UNIX System Administration Handbook - Evi Nemeth [229]
Table 15.3 ATM adaptation layers
It is unclear how AAL 2 would ever be used in real life. Currently, there is no defined standard for it. AALs 3 and 4 turned out to be very similar and were combined. A group of vendors that had to implement ATM were unhappy with AALs 3 and 4 because of their high overhead. They developed their own solution, the Simple and Efficient Adaptation Layer (SEAL), which soon became AAL 5.
15.5 FRAME RELAY: THE SACRIFICIAL WAN
Frame relay is a WAN technology that offers packet-switched data service, usually for a reasonable cost. Although the claim is not 100% accurate, frame relay is often said to be remarketed X.25, a scary packet-switched technology from the mid-1970s. Fortunately, it’s in such widespread use that the equipment, software, and staff that support it have evolved to be robust and to perform well.
Traditionally, users who wished to connect to remote sites would purchase a dedicated circuit from the phone company, such as a 56 Kb/s DDS line or a T1 line. These are point-to-point data circuits that are connected 24 hours a day. Unfortunately, this type of connection is often expensive, as it requires that the phone company dedicate equipment and bandwidth to the link.
In contrast, frame relay is an “economy of scale” approach. The phone company creates a network (often referred to as a “cloud”6
) that connects its central offices. Users such as yourself provide data for remote sites in small packets. The phone company switches the packets through the appropriate central offices, ultimately delivering them to their destinations. In this model, you and the phone company are gambling that at any given second, the total amount of traffic won’t exceed the bandwidth of the network (a condition known euphemistically as “being oversubscribed”).
A router encapsulates IP traffic over frame relay connections. Packets are switched over invisible “permanent virtual circuits” (PVCs), which allow your packets to travel only to the sites you’ve paid for them to reach. These PVCs provide some degree of privacy protection from the other sites connected to the frame relay network.
The biggest advantage of frame relay is that it is usually inexpensive. But in the world of “you get what you pay for,” you may find that frame relay’s performance is sometimes poor. Frame relay connections have some packet switching overhead, and link speed may degrade during periods of heavy use.
15.6 ISDN: THE INDIGENOUS WAN
Integrated Services Digital Network (ISDN) is a phone company offering that takes many forms. In its most common and usable form, called Basic Rate Interface (BRI) ISDN, it is essentially an all-digital phone line that provides two dial-up 64 Kb/s “B” channels and a single 16 Kb/s signaling “D” channel. Each B channel can be used for either voice or data (a voice line can be carried on a single 64 Kb/s channel).
ISDN offers a relatively high-speed digital line at a reasonable cost ($30–$150 per month, depending on where you live). Devices called terminal adaptors convert the phone line into a more familiar interface such as RS-232. They are used (and priced) much like modems. Most adaptors can aggregate the two B channels, yielding a 128 Kb/s data channel.
ISDN can be used in place of normal dial-up networking and also as a wide-area technology that uses a router or bridge to connect remote sites across the line.
Although many U.S. phone companies have installed switches that are compatible with ISDN, they still haven’t figured out how to market or support them.7
Only in a few areas can you just call up the phone company and order an ISDN line. Some tips: make sure you deal with the branch of the phone company that handles business services, since that is how ISDN is usually classified. In many regions, you will have to argue your way past several waves of drones before you reach someone who has heard of ISDN before, even if the service really is available.
15.7 DSL: THE PEOPLE’S WAN
It’s easy to move large amounts of data among