PPP in the Washington School District WAN

            In data communications across LANs, data is encapsulated as it moves down the OSI protocol stack.  Similarly, data across a WAN must be encapsulated

in order to traverse the network.  The OSI encapsulation used for LAN packets is insufficient for use over WAN links, so different encapsulation techniques have

been developed to meet the needs of these wide area links.  The most prominent are High Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP).

Since PPP is the all around superior encapsulation format, the Washington School District has asked that it be used.
 

            Before transmission along a data link, PPP makes sure that a link is available.  It does this through the use of Link Control Protocol (LCP) frames.

Basically, both ends of the WAN link send LCP frames to each other containing agreed upon rules for the communication session.  Some of these rules include the

maximum size of a packet to be sent along the link and the authentication technique the communicating partners will use.  After the rules of the link have been agreed

upon, the link is ready.
 

            What happens after the link is ready is really the heart of what makes PPP superior to other encapsulations (such as SLIP and HDLC).  With the link ready,

PPP can optionally take steps to check upon the quality of the link, to see if it is good enough to handle layer 3 protocols.  This second step of PPP also allows

security through Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).  With PAP, the sending router keeps ending

a login and password until the receiver acknowledges receipt of the correct login and password.  Hackers can attempt to break PAP authentication through

repeated tries, so PAP is fairly insecure.  CHAP, on the other hand, operates with limited chances for the remote router to authenticate itself.  Through CHAP, the

router being accessed sends a challenge of sorts, asking the remote accessing router "What is the correct login and password?".  The remote router then responds

with a value.  If the remote router responds correctly, it is authenticated, otherwise, the link is terminated.  This authentication is unavailable in older WAN

encapsulations, so it furthers the superiority of PPP.
 

            With authentication and link quality testing completed, the PPP link is ready to negotiate network layer protocol types to be sent.  By sending Network

Control Protocol (NCP) packets back and forth, nodes using the PPP WAN link become aware of which network layer protocols they can send.  Different NCPs

are needed to send different protocols, so if a node wished to send IP and IPX packets across the link, it would need separate NCP packets for both protocols.
 

            Basically, PPP has become the protocol of choice for WAN connections, as it offers the following features:
 

• Control of link setup through the PPP link establishment phase.

• Multiplexing--that is, sending several different network layer protocols over a WAN link at once--through the use of NCP packets.

• Error detection and correction in the optional link quality establishment phase.

• Negotiation of a link's capabilities through the LCP packets in the link establishment phase.

            As it has already been established that PPP will used to encapsulate data over the WAN link, documentation of the commands necessary to implement PPP

is all that is left.  To enable PPP on the routers of the Washington network, the following steps need to be taken:
 

1. Login to the router through the console port, telnet, or remotely through the auxilary port.

 
2. Enter privileged mode with the enable command.

 
3. Enter global configuration mode with the configure terminal command.

 
4. Since PPP is to be implemented on the WAN link, it will have to be set on the serial interface that connects to the nearest regional hub.  For Acacia, this will be the Serial 0 interface.  To configure PPP on this interface, enter interface configuration mode with the interface serial 0 command.

 
5. Within interface configuration mode, enter the command encapsulation ppp to set PPP as the interface's encapsulation type.

 
6. Exit interface configuration mode by hitting Ctrl Z.

 
7. Enter the command copy run start to copy the modified configuration into NVRAM.

        Once these steps have been taken, PPP should override the default WAN  encapsulation of HDLC and become the new encapsulation method for the serial

interface leading out to the Washington School District WAN.  The configuration file will change very little.  There is nothing more than the additional line

"encapsulation PPP" under the information for interface serial 0.