User guide

Internetworking Primer A-15
LANs and WANs
Figure A-4 A Typical WAN Architecture
WAN Physical Layer (Layer 1)
Most WANs require an interconnection that is provided by a communications service
provider, such as a phone company. This physical layer uses many of the technologies that
have evolved over the last few decades for carrying voice communications. The service
provider circuit typically terminates at a channel service unit (CSU) at the customer site.
Master Clock Timing
Two communicating devices on a synchronous serial cable, such as a CSU and an access
server, must agree on which device will provide the clocking for data transmission timing.
That device (typically an external channel service unit/data service unit [CSU/DSU]) is
called data circuit-terminating equipment (DCE). The other device will be data terminal
equipment (DTE), and is usually an access server or router. The physical layer connectors
(such as EIA/TIA-232 or V.35) together with the mode (DTE or DCE) will determine the
required pinout on the cable connecting the two devices. (See Figure A-5.)
Figure A-5 Master Clock Timing
H3569
WAN
CSU/DSU CSU/DSU
Layer 1
Layer 2
Layer 3
Layer 1
Layer 2
Router
Layer 1
Layer 2
Layer 3
Layer 1
Layer 2
Router
H3115
SCTE (suggested)
Clock
DSU/CSU
Router