Cabletron Systems 3Com Network Control Protocol NCP as described in, PPP Extensions for Bridging

Theory of Operation 19

The DB-9,9-pin,RS-232 serial port provides a User Interface (UI) or a Point-to-Point Protocol (PPP) connection. The UI provides basic management tools for the AP. The PPP provides a link between APs using a serial connection. The serial link supports short haul (direct serial) or long haul (telephone line) connections. The AP is a Data Terminal Equipment (DTE) device with male pin connectors for the RS-232 port. Connecting the AP to a PC requires a null-modem cable; connecting the AP to a modem requires a straight-through cable.

Bridging Support The AP PPP interface, accessible from the serial port at the rear of the AP, provides two types of bridging operations: Internet Protocol (IP) bridging between an AP and a computer, and between two APs (with one AP connected to a LAN). To establish an Internet Protocol bridge with an AP, ensure that the computer includes the appropriate Telnet software with PPP and TCP/IP protocols. Using Telnet, a remote computer can connect to any AP on an Ethernet network, as long as data transfers through IP packets.

A PPP link provides the option of using a direct serial link or modem to extend wired Ethernet topologies. Once in PPP mode, the AP automatically attempts to communicate with the other device using the Data-Link Bridging (DLB) protocol. An AP using DLB communicates on the MAC level, and receives and transmits Ethernet frames.

If the other device does not support DLB, the AP attempts to communicate using Internet Protocol Control Protocol (IPCP). An AP using IPCP communicates on the IP level, and receives and transmits IP packets.

The PPP implementation in the AP uses the Link Control Protocol (LCP) and

Network Control Protocol (NCP) as described in:

■RFC 1171: the Point-to-Point Protocol, July 1990.

■RFC 1220: PPP Extensions for Bridging, April 1991.

■RFC 1332: The PPP Internet Protocol Control Protocol, May 1992.

■RFC 1661: The Point-to-Point Protocol, July 1994.

(RFCs are Requests For Comments used in Internet Communities.)

The AP database dynamically tracks wireless clients and APs on the PPP interface. Packets forward to the PPP link after the AP determines their destination.

The PPP implementation in the AP uses the NCP as described in RFC 1220: PPP Extensions for Bridging to encapsulate packets at the Ethernet level. The PPP provides IP bridging control as defined by RFC 1172 and MAC-level bridging. The protocol provides support for PPP negotiations conforming to RFC 1661. Users cannot plug a non-AP node directly into the AP serial port, only AP-to-AP PPP links.

For detailed information, refer to RFC 1171: The Point to Point Protocol and RFC

1220: PPP Extensions for Bridging.