command lists output for call setup and disconnect; the output in the example shows output
typical of what happened on SanFrancisco when the call to LosAngeles was made. The debug
dialer events and debug dialer packets commands provide similar information when a packet
is a candidate for causing the dial to occur--in other words, when a packet is routed out the dial
interface.
Certainly, Cisco requires CCNAs to have a solid foundation of the WAN technologies in this
chapter. Cisco also expects CCNAs to be able to compare and contrast these different WAN
technologies. This section summarizes many of the concepts found earlier in this chapter, with
a focus on comparison.
synchronous serial leased lines and packet switching services. PPP, HDLC, and LAPB are
the three data link protocols most typical on leased lines, with Frame Relay being the most
pervasive packet-switched service. In fact, Frame Relay is better named a frame switching
service to imply that the protocol is a Layer 2 protocol, but when talking similar services as a
group, packet switching is the typical term used. X.25 and ATM services also fall into the packet
switching category.
to the documents referenced in Appendix B, "Decimal to Hexadecimal and Binary Conversion
Table.") X.25 is very similar to Frame Relay because it uses VCs and has error-recovery built
in to each link and each end-to-end VC. ATM is similar in its use of virtual connections, which
are conceptually equivalent to VCs. However, ATM includes the concept of segmentation and
reassembly (SAR), in which the device at the edge of the ATM network breaks the frames to be
sent into smaller cells (53 bytes) that are reassembled at the other end of the VC. The details of
how each of these three packet-switching protocols are implemented is different; however, each
creates a multiaccess network, with direct packet forwarding allowed only between pairs of
devices that have a VC between them.
and weaknesses.