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

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.

The ADSL router or modem at the client will interface with a standard 2 pair telephone line, which is terminated at the service provider DSL Access Multiplexer (DSLAM). At that point the service provider will cross connect their DSLAM with a variety of different equipment such as T1, T3, SONET, Frame Relay, ATM or DSL circuits for transmission across the Internet or to a different central office. As mentioned some clients will utilize ADSL for line sharing their phone calls as well. The DSLAM will split off voice traffic from the data traffic and routed to a Class 5 switch where it is sent across the PSTN using a protocol such as SS7. Many business clients will opt for an additional data line, which is an increased cost however if the voice line isn't available that doesn't affect their Internet connection.

Very High Rate DSL (VDSL) is a higher speed DSL specification that will transmit data at distances between 1,000 feet and 4,500 feet across copper telephone lines. Distances of approximately 1000 feet will support speeds of 55 Mbps while distances of 4,500 feet will support an approximate speed of 12 Mbps. There are some technical differences with line encoding however the modem will split off the telephone service as does ADSL for phone calls.

Symmetric DSL (SDSL) is somewhat new and as the name suggests transmits data in both directions at T1 speeds. The distance specification from the central office is 21,000 feet and it should be noted that there must be a separate phone line since SDSL won't split off phone traffic. That is currently an issue that is being addressed and should be available this year. SDSL is an always-on service, which reduces the issues with call setup. That and the higher upstream traffic rates make it better suited for web hosting applications since your file downloads sent with downstream traffic is sent with their upstream traffic. There are tremendous opportunities for service providers to sell cost effective high speed Internet access to many clients across the United States today. Telecommuters and business clients can reduce costs with higher speed circuits for sending voice and data from home, access and distribution offices across the Internet.   

SONET/SDH

The Synchronous Optical Network (SONET) specification describes a high speed fiber technology used by service providers for transporting voice and data traffic. A SONET network is built with a series of ring segments that are inter-connected. Each SONET segment is comprised of dual counter rotating rings for link diversity should one of the rings be unavailable. The standard OC-1 interface is 51.8 Mbps. The SONET network ring is built with Add/Drop Multiplexers (ADM) which terminate the SONET signal at various metropolitan and national locations. They are Time Division Multiplexers that mux/demux SONET signals from an OC-12 to OC-48 traffic stream. Each ADM has an active and a standby connection to the SONET ring. When a network failure with the active connection is detected, the standby connection is immediately activated. The SONET frame structure is 810 bytes that is comprised of overhead and payload bytes. The overhead is comprised of section and line signaling. The payload bytes are comprised of path signaling and payload. Customer routers such as the Cisco 7507 that support OC-3 interfaces that can interface with an ADM.

Dense Wave Division Multiplexing (DWDM)

Dense Wave Division Multiplexers (DWDM) are used to multiplex optical signals at various wavelengths onto a single fiber strand for transport across an optical network at speeds from OC-48 to OC-192. Each wavelength can run at speeds of up to 10 Gbps. Current optical systems can multiplex as many as 100 wavelengths or channels per fiber strand which is almost 1 Terabit (1000 Gbps) aggregate speed. Current efforts are focused on developing multi-terabit transport on 1 fiber strand. This technology is somewhat of a demarcation between the fastest enterprise core networks and the long haul ISP core networks which aggregate hundreds of enterprise customers. The enterprise customer can connect with DWDM networks using ATM switches and IP routers with OC-48 interfaces. Public and private SONET network providers have rings that connect using equipment with interfaces running at OC-48 speeds as well. The Cisco 12016 Gigabit switch router is available with OC-48 interfaces. Companies today are utilizing 400 OC-48 router interfaces at the 12016 to build a Terabit WAN core.

Data Link Switching (DLSW+)   

Data Link Switching is a Layer 2 protocol used for encapsulating SNA frames across an IP WAN. It is an IP encapsulation method that integrates SNA workstations and servers with the IP enterprise network. Encapsulation of SNA is required since there is no Layer 3 addressing defined with its protocol stack. Routers that are configured with DLSW+ will establish peer relationships with local and remote DLSW+ routers. Each SNA Frame is encapsulated in an IP packet before it is sent across the WAN to a peer router configured with DLSW+. Each local router will terminate LLC2 data link layer frames from each workstation and send local acknowledgments to each local workstation as packets are sent and received. That eliminates LLC2 timeout issues that can occur between workstation and server when acknowledgments must travel across a congested WAN circuit.

SNA is connection-oriented and must receive data link acknowledgments every few seconds or the session will timeout. Bridge protocols such as Source Route Bridging (SRB) limit the number of bridges and rings that an SNA packet can span. There is no issue with the number of Rings utilized with DLSW+ since the RIF field is terminated at the router. DLSW+ integrates many different data link technologies such as Ethernet, Token Ring, SDLC and Frame Relay. There is a translation as well between different frame types such as Ethernet and Token Ring at each router for those data link technologies. Promiscuous mode is configured at DLSW+ routers which allows for many connections from remote peer routers. An example would be 7500 Data Center routers that have peer connections from many distribution offices.

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Shaun Hummel is an author of various technical books and has a web site focused on information technology job search solutions and certifications.

http://www.networkjobsolutions.com