Basics of Data Communication Part: 2

Data Communication Networking

Data Communication takes place between two devices that are directly connected by some form of point-to-point transmission medium. The devices are very far apart. It would be inordinately expensive if string a dedicated link between two devices which is thousands of miles apart. There is set of devices used to establish such networks. There are following types of networking available:

Wide Area Network (WAN)

Wide area networks have traditionally been considered to be those that cover a large geographical area. Typically, a WAN consists of a number of interconnected switching nodes. Transmission from any one device is routed through these internal nodes to the specific destination device. WAN has been implemented using one of the following technologies:

Circuit Switching

In circuit-switching, this path is decided upon before the data transmission starts. The system decides on which route to follow, based on a resource-optimizing algorithm, and transmission goes according to the path. For the whole length of the communication session between the two communicating bodies, the route is dedicated and exclusive, and released only when the session terminates.

Packet Switching

In packet-switching, the packets are sent towards the destination irrespective of each other. Each packet has to find its own route to the destination. There is no predetermined path; the decision as to which node to hop to in the next step is taken only when a node is reached. Each packet finds its way using the information it carries, such as the source and destination IP addresses.

Frame Relay

Frame relay was developed to take advantage of these high data rates and low error rates. Whereas the original packet switching networks were designed with a data rate to the end user of about 64kbps, frame relay networks are designed to operate efficiently at user data rates of up to 2Mbps.

ATM

Asynchronous Transfer Mode (ATM) can be viewed as an evolution from frame relay. The most obvious difference between frame relay and ATM is that frame relay uses variable-length packets, called frames, and ATM uses fixed-length packets, called cells. As with frame relay, ATM provides little overhead for error control, depending on the inherent reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed packet length, the processing overhead is reduced ever further for ATM compared to frame relay. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, compared to the 2Mbps target of frame relay.

Local Area Network (LAN)

LAN is a communication network that interconnects a variety of devices and provides a means for information exchanged among those devises. The scope of the LAN is small, typically a single building or a cluster of buildings. It is usually the case that the LAN is owned by the same organization that owns the attached devices. The internal data transfer rates of the LANs are greater than WANs. There are various topologies are possible like Bus, Right etc.

Ethernet

In Bus topology, at any instance one machine is the master and is allowed to transmit. An arbitration mechanism is needed to resolve conflicts when two or more machine wants to transmit simultaneously. The arbitration mechanism may be centralized or distributed. IEEE 802.3 CSMA/CD popularly known as Ethernet, is a bus based broadcast network with decentralized control operating at 10 or 100 Mbps. Computers on an Ethernet can transmit whenever they want to. If two or more packets collide, each computer just waits a random time and tries again later to send.

Token Ring

Unlike Ethernet, Token Ring uses a ring topology whereby the data is sent from one machine to the next and so on around the ring until it ends up back where it started. It also uses a token passing protocol which means that a machine can only use the network when it has control of the Token; this ensures that there are no collisions because only one machine can use the network at any given time.

ISDN and Broadband ISDN

Through Integrated Service Digital Network (ISDN) user with a single access point to ISDN network can avail of different kinds of communication like his computer can access the internet, he can use the network for his telephone usages and also probably video communication. The ISDN is intended to be a worldwide public telecommunications network to replace existing public telecommunications networks and deliver a wide variety of services.
The second generation referred to as Broadband ISDN, supports vary high data rates (100s of Mbps) and has a packet switching oriented.

Protocols and Protocol Architecture

To transfer any file or say data between two or more computers there must be a path either directly or any communication network known as computer communication. Similarly when two or more computers are interconnected via a communication network referred as computer network. Computer communication and computer network has some protocols and communication architecture.

A protocol is set of rules for communicating between computers. Protocol includes the key like data format, timings, sequencing, error controls etc. Without these rules, the computer can not make sense of the stream of incoming bits.

Basically, protocol is software that resides either in a computer's memory or in the memory of a transmission device, like network interface card. When data is ready for transmission, this software is executed.

Two protocol architectures have served as the basis for the development of interoperable communications standards: the TCP/IP protocol suite and the OSI reference model. TCP/IP is the most widely used interoperable architecture, and OSI has become the standard model for classifying communication functions. Here is the brief introduction of them.



The OSI Protocol Architecture

Open System Interconnection (OSI) includes set of protocols that attempt to define and standardize the data communication process defined by International Standardization for Organizations. The OSI model has seven layers as discussed below.

1. Physical Layer
   
   Physical Layer deals with the hardware level like, transmission media, connections and the voltage for digital signals. In other word physical layer provides the electrical and mechanical interface to the network medium (cables).


2. Data Link Layer
   
   Data Link Layer deals with physical transfer, framing (the assembly of data bits into single unit), flow control and error control functions. It is responsible for getting the data packaged for the physical layer. Data Link layer subdivided into two parts LLC (Logical Link Control) and MAC (Medium Access Control).
   
3. Network Layer
   
   Network Layer is very important layer in OSI Protocol Architecture. Network layer deals with the transfer of data in the form of packets over the communication networks. A key aspect of this transfer is the routing of packets from the source to destination machine. Routing is the process by which a path is selected out of many available paths to the destination so that data packets reach the destination fast, efficiently, reliably as required. Network layer is also responsible for translating logical address or names into physical (or data link) addresses.
   
4. Transport Layer
   
   Transport Layer ensures that data is successfully sent and received between two end nodes. If data is sent incorrectly, this layer has the responsibility to ask for re-transmission of the data. Also it ensures data are passes onto the upper layers in the same order in which they where sent. Specially, it provides a reliable, network-independent message interchange service to the top three application oriented layers.
   
5. Session Layer
   
   Session Layers decides when to turn communication on or off between two computers. It also deals with the programs running in each machine to establish conversations between them.
   
6. Presentation Layer
   
   Presentation Layer performs code conversation and data re-formatting (translating). It is translator of the network, making sure the data is in the correct form for the receiving application.
   
7. Application Layer
   
   This layer provides the interface between the software running in a computer and the network. It provides functions to the user's software, including file transfer access and management and electronic mail services.

TCP/IP Protocol Architecture

TCP/IP has no any official protocol model as there is in case of ISI model. OSI model where defined by International Standardization for Organizations (ISO). But in case TCP/IP, has no any such. However, based on protocol standards TCP/IP have been developed and it has five layers.

1. Physical Layer
   
   Physical Layer covers the interface between a data transmission device (computer) and a transmission medium or network.


2. Network Layer
   
   This layer covers the exchange of data between end systems and the network to which it is attached. The transmitter computer provide the network path and destination address.
   
3. Internet Layer
   
   Internet Protocol is used by this layer to provide the routing function across multiple networks.
   
4. Transport Layer
   
   This layer ensures that data is sent and received successfully or not. If any error occurred then it sends the re-transmission request.
   
5. Application Layer
   
   It provides user friendly interface between user and transmission devices. For example, file transfer, electronic mail services etc.