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• access to Internet
• integration of LANs
• creation of trunk lines between PABX
• increase of subscribers’ lines with the help of Digital Loop Carrier systems
• high-speed access to SDH networks
4.2 Local area network integration. Access to Internet
Local area networks facilitate documentation-processing, access to data in modern companies
but the Ethernet technology does not allow to transfer data at long distances and create Wide
Area Networks (WANs). xDSL can be used for the solution of this problem.
4.2.1 TCP/IP stack structure
TCP/IP became widely practiced with the development of the Internet all over the world. It was
engineered earlier than the OSI model, and that is why differs greatly.
Fig.1 shows the TCP/IP structure.
TCP/IP protocols are composed of 4 layers:
Layer IV
The lowest layer (Layer IV) corresponds to the physical and data link layers of the OSI reference
model. This layer in TCP/IP is not regulated, but it supports all the popular physical and data link
layer standards: for LANs, this is Ethernet, for WANs, these are Point-to-Point Protocols, SLIP,
Frame Relay. However, when a new LAN and WAN technology appears, it is usually included
into TCP/IP stack because of a specially engineered request for comments (RFC), which
determines the encapsulation method of IP packets into its frames. Thus, for the encapsulation of
IP protocols into ATM cells, there was engineered a special RFC 1483 method. This method is
used in Black Box PAM modems as well.
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