until now, we assumed to a Network means homogeneous, which consists of number of machines all are using same protocol in each layer.
But in real a number of networks like LANs, WANs with numerous protocols in each layer are connected together to form internet.
The main reasons for internetworking are:
- Some computers use TCP/IP, but some may use IBM’s SNA and a substantial number of telephone companies uses ATM N/W’s and some are still using Novell NCP/IPX (or) Apple talk.
i.e, The installed base of different N/W’s is large.
- The cost of computers and N/W’s cheaper there is a possibility of having more N/W’s and protocols.
Let’s see an example, How Networks are connected?
In this figure we have wide area ATM network , FDDI ring, wireless LAN and SNA’s main frame Network all are connected together.
The main reason for interconnecting all these Network’s is to
- Allow users in one network will communicate with users in other networks.
- and also, to access data (from one network to other).
This happens when we exchange packets between number of networks (which is not a simple task).
How Networks differ?
Networks may differ in different ways,
i.e, at physical layer- The modulation techniques differ at DLL – frame formats.
we just see here the differences in network layer.
when a sender in one network need to send packets to different Networks at the interfaces between Networks many problems may occur.
when packets from a CO-N/W to CL-N/W flows, we may require protocol conversion.
In some cases, Address conversions also be needed (i.e, a kind of directory system is required). Passing Multicast packets to a Network, which may not support Multicasting is a problem.
Packet size in different N/W’s is a big issue, a N/W supports a packet of size 8000 bytes when given to a N/W packet size as 1500 bytes.
i.e, The problems we may face when N/W’s differ are listed below.
