subpieces and design a separate
protocol for each subpiece.
OSI model (for reference only, not for the exam)
A good way of understanding the various protocols and technologies associated with networks is to look
at the ISO (International Standards Organisation) seven-layer model of networks, also known as the OSI
(Open Systems Interconnect) model[Travis, Chapter 1].
This model sets the standards for networks by defining how each aspect should work – from the physical
interconnections on the network through to how data is communicated. The seven-layer ISO OSI model
is essential as it dictates how each computer on the Internet (i.e. corporate LAN, home computer etc.)
must behave in order to facilitate data transfer.
Without it, Internet computers could (and most probably would) have incompatible network architectures.
The topmost layers are the most abstract, each one built upon the more primitive lower layers. The layers
can be independent of each other so, for example, programmers can concentrate on writing network
software (higher level) without having to worry about the physical connectivity (lower level).
A brief description of each layer is as follows:
1. Physical: Information is encoded ultimately into electrons or electromagnetic waves. The physical
layer defines the voltages and signal levels for a type of network. Part of the Ethernet standard, for
example, defines the Physical layer. Standards such as RS232, 10-Base-T, Twisted Pair, Fibre-Optic,
modems and others, are also involved in this layer.
2. Data Link: This layer is concerned with reliable and error-free data transfer across nodes, with
facilities such as bit error detection.
3. Network: The network layer is a type of kernel service that uses unique source and destination
addresses to send data over the network. IP (Internet Protocol) is part of this layer, since is specifies
how to identify source and destination addresses with “IP numbers”, e.g. 128.34.22.110
4. Transport: The transport layer segments messages into smaller units called packets for transmission.
It controls the transmission of packets, making sure that they arrive in sequence and that no packets
are missing. TCP (Transmission Control Protocol) is part of this layer.
5. Session: This layer connects and manages the sessions between two end users. Services like DNS
(Domain Name Service), login, authentication using passwords, etc. are part of this layer.
6. Presentation: The Presentation layer helps with initiating and terminating connections, and isolates
the Application layer from differences between various types of computers in the way they represent
data internally.
7. Application: This is the top-end user layer. It provides approximate meaning and functionality to the
transferred data. For example, FTP and HTTP protocols (both described later) fall into this category.
The historic ISO 7-Layer Reference Model.
A layering model is a tool to help protocol
designers construct a suite of protocols that solves
all communication problems
[Comer 2001 Chapter 16]
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ES154 Communications and Media 2001-2002
UNIVERSITY OF WARWICK
Lecture 11
School of Engineering
Basic networks; WANs; Network connections
D.D.Udrea
Physical layer
One of the major limitations to building Internet-enabled systems is the available bandwidth, or how
many bits per second can be transmitted. An example of limited bandwidth can be seen when you watch
real-time video with an analogue modem. Often special compression techniques have to be used in order
to enable the video to appear smooth and if the connection is not consistent, then the video stops. This
restriction in available bandwidth is generally due to limitations of the Physical layer.
Internet Hardware Interface – physical layer
Cellular modem (wireless) e.g. mobile phone
Serial modem (using analogue phone line)
ISDN modem
T1
ADSL modem
Cable modem
T3
10-BaseT Ethernet*
100-Base-T Ethernet*
Gigabit Ethernet*
Theoretical Maximum Speed
19 Kbit/s
56 Kbit/s
128 Kbit/s
1.544 Mbit/s
9 Mbit/s
30 Mbit/s
44.736 Mbit/s
10 Mbit/s
100 Mbit/s
1000 Mbit/s
*LAN (Local Area Network) technologies
References:
Comer, Douglas - Computer networks and Internets with Internet applications / Douglas E. Co. - 3rd ed. - Upper Saddle River,
N.J.; London: Prentice Hall, 2001. – ISBN 0-13-091449-5 QA 74.2.C6
Leeson Mark – ES154 Communications and Media – lecture notes : es154-0001-1.pdf, 2001
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