NETWORK PROTOCOLS

To avoid chaos in computer communications, rules must be established for the exchange of data from one site to another. These rules are known as line protocol. Communications software packages control the speed and mode of communications between computer systems.

Many different standard network protocols exist to perform addressing, routing, and packetizing. All provide formal definitions for how addressing and routing is to be executed, and specify packet structures to transfer this information between computers.OSI, TCP/IP, IPX/SPX, and X.25 are commonly used routing protocols.

Open Systems Interconnection (OSI):

A major problem of early networked computer systems was that a lack of consistency existed among the protocols of different types of computers. Consequently, various efforts have resulted in the establishment of standards for data transmission protocols. For example, the International Standards Organization (ISO) developed a set of standard protocols called the Open Systems Interconnection (OSI). The OSI model separates each network's functions into seven layers of protocols, or communication rules. This model identifies functions that should be offered by any network system.It is important to note that the physical layer, data link layer, and network layer appear in the user and host computers as well as units such as the front-end processor and the cluster control unit. The remaining layers appear only in the user and host computers.

TCP/IP:
TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of communications protocols developed for internetworking dissimilar systems. This is supported by many hardware vendors from microcomputers to mainframes. It is used by most universities, federal governments, and many corporations. TCP/IP has two parts. TCP protocol controls data transfer that is the function of the transport layer in the OSI model. IP protocol provides the routing and addressing mechanism that are the roles of the network layer in the OSI model.
The TCP/IP may be the oldest networking standard, and is also the most popular network protocol, used by almost 50 percent of all installed backbone, MAN (metropolitan area network), and WAN (wide area networks). TCP/IP is widely compatible with many other protocols. Although TCP/IP supports many protocols, it is usually associated with Ethernet. TCP/IP is also the network protocol used on the Internet.

IPX/SPX:
IPX/SPX (lnternetwork Packet Exchange/Sequenced Packet Exchange) is a Novell NetWare communications protocol used to route messages from one end to another. It is the major network protocol used by Novell NetWare, and about 40 percent of all installed LAN (local area networks) use this protocol.
IPX/SPX has two parts, and is similar to TCP/IP. SPX controls the transport layer in the OSI model. It guarantees that an entire message arrives intact. IPX manages the role of the network layer in the OSI model and is used as delivery mechanism for SPX. IPX/SPX can be linked with many other protocols.

X.25:
X.25 is a CCITT standard developed by ITU- TSS for WAN (wide area networks). It defines the interface between an end user computer and packet switching network. This is an international standard used for many worldwide corporations. It also has two parts. Packet layer protocol (PLP) is the routing protocol that manages the network layer and X.3 controls the transport layer.

Data Link Layer Definition

The data link layer is the second layer in the OSI seven-layer reference model. It responds to service requests from the network layer above it and issues service requests to the physical layer below it.

The data link layer is responsible for encoding bits into packets prior to transmission and then decoding the packets back into bits at the destination. Bits are the most basic unit of information in computing and communications. Packets are the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well.

The data link layer is also responsible for logical link control, media access control, hardware addressing, error detection and handling and defining physical layer standards. It provides reliable data transfer by transmitting packets with the necessary synchronization, error control and flow control.

The data link layer is divided into two sublayers:

The media access control (MAC) layer and the logical link control (LLC) layer. The former controls how computers on the network gain access to the data and obtain permission to transmit it; the latter controls packet synchronization, flow control and error checking.

The data link layer is where most LAN (local area network) and wireless LAN technologies are defined. Among the most popular technologies and protocols generally associated with this layer are Ethernet, Token Ring, FDDI (fiber distributed data interface), ATM (asynchronous transfer mode), SLIP (serial line Internet protocol), PPP (point-to-point protocol), HDLC (high level data link control) and ADCCP (advanced data communication control procedures).

The data link layer is often implemented in software as a driver for a network interface card (NIC). Because the data link and physical layers are so closely related, many types of hardware are also associated with the data link layer. For example, NICs typically implement a specific data link layer technology, so they are often called Ethernet cards, Token Ring cards, etc. There are also several types of network interconnection devices that are said to operate at the data link layer in whole or in part, because they make decisions about what to do with data they receive by looking at data link layer packets. These devices include most bridges and switches, although switches also encompass functions performed by the network layer.Data link layer processing is faster than network layer processing because less analysis of the packet is required.