1. Communications Processors and Software
Front-end processor => A small computer dedicated to communication management. It is attached to the main or host computer in a network. It performs such special communications processing as error control, formatting, editing, controlling and routing, and speed and signal conversion.
Concentrator => A programmable telecommunication computer that collects and temporarily stores messages from terminals for batch transmission to the host computer.

2. Communications Processors and Software
Controller => A specialized computer that supervises communications traffic between the CPU and the peripheral devices such as terminals and printers. It routes output from the CPU to the appropriate peripheral device
Multiplexer => A device that enables a single communications channel to carry data transmissions from multiple sources simultaneously. The multiplexer divides the communications channel so that it can be shared by multiple transmission devices. The multiplexer can divide a high-speed channel into multiple channels of slower speed .

3. Private Branch Exchanges
PBX is a special-purpose computer designed for handling and switching office telephone calls at company site.
PBX can carry voice and data to create local networks.
PBX can store, transfer, hold, and redial telephone calls.
PBX can switch digital information among computers and office devices.
The advantage of digital PBX over local networking is that PBX does not need special wiring.

4. Local Area Network Server Network Operating System Computer1 Network
Gateway
Other
Network
Computer1
Printer
Computer1

5. Local Area Network
LAN is used to connect PCs or resources (e.g. printers) in one building or several buildings in close proximity.
LAN has higher transmission capacity than PBX (Private Branch Exchange).
LAN can transmit video and graphics.
LAN is more expensive to install than PBX and less flexible. It requires new wiring each time a LAN is moved.
The server acts as a librarian. It stores programs and data files for network users. The server determines who will get access to what and in what sequence.
Sever can be powerful PCs with large hard-disk capacity, workstations, minicomputers, or mainframes.
The network gateway connects the LAN to public networks, such as the telephone network, or to other corporate networks.

6. Local Area Network
A gateway is a communications processor that can connect dissimilar networks by translating from one set of protocols to another.
LAN can use twisted wire, coaxial , or fiber-optic cable.
LAN also can use wireless technology.
The network operating system (NOS) can reside on every computer in the network, or it can reside on a single server.
The NOS routes and manages communications on the network and sharing of network resources.

7. Wide Area Network
It spans a broad geographical distance, ranging from several miles to the span of entire continent.
Wan may consists of a combination of switched and dedicated lines, microwave, and satellite communications.
Switched lines are telephone lines that a person can access from his or her terminal to transmit data to the designated destination.
Dedicated lines are continuously available for transmission. This lines can be leased or purchased from a common carriers or private communications media vendors.
Most WANS are switched.

8. Value-Added Networks (VANS)
Value-added networks are private, multipath, data-only, third-party-managed network that is used by multiple organizations on a subscription basis.
The VAN is set up by a firm that is in charge of managing the network.
The subscribers pay only for the amount of data they transmit plus a subscription fee.
Customers do not have to invest in network equipment and software or perform their own error checking, editing, routing, and protocol conversion.
The network can use twisted-pair lines, satellite links, and other communication channels leased by the value-added carrier.

9. Packet Switching (16 bits) (24 bits) (16 bits) Start Framing End
Error
Control
(64 bits)
Check
Bits
Header
Text
Message
No.
Link
No.
Destination
Source
Packet No.
Data are grouped into small packets, framed by identifying information,
which are transmitted independently via various communication
channels to maximize the potential of the paths in a network.

10. Frame relay A shared network service.
It is faster and less expensive than packet switching. It can achieve transmission speed upto megabits per second.
Frame relay packages data into frames that are similar to packets.
But it does not perform error correction.
It works well on reliable lines that do not require frequent retransmission because of error.

11. Asynchronous Transfer Mode (ATM)
ATM technology parcels information into uniform cells, each with 53 groups of eight bytes, eliminating the need for protocol conversion.
It can seamlessly and dynamically switch voice, data, images, and video between users.
It can pass data between computers from different vendors.
It permits data to be transmitted at any speed the network handles. ATM can transmit up to 2.5 GBPS.
It can connect LAN and WAN together more easily.

12. Internetworking, Connectivity & Open System
Internetworking => The linking of separate networks, each of which retains its own identity, into an interconnected network.
Connectivity => The ability of computers and computer-based devices to communicate with one another and share information in a meaningful way without human intervention.
Open Systems => Open systems promote connectivity because they can operate on different hardware platforms. They are built on public non-proprietary operating systems, user interfaces, application standards, and networking protocols.

13. Models of Connectivity for Networks: TCP/IP
Host A
Host B
Application
Application
Identical
message
Transport (TCP)
Transport (TCP)
Identical
message
Internet Protocol
(IP)
Internet Protocol
(IP)
Identical
datagram
Network
Interface
Network
Interface
Identical
frame
Physical net

14. Open System Interconnect (OSI)
This model is an alternative model developed by the International Standards Organization for linking different types of computers and networks.
It is designed to support global networks with large volume of transaction processing.
Like TCP/IP , OSI enables a computer connected to a network, regardless of the manufacturer, by establishing communication rules that permit the exchange of information between dissimilar systems.
OSI divides the telecommunications process into seven layers.

15. Electronics Commerce & its applications
Electronic mail
Voice mail
Facsimile Machine (fax)
Digital Information Services
Teleconferencing, Dataconferencing, and Videoconferencing
Groupware: The leading commercial groupware product has been Lotus Notes from Lotus Development Corporation.
Group writing and commenting, distribution, scheduling meeting and appointments, shared files and databases, shares timelines and plans, electronic meeting