Ch1 ,2 ,3,4

4123702
Data Communications System
By
Ajarn Preecha Pangsuban

4123702 Data Communications System @YRU 2
Part 1
Overview of Data Communications and
Networking
 Chapter 1

Brief description of data communication, networking,
and protocol standards
 Chapter 2

Network models

Data Communication
 Why study data communications?
 Data communication is the exchange of data
between two devices via a transmission medium
 Effectiveness depends on:

Delivery

Accuracy

Timeliness

Communication System Components

Data Representation
 Text – represented as a bit pattern, a sequence
of bit (0s or 1s); codes often used:

ASCII; Extended ASCII; Unicode; ISO
 Numbers – represented by binary equivalent
 Images – bit patterns representing pixels
 Audio
 Video

DTE: Data Terminal
Equipment
T ra n s m is s io n
N e tw o rk
D T E D T ED C E D C E
DCE: Data
communicatio
Equipment
A Communications Model

Simplified Communications Model

Simplified Data Communications Model

Simplified Network Model

Direction of Data Flow
 Simplex – unidirectional; one transmits, other
receives

Direction of Data Flow (cont)
 Half-duplex – each can transmit/receive;
communication must alternate

Direction of Data Flow (cont)
 Full-duplex – both can transmit/receive
simultaneously

Networks
 A network is set of devices (nodes) connected
by communication links (media)
 A node can be a computer,printer,or other
device capable of sending and/or receiving data
 Link connecting the devices are often called
communication channels
 Most network use distributed processing
 Advantages?

Network Criteria (Standard)
 Performance – depends on number of users, type
of medium, HW/SW
 Reliability – measured by freq of failure, recovery
time, catastrophe vulnerability
 Security – protection from unauthorized access,
viruses/worms

Types of Connections
 Point-to-point – dedicated link

Types of Connections (cont)
 Multipoint (Multidrop) – shared a single link

Physical Topology
 The way in which a network is laid out physically
 4 basic types: mesh, star, bus, ring
 May often see hybrid

Mesh Topology
 Dedicated point-to-point
links to every other device
 Has n(n-1)/2 physical
channel to link n devices
 Devices have n-1 I/O
 Advantages?
 Disadvantages?

Advantages
 Data can be transmitted from different devices
simultaneously. This topology can withstand high
traffic.
 Even if one of the components fails there is
always an alternative present. So data transfer
doesn’t get affected.
 Expansion and modification in topology can be
done without disrupting other nodes.

Disadvantages
 There are high chances of redundancy in many of
the network connections.
 Overall cost of this network is way too high as
compared to other network topologies.
 Set-up and maintenance of this topology is very
difficult. Even administration of the network is
tough.

Star Topology
 Dedicated point-to-point
links to central controller
(hub)
 Controller acts as
exchange
 Advantages ?
 Disadvantages ?

Advantages
 As compared to Bus topology it gives far much better performance,
signals don’t necessarily get transmitted to all the workstations. A
sent signal reaches the intended destination after passing through no
more than 3-4 devices and 2-3 links. Performance of the network is
dependent on the capacity of central hub.
 Easy to connect new nodes or devices. In star topology new nodes
can be added easily without affecting rest of the network. Similarly
components can also be removed easily.
 Centralized management. It helps in monitoring the network.
 Failure of one node or link doesn’t affect the rest of network. At the
same time its easy to detect the failure and troubleshoot it.

Disadvantages
 Too much dependency on central device has its
own drawbacks. If it fails whole network goes
down.
 The use of hub, a router or a switch as central
device increases the overall cost of the network.
 Performance and as well number of nodes which
can be added in such topology is depended on
capacity of central device.

Bus Topology
 Multipoint configuration
 One cable acts as a backbone to link all devices
 Advantages ?
 Disadvantages ?

Advantages
 It is easy to set-up and extend bus network.
 Cable length required for this topology is the least
compared to other networks.
 Bus topology costs very less.
 Linear Bus network is mostly used in small
networks. Good for LAN.

