A copper or ?¬?ber
cable connects the CPE to the nearest exchange or central of?¬?ce (CO) of the service
provider. This cabling is often called the local loop, or ???last mile.??? Transmission of analog
data (such as a telephone call) is connected locally to other local loops, or nonlocally
through a trunk to a primary center. Analog data then goes to a sectional center and on to a
regional or international carrier center as the call travels to its destination.
For the local loop to carry data, however, a device such as a modem or DSU/CSU is needed
to prepare the data for transmission. Devices that put data on the local loop are called data
communications equipment (DCE). The customer devices that pass the data to the DCE are
called data terminal equipment (DTE). The DCE primarily provides an interface for the
DTE into the communication link on the WAN cloud.
The WAN access physical layer describes the interface between the DTE and the DCE.
Figure 5-4 shows the location of DTE and DCE equipment and some Layer 1 connectivity
supported between those devices.
WAN Cabling
Cisco routers support the EIA/TIA-232, EIA/TIA-449, V.35, X.21, and EIA/TIA-530
standards for serial connections.
When you order the cable, you receive a shielded serial transition cable that has the
appropriate connector for the standard you specify.
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