2. Transmission of Data
• Data is transferred between devices in either serial or parallel
transmission.
– Parallel transmission: is the transfer of data simultaneously
using separate channels.
– Serial transmission: is the transmission of data one after the
other.
3. Serial Data Transmission
• Serial data transfer may be either synchronous or asynchronous.
– Asynchronous transmission: the sending of data by identifying each
byte with special start and stop bits. The protocol agreement decides
how many start and stop bits (between 0 and 2). Asynchronous
transmission may occur at any time, as long as both the transmitter and
receiver are working correctly.
– Synchronous transmission: requires all of the data to be sent at the
same rate at an agreed time. This is synchronized between two devices
by using a clock. Synchronous transmission is faster then asynchronous
transmission as there are no extra bits however it requires more
planning and co-ordination between the two systems.
4. Direction of Data flow
• The direction of data flow can be simplex, half duplex or full duplex.
• Simplex: only allows transmission in one direction only, from the
sender to the receiver.
• Half-duplex: allows transmission in both directions, but not at the
same time. The sender and receiver must take turns at sending and
transmitting data.
• Full-duplex: allows the transmission of data in both directions
simultaneously.
5. Protocols
• Protocols in communication – a protocol is needed when one
computer communicates with another.
• A protocol is a set of rules that control a communications link.
• A protocol’s tasks is to ensure that the data send by the sending
device is the same as the data received by the receiving device.
6. Protocols cont’d…
• A protocol defines the format and meaning of information
exchanged by describing parameters such as:
– speed of transmission (bps, baud)
– transmission mode (answer or originate)
– handshake
– parity
– error detection
– correction
7. Protocols cont’d…
• Elements that a protocol controls include:
– The type of language used (e.g. ASCII)
– Type of error checking method used (e.g. checksum)
– What to do when error is detected (e.g. resend data)
– Way data are sent (asynchronous or synchronous transmission)
– And so on…
8. Handshaking
• Handshaking – is the exchange of signals between two devices to
establish a communication link for senders and receivers.
• The computer handshakes to a printer or a modem in order to
establish a connection for communicating.
• For example
– For a link between the computer and modem, handshaking
involves the signalling of both to each other that they are
switched on and ready for work
– The computer sends a DTR (Data Terminal Ready) signal to
modem through a particular wire used only for this signal. When
the modem is ready it sends a DSR (Data Set Ready) signal to
computer through a different wire in the connecting cable.
9. Networks
• A network is how a number of computer systems and peripherals
are connected together.
10. LANS
• Local area networks (LAN): connect computers within a building or
complex. LAN's cover a small geographical area and are connected
via coaxial or fibre-optic cables.
Advantages of LAN's:
• Sharing limited hardware resources (such as printers).
• Sharing application software.
• Improved communication among users sending electronic
messages.
12. Bus Topology
Bus topology uses a trunk or backbone to
which all of the computers on
the network connect.
• Systems connect to this backbone using
T connectors or
taps.
Advantages
- Cheap and easy to implement
- Require less cable
- Does not use any specialised
network equipment.
Disadvantages
- Network disruption when
computers are added or removed
- A break in the cable will prevent all
systems from accessing the network
- Difficult to troubleshoot.
13. Ring Topology
- Ring topology consists of nodes in a
closed loop or circle.
- Each node is part of a loop linking
all the nodes together and nodes
are attached to the cable at various
points around the ring.
- Traffic can be one-way around the
loop or can travel in both directions
if a double loop is used.
Advantages
- Cable faults are easily located,
making troubleshooting easier
- Ring networks are moderately
easy to install
Disadvantages
- Expansion to the network can
cause network disruption
- A single break in the cable can
disrupt the entire network
14. Star Topology
• Star topology is a network topology that
consists of computers attached to a central
point or node (a device such as a computer
connected to a network) which can be a
computer like a mainframe, hub with links
going out to computers that are workstations
and servers.
- Star networks are popular and most common
in LANs.
Advantages
- Easily expanded without disruption to
the network
- Cable failure affects only a single user
- Easy to troubleshoot and isolate
problems
Disadvantages
- Requires more cable
- A central connection device
allows for a single point of
failure
- More difficult to implement
15. WANS
• Wide area networks (WAN): connect computer spread over a
large geographical area.
• They usually consist of a mainframe computer (called the
host) and a number of terminals.
• WAN's may use private leased lines or normal telephone lines
(PSTN – public switch telephone network) (or a combination).
– Private leased lines are dedicated to a single network.
17. • NICSs: Network interface cards converts data between the
computer into a form suitable for transmission across the network.
The conversion uses the rules of the data link and physical link
protocols in operation.
• Servers: Help to manage the network and the resources of that
network. On larger networks servers commonly have specialised
tasks such as: File Servers: stores and manages files, Print Servers:
manages printers and print jobs, Mail Server: Manages email, Web
Server: manages web access.
• Routers: connects multiple networks and are protocol independent.
can be used in place of a switch or bridge.
• Switches: Smart hubs which transmit packets to the destination
port only
18. • Hubs: like double adapters /power boards in the home except
instead of plugging in extension cords we are plugging in computers
to allow them to communicate.
• Bridges: This type of network is called a segmented LAN and has
largely been superseded by the use of switches which can transfer
data straight to a computer and thus avoid bottleneck jams which
bridges were designed to fix.
• Gateways: Often used to connect a LAN with a WAN. Gateways join
two or more different networks together.
19. • Transmission media:
– twisted pair – telephone cable
– coaxial cable –Thick black cable used for higher bandwidth
communications than twisted pair (i.e. Optus cable)
– fibre optic – data transferred through pulses of light. Extremely
fast.
– Non cable methods such as satellite, microwave, wireless and
bluetooth
21. • Network operating systems allow networks to be centrally
controlled by network administrators. The ability to centrally control
networks improves the security and efficiency of access to the
network’s various resources. It furthermore simplifies the tasks
performed by network administrators.
• Network operating systems operate at the network and above
layers of the OSI model. The NOS is installed on one or more servers
where it provides various services to secure and support the
network’s resources and users – one vital NOS service being the
authentication of users based on their user names and passwords.
22. Network Operating System Tasks
• Network administrators are the personnel responsible for the ongoing
maintenance of network hardware and software. This includes
installation and configuration of switches, routers and other active
hardware devices.
• Network administrators spend much of their time providing support
to new and existing users. This includes configuring new
workstations (clients) and controlling and monitoring access to
network resources as needs change.
• Network Operating System Tasks deals with:
– Directory (or folder) Permissions
– File Permissions
– Installation of software and sharing with users
– Client installation and protocol assignment
23. • Logon: login or logon refers to the credentials required to obtain
access to a computer system or other restricted area. Logging in or
on and signing in or on is the process by which individual access to a
computer system is controlled by identifying and authenticating the
user through the credentials presented by the user.
• Logoff: Once a user has logged in, they can then log out or log off
when access is no longer needed. To log out is to close off one's
access to a computer system after having previously logged in.
24. Intranets and Extranets
• Intranet
– private network (LAN or WAN) used to share resources in secure
environment
– uses web pages (HTML to view) and TCP/IP protocols (to make
connection)
• Extranet
– intranet that has been extended to include access to or from
selected external organizations such as customers, but not
general public.
