Technical data

www.westermo.com Theoretical and general applications 119
This is why devices A and B have their own ARP-table of IP addresses and associated
MAC-addresses.
ARP Address Resolution Protocol, manages a dynamic update of the ARP-tables so
that the association between IP and MAC-addresses is always known.
… Assume that computer (A) wants to communicate with the PLC (B).
Computer (A) already knows (B’s) IP address (can e.g. have been manually con-
figured by an operator) but (B’s) MAC address is unknown to (A).
Communication can not begin until (A) knows (B’s) MAC-address.
… A discovers that B is on the same network by comparing the destination’s IP
address and the network mask.
… A sends out an ARP request in the form of a broadcast message. The enquiry
contains (A’s) IP and MAC address as well as B’s IP address.
… All units on the network understand the message, but only B recognises its IP
address and sends an ARP reply in response, which contains B’s MAC-address.
… A’s ARP-table can now be updated so that it also contains B’s MAC-address.
Point to Point (PPP)
There are also occasions when you need to connect and communicate using TCP/IP
via a serial connection. This concerns connections to the Internet via a modem or
when you need to connect to a local area network. How you communicate varies
from application to application. On these occasions you use the PPP protocol
(Point to Point Protocol.) which is without doubt the most used link protocol for com-
puters that remotely connect to a network. Examples of serial communications are:
telecom modem, modem with own leased line, ISDN, GSM, radio or short-haul
modems.
Security (CHAP and PAP)
The protocol PPP is frequently used for remote point to point connections, irrespec-
tive of whether it is a dial-up, ISDN or leased line. In general some form of security
between the communicating parties is required. PPP supports two methods of user
verification, PAP (Password Authentication Protocol) and CHAP (Challenge
Handshake Authentication Protocol) for this purpose. Authentication, verification of
messages, is not compulsory in PPP, so the parties are free to communicate without
identification or negotiating on which protocol to use. The principal rule is first and
foremost to choose CHAP. PAP is generally only chosen when one of the parties does
not support CHAP.
B A C K