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   An Introduction to TCP/IP ...page 2

1.Introduction:

 An Internet consists of a set of connected networks. The chief advantage of an Internet is that it provides universal interconnection while allowing individual groups to use whatever network hardware is best suited to their needs. The technology TCP/IP provides the basis for the global Internet, which helps in connecting individuals, universities, corporation and government department in many countries around the globe. The global Internet is expanding rapidly.

 2.Underlying Network Technologies:

(i) Approaches to Network Communication:

 The two approached to network communications are

•  Circuit switched (Connection Oriented)

• Packet switched (Connectionless)

Circuit switched:

            Circuit switched network operates by forming a dedicated connection between two points.

Packet switched:

            In a Packet switched network, data to be transferred across a network is divided into small pieces called packets that are multiplexed onto high capacity intermachine connections

(ii) Wide Area and Local Area Networks

 The Packed switch technology is divided into two broad categories

•       Wide Area Network

•       Local Area Network

Wide Area Network:

            WAN technologies, sometimes called long haul networks, provide communication over large distances. Most WAN technologies do not limit the distance spanned. A WAN can allow the endpoints of a communication to be arbitrarily far apart. For example, a WAN can span a continent or can join computers across an ocean. Usually WANs operate at slower speeds that LANs, and have much greater delay between connections. The typical speed of WAN ranges from 56 Kbps to 155 Mbps. Delay across a WAN can vary from a few milliseconds to several tenths of second.

Local Area Networks:

            LAN technologies provide the highest speed connections among computers, but sacrifice the ability to span large distances. For example, a typical LAN spans a small area like a single building or a small campus and operates between 10 Mbps and 2 Gbps (Billion bits per second). Because LAN technologies cover short distances, they offer lower delays than WANs. The delay across a LAN can be as short as a few tenths of a millisecond, or as long as 10 milliseconds.

(iii) Ethernet Technology:

            Ethernet is the name given to a popular packet switched LAN technology; most medium or large corporations use Ethernet. The Ethernet is a 10/100 Mbps (Recently extending in to Gbps range) broadcast bus technology with distributed access control. It is a bus because all stations share a single communication channel; it is a broadcast because all transceivers receive every transmission.

 (iv) Fiber Distributed Data Interconnect (FDDI):

            FDDI is a popular local area networking technology that provides higher bandwidth than Ethernet. Unlike Ethernet and other LAN technologies that use cables to carry electrical signals, FDDI uses glass fibers and transfers data by encoding it in pulses of light. FDDI has ability to detect and correct network problems, such as a break in the network. The network is called Self-healing because the hardware can automatically accommodate failure

 (v) Asynchronous Transfer Mode (ATM):

            ATM is a high-speed connection oriented networking that has been used in both local area and wide area networks. ATM can switch data at gigabit speeds. To achieve high transfer speeds, an ATM network uses special-purpose hardware and software techniques.

3. Internet Architecture

            An Internet is more than a collection of networks interconnected by computers. Internetworking implies that the interconnected systems agree to conventions that allow each computer to communicate with every other computer.

The networks interconnect to form an internetwork.  Then, how a packet flows from one network to another? Physically, a device that attaches to both of them can only connect two networks.  Devices that interconnect two networks and pass packets from one to the other are called Internet gateways or routers. A router reads the destination address, and routs the packet to the destination.

Consider an example consisting of two physical networks shown as below. In the figure, router R connects to both network 1 and 2. For R to act as a router, it must capture packets on network 1 that are bound for machines on network 2 and transfer them. Similarly, R must capture packets on network 2 that are destined for machines on network 1 and transfer them.

In the above figure, two physical networks interconnected by R, a router (IP gateway). In a TCP/IP Internet, computers called routers or gateways provide all interconnections among physical networks. Routers use the destination network, not the destination host, when routing a packet.

4. Internet Addresses:

           Each host on a TCP/IP Internet is assigned a unique 32-bit Internet address that is used in all communication with that host. Conceptually, each IP address consists of a network id that identifies a network, and host id identifies a host on that network. Given an IP address, its class can be determined from the three high-order bits, with two bits being sufficient to distinguish among the three primary classes.

   (i) ARP (Mapping Internet Addresses to Physical Addresses)

        Address Resolution Protocol, ARP, allows a host to find the physical address of a target host on the same physical network, given only the target’s IP address.  

        ARP is a low-level protocol that hides the underlying networks physical addressing, permitting one to assign an arbitrary IP address to every machine. ARP is a part of physical network system, and not as part of Internet protocols.

  (ii) RARP (Reverse Address Resolution Protocol)

             At system startup, a computer that does not have a disk (diskless computers) must contact a server to find its IP address before it can communicate using TCP/IP. It is found that the RARP protocol that uses physical network addressing to obtain that machine’s Internet address. The RARP mechanism supplies the target machine’s physical hardware address to uniquely identify the processor and broadcasts the RARP request. Servers on the network receive the message, look up the mapping in a table, and reply to the sender. Once a machine obtain its IP address in memory and does not use RARP again it reboots.