WAN Terminology
Now that you have a working scenario with r1 to r6, I want to discuss WAN concerns and terminology a bit more. The Telecommunications Act of 1996 drastically changed the way of the WAN. However, there are still three main concerns when connecting your sites:
Availability
Bandwidth
Cost
Whether you select leased lines, circuit-switched networks, packet-switched networks, cell-switched networks, Digital Subscriber Lines (DSL), cable modems, or wireless, you must interface with the provider(s). Interfacing with the provider is a big part of supporting the WAN. The central office (CO) is the entry point of the cloud for calling devices and the exit point of the cloud for called devices. It is the switching point for calls that traverse the service provider's toll network. The last mile or local loop extends from the demarcation point (demarc) to the CO. Essentially the demarc is known in the support world as the “line of blame.” Customer premises equipment (CPE) resides at the customer location, although it may be owned by the subscriber or leased from the provider. More than one provider may be involved for your various primary, sectional, regional, and international trunks and switches. Knowing who to call for what is an important part of troubleshooting methodology.
Use Figure 9-2 to review the DTE/DCE specifications and Table 9-2 to review WAN connection types and encapsulations.
Figure 9-2. Serial DTE/DCE Connections
NOTE
Although the lab scenarios and Trouble Tickets you work through in this book (and in any lab for that matter) make use of back-to-back serial cables, in a practical environment you must order the appropriate cable. One end is the standard DB60, but the other end could be EIA/TIA-232, EIA/TIA-449, EIA/TIA-530, V.35, X.21, and so on. Figure 9-2 shows an example of the Physical Layer DTE/DCE requirements in the lab compared to the practical application of such.
| Layer 1 Connection Types | Layer 2 Encapsulations | Examples |
|---|---|---|
| Leased line Synchronous serial | HDLC
PPP SLIP[*] | Point-to-point or dedicated connections (pre-established path)
Private use No service provider (no cloud) |
| Circuit switched
Asynchronous serial Synchronous serial (legacy 56 kbps) | HDLC
PPP SLIP |
Phone call (dedicated for the call duration)
ISDN (totally synchronous) Service provider |
| Packet switched Synchronous serial | Frame Relay
ATM X.25 | Store and forward
Service provider (Share physical connections to reduce cost, virtual circuits) |
[*] SLIP = Serial Line Internet Protocol
HDLC is the first of this chapter's specific WAN topics I want to focus on. The section starts with a brief overview, then looks at the layers, and finishes up with shooting HDLC troubles.