Leased-Line PPP Overview (System Administration Guide: Resource Management and Network Services)

System Administration Guide: Resource Management and Network Services

Leased-Line PPP Overview

A hardwired, leased-line PPP configuration involves two peers that are connected by a link that consists of a switched or unswitched digital service leased from a provider. Solaris PPP 4.0 works over any full-duplex, point-to-point leased-line medium. Typically, a company rents a hardwired link from a network provider to connect to an ISP or other remote site.

Comparison of Dial-Up and Leased-Line Links

Both dial-up and leased-line links involve two peers that are connected by a communications medium. The next table summarizes the differences between the link types.

Leased Line	Dial-up Line
Always connected unless a system administrator or power failure takes it down	Initiated on demand, when a user tries to call a remote peer
Uses synchronous communications	Uses asynchronous communications
Rented from a provider	Uses existing telephone lines
Requires synchronous units	Uses less costly modems
Requires specialized interfaces	Uses standard serial interfaces that are included on most computers

Parts of a Leased-Line PPP Link

Figure 29-3 Basic Leased-Line Configuration

The graphic shows the parts of a leased-line-link, which are described in the following context.

The parts of the leased-line link include:

Two peers, one at each end of the link. Each peer might be a workstation or server. Often the peer functions as a router between its network or the Internet, and the opposite peer.
Synchronous interface on each peer. Some machines that run Solaris software require you to purchase a synchronous interface card, such as HSI/S, to connect to a leased line. Other machines, such as UltraSPARC^TM workstations, have built-in synchronous interfaces.
CSU/DSU synchronous digital unit on each peer, which connects the synchronous port to the leased line.

A CSU might be built-in to the DSU, or owned by you, or leased from a provider, depending on your locale. The DSU gives the Solaris machine a standard synchronous serial interface. With Frame Relay, the Frame Relay Access Device (FRAD) performs the serial interface adaptation.
Leased line, providing switched or unswitched digital services. Some examples are SONET/SDH, Frame Relay PVC, and T1.

Note -

SONET is called an octet synchronous link. PPP uses a framing mechanism similar to asynchronous framing over a SONET line. PPP does not use the expected bit-synchronous protocol.

What Happens During Leased-Line Communications

On most types of leased lines, peers do not actually dial each other. Rather, a company purchases a leased-line service to explicitly connect between two fixed locations. Sometimes the two peers at either end of the leased line are at different physical locations of the same company. Another scenario is a company that sets up a router on a leased line that is connected to an ISP.

Leased lines are less commonly used than dial-up links, though the hardwired links are easier to set up. Hardwired links do not require chat scripts. Authentication is often not used because both peers are known to each other when a line is leased. After the two peers initiate PPP over the link, it stays active unless the leased line fails or either peer explicitly terminates the link.

A peer on a leased line that runs Solaris PPP 4.0 uses most of the same configuration files that define a dial-up link.

The following process occurs to initiate communication over the leased line:

Each peer machine runs the pppd command as part of the booting process or other administrative script.
The peers read their PPP configuration files.
The peers negotiate communications parameters.
An IP link is established.