WAN physical-layer protocols describe
how to provide electrical, mechanical, operational, and functional
connections for WAN services. Most WANs require an interconnection
that is provided by a communications service provider (such as an RBOC),
an alternative carrier (such as an Internet service provider), or a
post, telephone, and telegraph (PTT) agency.
The WAN physical layer also describes
the interface between the DTE and the DCE. Typically, the DCE is the
service provider, and the DTE is the attached device, as shown in the Figure.
Several physical-layer standards define
the rules governing the interface between the DTE and the DCE:
- EIA/TIA-232 -- A common physical-layer
interface standard, developed by EIA and TIA, that supports
unbalanced circuits at signal speeds of up to 64 kbps. It closely
resembles the V.24 specification, and was formerly known as
RS-232. This standard has been in place for many years.
- EIA/TIA-449 -- A popular physical-layer
interface developed by EIA and TIA. It is essentially a faster (up
to 2 Mbps) version of EIA/TIA-232, capable of longer cable runs.
- EIA/TIA-612/613 -- A standard
describing High Speed Serial Interface (HSSI), which provides
access to services at T3 (45 Mbps), E3 (34 Mbps), and Synchronous
Optical Network (SONET) STS-1 (51.84 Mbps) rates. The actual rate
of the interface depends on the external DSU and the type of
service to which it is connected.
- V.24 -- An ITU-T standard for a
physical-layer interface between DTE and DCE.
- V.35 -- An ITU-T standard describing a
synchronous, physical-layer protocol used for communications
between a network access device and a packet network. V.35 is most
commonly used in the United States and in Europe, and is
recommended for speeds up to 48 kbps.
- X.21 -- An ITU-T standard for serial
communications over synchronous digital lines. The X.21 protocol
is used primarily in Europe and Japan.
- G.703 -- An ITU-T electrical and
mechanical specification for connections between telephone company
equipment and DTE using British Naval connectors (BNCs) and
operating at E1 data rates.
- EIA-530 -- Two electrical
implementations of EIA/TIA-449: RS-422 (for balanced transmission)
and RS-423 (for unbalanced transmission).
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