|  | 2.3 | How WANs relate to the OSI Model
| 2.3.2 | WAN physical layer standards

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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