ELSA LANCOM Business 6000, Manual

Page: 120 / 174

Technical basics
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ELSA LANCOM Business 6000
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120
EN
7.2.2 Expansion through local networks
Medium Access
Control
Up to now we have only considered the point-to-point connections. However,
many computer networks are based on multipoint cabling such as Ethernet.
All computers connected to the same network can then receive the signals of
all other computers (so-called broadcast transfer to a shared medium). If
several computers are sending simultaneously, the superimposed signals are
destroyed. A variety of access methods such as CSMA/CD or Token Ring are
implemented in the MAC layer (
M edia A ccess C ontrol, MAC) for the
avoidance and resolution of such collisions.
LAN and IP network The connection of all computers communicating through a shared medium
using a MAC protocol is called a LAN. A LAN forms an independent network
and is subordinate to the IP network, i.e. IP networks can use the physical
connections of the LAN to establish connections between the hosts and the
routers. A LAN—Local Area Network—is, as the name implies, restricted in
its area.
MAC-address Specific LAN addresses hardwired into the interfaces by their manufacturers
are used to manage the transfer in the LAN. Since the LAN addresses are
used for communication via the MAC protocol, they are called MAC
addresses. They can be thought of as the fingerprint of the interface
hardware. MAC addresses can look like this, for example: 00-80-C7-6D-A4-
6E.
MAC addresses are independent of IP addresses. An IP host whose interface
works through a LAN has an IP and a MAC address. Whereas the structure of
IP addresses with its similarity to postal addresses is supposed to simplify
routing in enormous IP networks, the fingerprint-like MAC addresses are
designed to make the connection to a LAN as easy as possible.
Transfer in LAN is also packet-oriented. Every MAC packet contains the MAC
addresses of the source and of the recipient. Although every packet is
received by all computers, it is processed only by the target computer. There
is an additional MAC broadcast address that is processed by all computers in
the LAN.
IP in the LAN Every LAN packet contains an entry with the type of the network protocol. An
IP packet can be transferred through a LAN by packing it in a LAN packet and
adding the 'IP' protocol type to it. Because of the IP entry, the LAN interface
at the receiving host recognizes that the LAN packet contains an IP packet,
extracts it and processes it as an ordinary IP packet. In this way, IP packets
and packets of other network protocols like IPX can be transferred