| Overview | | | | should be noted that there must be a separate |
| This article discusses some of the most | | | | phone line since SDSL won't split off phone traffic. |
| implemented WAN protocols in enterprise | | | | That is currently an issue that is being addressed |
| networking environments today including HDLC, | | | | and should be available this year. SDSL is an |
| DSL, SONET, DWDM, and DLSW+ | | | | always-on service, which reduces the issues with |
| High Level Data Link Control (HDLC) | | | | call setup. That and the higher upstream traffic |
| HDLC is a Cisco proprietary protocol for designed | | | | rates make it better suited for web hosting |
| for sending data across serial links. It defines an | | | | applications since your file downloads sent with |
| encapsulation method at the data link layer for | | | | downstream traffic is sent with their upstream |
| transporting data over a public or private network. | | | | traffic. There are tremendous opportunities for |
| This protocol is utilized for Leased Line TDM | | | | service providers to sell cost effective high speed |
| circuits. TDM circuits are probably the oldest circuit | | | | Internet access to many clients across the United |
| types originating from circuit switching technology | | | | States today. Telecommuters and business clients |
| used by the public switched telephone network | | | | can reduce costs with higher speed circuits for |
| that carries your phone traffic. The difference is | | | | sending voice and data from home, access and |
| that companies transporting data traffic require | | | | distribution offices across the Internet. |
| circuit speeds of 256 Kbps to 45 Mbps. That is a | | | | SONET/SDH |
| data grade circuit which requires equipment at | | | | The Synchronous Optical Network (SONET) |
| both ends of what is a phone line, for conditioning | | | | specification describes a high speed fiber |
| and formatting the data for those speeds. | | | | technology used by service providers for |
| The TDM network works with increments called | | | | transporting voice and data traffic. A SONET |
| digital signal zero's (DS-0). A DS-0 is a 64 Kbps | | | | network is built with a series of ring segments |
| channel (56 Kbps if in-band signaling used) that is | | | | that are inter-connected. Each SONET segment is |
| part of the DS-1 industry standard specification. A | | | | comprised of dual counter rotating rings for link |
| DS-1 defines a framing standard for transmission | | | | diversity should one of the rings be unavailable. |
| across a T1 circuit at 1.544 Mbps with 24 DS-0 | | | | The standard OC-1 interface is 51.8 Mbps. The |
| channels. A DS-3 defines a framing standard for | | | | SONET network ring is built with Add/Drop |
| transmission across a T3 circuit at 44.736 Mbps | | | | Multiplexers (ADM) which terminate the SONET |
| with 28 DS-1 channels. Some service providers | | | | signal at various metropolitan and national locations. |
| offer what is called Fractional T1 (Frac T1). It is a | | | | They are Time Division Multiplexers that mux |
| circuit that runs at a speed less than 1.544 Mbps | | | | demux SONET signals from an OC-12 to OC-48 |
| since it is a subset group of the 24 channels. | | | | traffic stream. Each ADM has an active and a |
| The common Fractional T1 speed is 384 Kbps, | | | | standby connection to the SONET ring. When a |
| which is comprised of 6 DS-0 channels. Many | | | | network failure with the active connection is |
| router serial interfaces have a feature that split or | | | | detected, the standby connection is immediately |
| channelize a T1 circuit. That is useful if you don't | | | | activated. The SONET frame structure is 810 |
| want to pay for a full T1 circuit. It does that by | | | | bytes that is comprised of overhead and payload |
| differentiating each specific channel from the full | | | | bytes. The overhead is comprised of section and |
| T1 circuit. Europe uses somewhat different circuit | | | | line signaling. The payload bytes are comprised of |
| speed standards called E1and E3. The E1 circuit is | | | | path signaling and payload. Customer routers such |
| comprised of 30 DS-0 channels and runs at 2.048 | | | | as the Cisco 7507 that support OC-3 interfaces |
| Mbps while E3 is comprised of 20 T1 channels and | | | | that can interface with an ADM. |
| runs at 34.368 Mbps. | | | | Dense Wave Division Multiplexing (DWDM) |
| Digital Subscriber Line (DSL) | | | | Dense Wave Division Multiplexers (DWDM) are |
| Digital Subscriber Line is a newer broadband | | | | used to multiplex optical signals at various |
| technology being utilized for remote dial and | | | | wavelengths onto a single fiber strand for |
| access office connectivity. It is very cost | | | | transport across an optical network at speeds |
| effective when compared with ISDN and | | | | from OC-48 to OC-192. Each wavelength can run |
| T1circuits since it is faster and less expensive. The | | | | at speeds of up to 10 Gbps. Current optical |
| issue with DSL is that you must be located no | | | | systems can multiplex as many as 100 |
| more than 18,000 feet from the service provider | | | | wavelengths or channels per fiber strand which is |
| central office. The demand for high speed | | | | almost 1 Terabit (1000 Gbps) aggregate speed. |
| Internet access has service providers installing | | | | Current efforts are focused on developing |