Disadvantages
 1) There is a limit on central cable length and number of nodes that can be
connected.
 2) Dependency on central cable in this topology has its disadvantages. If the
main cable (i.e. bus ) encounters some problem, whole network breaks
down.
 3) Proper termination is required to dump signals. Use of terminators is must.
 4) It is difficult to detect and troubleshoot fault at individual station.
 5) Maintenance costs can get higher with time.
 6) Efficiency of Bus network reduces, as the number of devices connected to
it increases.
 7) It is not suitable for networks with heavy traffic.
 8) Security is very low because all the computers receive the sent signal from
the source.

Ring Topology
 Dedicated point-to-point configuration to neighbors
 Signal is passed from device to device until it reaches
destination
 Each device functions as a repeater
 Advantages ?
 Disadvantages?

Advantages
 This type of network topology is very organized. Each node gets to
send the data when it receives an empty token. This helps to
reduces chances of collision. Also in ring topology all the traffic flows
in only one direction at very high speed.
 Even when the load on the network increases, its performance is
better than that of Bus topology.
 There is no need for network server to control the connectivity
between workstations.
 Additional components do not affect the performance of network.
 Each computer has equal access to resources.

Disadvantages
 Each packet of data must pass through all the
computers between source and destination. This
makes it slower than Star topology.
 If one workstation or port goes down, the entire
network gets affected.
 Network is highly dependent on the wire which
connects different components.
 MAU’s and network cards are expensive as
compared to Ethernet cards and hubs.

Categories of Networks
 LAN – smaller geographical area
 WAN – large geographical area

Local Area Network
 Usually privately owned and links the devices in a
single office, building, or campus
 LAN size is limited to a few kilometers.
 LANs are designed to allow resources to be shared
(hardware , software and data )
 Today LANs to have data rates of 100 Mbps to 10Gbps
 Backbone Networks (BN), have a scale of a few
hundred meters to a few kilometers. Include a high
speed backbone linking the LANs at various locations.

Figure 1-2
Local Area Network (cont)

Wide Area Network
 WAN provides long distance transmission of
data, voice, image, and video information over
large geographical areas
 Comprise a country, a continent, or even the
whole world (Interlink age of many LANs and
MANs)
 Low data transmission rate (below 1 Mbps)
 Unlimited number of miles example: Internet
Network

Figure 1-4
Wide Area Network (cont)

Internetworks
 Connection of two or more networks by the use of
internetworking devices which include routers and
gateways
 Internet is a generic term used to mean an
interconnection of networks
 The Internet is the name of a specific worldwide
network.

Figure 1-5
Internetworks (cont)

The Internet
 Collaboration of more than hundreds of
thousands interconnected networks
 1969 – started as ARPAnet, a small network of
connected computers
 1972 - Cerf and Khan – packet delivery and
Transmission Control Protocol (TCP)
 Shortly thereafter – evolution of TCP/IP

The Internet today

Protocols and Standards
 Why do we need them?
 A protocol is a set of rules that governs data
communication; the key elements of a protocol are

Syntax – data formats and Signal levels

Semantics – control information and error handling

Timing – speed matching and sequencing
 Standards are necessary to ensure that products
from different manufacturers can work together as
expected.

Standards
 Types –

De jure (Formal) – legislated by an officially recognized
body

De facto – by convention or widespread use
 Standards Organizations

Committees – ISO, ITU-T, ANSI, IEEE, and EIA

Forums – special-interest groups that quickly evaluate
and standardize new technologies

regulatory agencies – FCC
 Internet Standards

Drafts (6 month lifetime)

RFC is an idea or concept that is a precursor to an
Internet standard.

Credits
 All figures obtained from publisher-provided
instructor downloads
Data Communications and Networking, 3rd edition by
Behrouz A. Forouzan. McGraw Hill Publishing, 2004

Ch1 ,2 ,3,4

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