CCNA Routing and Switching

334 CCNA Routing and Switching ICND2 200-105 Official Cert Guide
Back in the 1950s and 1960s, the U.S.-based Bell companies developed and deployed digital
voice and the T-carrier system. As part of that work, they standardized different transmission
speeds, including 64 Kbps, 1.544 Mbps, and 44.736 Mbps.
Those same Bell companies developed time-division multiplexing (TDM) technology that let
them combine multiples of these base speeds onto a single line. For instance, one popular
standard, a Digital Signal level 1 (DS1), or T1, combines 24 DS0s (at 64 Kbps) plus 8 Kbps
of overhead into one physical line that runs at 1.544 Mbps. However, to allow flexibility of
speeds offered to customers, the telco could install a T1 line to many sites, but run some
at slower speeds and some at faster speeds—as long as those speeds were multiples of 64
Kbps.
Now back to the idea of the speed of a leased line. What can you actually buy? Basically, at
slower speeds, you get any multiple of 64 Kbps, up to T1 speed. At faster speeds, you can
get multiples of T1 speed, up to T3 speed. Table 13-3 summarizes the speeds typically seen
in the United States, with a few from Europe.
Table 13-3 WAN Speed Summary
Names of Line
Bit Rate
DS0
64 Kbps
Fractional T1
DS1 (T1)
E1 (Europe)
Fractional T3
DS3 (T3)
E3 (Europe)
Multiples of 64 Kbps, up to 24X
1.544 Mbps (24 DS0s, for 1.536 Mbps, plus 8 Kbps overhead)
2.048 Mbps (32 DS0s)
Multiples of 1.536 Mbps, up to 28X
44.736 Mbps (28 DS1s, plus management overhead)
Approx. 34 Mbps (16 E1s, plus management overhead)
The Role of the CSU/DSU
For our last bit of discussion about WAN links in a working enterprise internetwork, next
consider the role of the CSU/DSU (called CSU for short). For the sake of discussion, the
next few paragraphs, leading up to Figure 13-7, assume a leased line with external
CSU/DSUs, like earlier in Figure 13-4.
The CSU sits between the telco leased line and the router; it understands both worlds and
their conventions at Layer 1. On the telco side, that means the CSU connects to the line
from the telco, so it must understand all these details about the T-carrier system, TDM,
and the speed used by the telco. On the router side of the equation, the CSU connects to
the router, with roles called the DCE and DTE, respectively. The CSU, acting as DCE (data
circuit-terminating equipment), controls the speed of the router serial interface. The router,
acting as DTE (data terminal equipment), is controlled by the clocking signals from the CSU
(DCE). That is, the CSU tells the router when to send and receive bits; the router attempts
to send and receive bits only when the DCE creates the correct electrical impulses (called
clocking) on the cable. Figure 13-7 shows a diagram of those main concepts of the role of
the CSU/DSU.