Integrated Services Digital Network
PPP is often used with Integrated Services Digital Network (ISDN), which is a digital data-transport service offered by regional telephone carriers. ISDN supports the transmission of text, graphics, video, music, voice, and other source material over telephone lines. PPP provides data encapsulation, link integrity, and authentication for ISDN.
ISDN offers a cost-effective, remote-access solution for telecommuters and remote offices that require higher transmission speeds and quicker connection establishment than analog dialup links can offer. ISDN is also a good choice as a backup link for another type of link (for example, a Frame Relay link).
The cost of an ISDN circuit usually is based on a monthly fee plus usage time. Various options exist for saving money on an ISDN circuit. Dial-on-demand routing (DDR) lets a user keep an ISDN circuit idle until data must be transmitted to the remote site. Snapshot routing lets a router learn its routing table without periodic routing-table updates, thus allowing a circuit to remain idle for a long period of time. Spoofing allows a local router to respond to keepalive packets rather than sending the packets across the link. Keepalive packets are used in the Novell NetWare Core Protocol (NCP) and Sequenced Packet Exchange (SPX) protocols.
There are also various methods for aggregating ISDN bandwidth to meet the needs of applications with variable bandwidth requirements. For example, MPPP, which was covered in the PPP section earlier in this chapter, is often used to aggregate ISDN
channels.
An ISDN circuit consists of 64-Kbps bearer (B) channels that carry user transmissions, and a signaling channel that carries control information, such as call setup and teardown signals. The signaling channel is called the D channel.
Figure 11-3 shows the two types of ISDN interfaces, which are as follows:
• The Basic Rate Interface (BRI) is two B channels and one 16-Kbps D channel.
• The Primary Rate Interface (PRI) is 23 B channels and one 64-Kbps D channel in the United States, and 30 B channels and one 64-Kbps D channel in Europe and other parts of the world.
Figure 11-3. The ISDN Basic Rate Interface (BRI) and Primary
Rate Interface (PRI)
Figure 11-3. The ISDN Basic Rate Interface (BRI) and Primary
Rate Interface (PRI)
Telecommuters and remote offices typically use a device that supports BRI (for example, an ISDN BRI router). A central site, such as headquarters for a company, can use a PRI device to connect multiple telecommuters and remote offices. PRI offers an elegant solution for connecting many small sites because it requires only one physical connection to support 23 (or 30) channels.
Continue reading here: Terminal equipment type 2 TE2 NonISDN terminals that predate the
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