WAN Network Protocols - DSL, SONET, HDLC, DWDM, DLSW+

By Shaun Hummel

Overview

This article discusses some of the most implemented WAN protocols in enterprise networking environments today including HDLC, DSL, SONET, DWDM, and DLSW+

High Level Data Link Control (HDLC)

HDLC is a Cisco proprietary protocol for designed for sending data across serial links. It defines an encapsulation method at the data link layer for transporting data over a public or private network. This protocol is utilized for Leased Line TDM circuits. TDM circuits are probably the oldest circuit types originating from circuit switching technology used by the public switched telephone network that carries your phone traffic. The difference is that companies transporting data traffic require circuit speeds of 256 Kbps to 45 Mbps. That is a data grade circuit which requires equipment at both ends of what is a phone line, for conditioning and formatting the data for those speeds.

The TDM network works with increments called digital signal zero's (DS-0). A DS-0 is a 64 Kbps channel (56 Kbps if in-band signaling used) that is part of the DS-1 industry standard specification. A DS-1 defines a framing standard for transmission across a T1 circuit at 1.544 Mbps with 24 DS-0 channels. A DS-3 defines a framing standard for transmission across a T3 circuit at 44.736 Mbps with 28 DS-1 channels. Some service providers offer what is called Fractional T1 (Frac T1). It is a circuit that runs at a speed less than 1.544 Mbps since it is a subset group of the 24 channels.

The common Fractional T1 speed is 384 Kbps, which is comprised of 6 DS-0 channels. Many router serial interfaces have a feature that split or channelize a T1 circuit. That is useful if you don't want to pay for a full T1 circuit. It does that by differentiating each specific channel from the full T1 circuit. Europe uses somewhat different circuit speed standards called E1and E3. The E1 circuit is comprised of 30 DS-0 channels and runs at 2.048 Mbps while E3 is comprised of 20 T1 channels and runs at 34.368 Mbps.

Digital Subscriber Line (DSL)

Digital Subscriber Line is a newer broadband technology being utilized for remote dial and access office connectivity. It is very cost effective when compared with ISDN and T1circuits since it is faster and less expensive. The issue with DSL is that you must be located no more than 18,000 feet from the service provider central office.

The demand for high speed Internet access has service providers installing DSL terminating equipment at many central offices. That will increase the chances for availability in your neighborhood if it isn't an option today. The current specification defines three primary technologies, which are Asymmetric DSL (ADSL), Very High Data Rate DSL (VDSL) and Symmetric DSL (SDSL).

Asymmetric DSL (ADSL) as the name suggests is asymmetric technology, which allows faster downstream speeds from the service provider to the client than upstream speeds from the client to the service provider. That design matches the flow of Internet and video applications since they typically have the client downloading more information than sending it. Depending on the distance from the service provider central office, downstream speeds can be faster than 6 Mbps and upstream speeds can be as much as 640 Kbps. Being conservative with bridge taps and using increased wire diameter (gauge) will increase traffic rates as well from client to service provider.

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