| DSL terminating equipment at many central | | | | multi-terabit transport on 1 fiber strand. This |
| offices. That will increase the chances for | | | | technology is somewhat of a demarcation |
| availability in your neighborhood if it isn't an option | | | | between the fastest enterprise core networks |
| today. The current specification defines three | | | | and the long haul ISP core networks which |
| primary technologies, which are Asymmetric DSL | | | | aggregate hundreds of enterprise customers. The |
| (ADSL), Very High Data Rate DSL (VDSL) and | | | | enterprise customer can connect with DWDM |
| Symmetric DSL (SDSL). | | | | networks using ATM switches and IP routers with |
| Asymmetric DSL (ADSL) as the name suggests | | | | OC-48 interfaces. Public and private SONET |
| is asymmetric technology, which allows faster | | | | network providers have rings that connect using |
| downstream speeds from the service provider to | | | | equipment with interfaces running at OC-48 |
| the client than upstream speeds from the client | | | | speeds as well. The Cisco 12016 Gigabit switch |
| to the service provider. That design matches the | | | | router is available with OC-48 interfaces. |
| flow of Internet and video applications since they | | | | Companies today are utilizing 400 OC-48 router |
| typically have the client downloading more | | | | interfaces at the 12016 to build a Terabit WAN |
| information than sending it. Depending on the | | | | core. |
| distance from the service provider central office, | | | | Data Link Switching (DLSW+) |
| downstream speeds can be faster than 6 Mbps | | | | Data Link Switching is a Layer 2 protocol used |
| and upstream speeds can be as much as 640 | | | | for encapsulating SNA frames across an IP WAN. |
| Kbps. Being conservative with bridge taps and | | | | It is an IP encapsulation method that integrates |
| using increased wire diameter (gauge) will increase | | | | SNA workstations and servers with the IP |
| traffic rates as well from client to service | | | | enterprise network. Encapsulation of SNA is |
| provider. | | | | required since there is no Layer 3 addressing |
| The ADSL router or modem at the client will | | | | defined with its protocol stack. Routers that are |
| interface with a standard 2 pair telephone line, | | | | configured with DLSW+ will establish peer |
| which is terminated at the service provider DSL | | | | relationships with local and remote DLSW+ |
| Access Multiplexer (DSLAM). At that point the | | | | routers. Each SNA Frame is encapsulated in an IP |
| service provider will cross connect their DSLAM | | | | packet before it is sent across the WAN to a |
| with a variety of different equipment such as T1, | | | | peer router configured with DLSW+. Each local |
| T3, SONET, Frame Relay, ATM or DSL circuits | | | | router will terminate LLC2 data link layer frames |
| for transmission across the Internet or to a | | | | from each workstation and send local |
| different central office. As mentioned some clients | | | | acknowledgments to each local workstation as |
| will utilize ADSL for line sharing their phone calls as | | | | packets are sent and received. That eliminates |
| well. The DSLAM will split off voice traffic from | | | | LLC2 timeout issues that can occur between |
| the data traffic and routed to a Class 5 switch | | | | workstation and server when acknowledgments |
| where it is sent across the PSTN using a protocol | | | | must travel across a congested WAN circuit. |
| such as SS7. Many business clients will opt for an | | | | SNA is connection-oriented and must receive data |
| additional data line, which is an increased cost | | | | link acknowledgments every few seconds or the |
| however if the voice line isn't available that doesn't | | | | session will timeout. Bridge protocols such as |
| affect their Internet connection. | | | | Source Route Bridging (SRB) limit the number of |
| Very High Rate DSL (VDSL) is a higher speed | | | | bridges and rings that an SNA packet can span. |
| DSL specification that will transmit data at | | | | There is no issue with the number of Rings utilized |
| distances between 1,000 feet and 4,500 feet | | | | with DLSW+ since the RIF field is terminated at |
| across copper telephone lines. Distances of | | | | the router. DLSW+ integrates many different |
| approximately 1000 feet will support speeds of 55 | | | | data link technologies such as Ethernet, Token |
| Mbps while distances of 4,500 feet will support an | | | | Ring, SDLC and Frame Relay. There is a |
| approximate speed of 12 Mbps. There are some | | | | translation as well between different frame types |
| technical differences with line encoding however | | | | such as Ethernet and Token Ring at each router |
| the modem will split off the telephone service as | | | | for those data link technologies. Promiscuous |
| does ADSL for phone calls. | | | | mode is configured at DLSW+ routers which |
| Symmetric DSL (SDSL) is somewhat new and as | | | | allows for many connections from remote peer |
| the name suggests transmits data in both | | | | routers. An example would be 7500 Data Center |
| directions at T1 speeds. The distance specification | | | | routers that have peer connections from many |
| from the central office is 21,000 feet and it | | | | distribution offices. |