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.

Physical Layer In the Network

Understanding the Role of the Physical Layer
The name “physical layer” can be a bit problematic. Because of that name, and because of what I just said about the physical layer actually transmitting data, many people who study networking get the impression that the physical layer is only about actual network hardware. Some people may say the physical layer is “the network interface cards and cables”. This is not actually the case, however. The physical layer defines a number of network functions, not just hardware cables and cards.

A related notion is that “all network hardware belongs to the physical layer”. Again, this isn't strictly accurate. All hardware must have some relation to the physical layer in order to send data over the network, but hardware devices generally implement multiple layers of the OSI model, including the physical layer but also others. For example, an Ethernet network interface card performs functions at both the physical layer and the data link layer.

Physical Layer Functions
The following are the main responsibilities of the physical layer in the OSI Reference Model:

Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer (although also partially the data link layer; see below).

Encoding and Signaling:  

The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.

Data Transmission and Reception: 

After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it. Note that this applies equally to wired and wireless networks, even if there is no tangible cable in a wireless network!

Topology and Physical Network Design: 

The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology.In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output. Devices like switches and routers operate at higher layers and look at the data they receive as being more than voltage or light pulses that represent one or zero.