'The Shadow Knows'
By Thomas G. Robinson, Executive Vice President,
To paraphrase the old radio drama format, when we last left this column, we had reviewed some of the pros and cons of common MAN (metropolitan area network)/WAN (wide area network) transport systems, especially as they apply to institutional users. We left off with the notion that a number of application-oriented issues would ultimately determine the best transport system, or mix of transport systems, to meet user requirements.
For example, the requirements of traditional voice systems could lead us to the use of a common transport system like ATM (Asynchronous Transfer Mode). In fact, some institutional organizations have established a common phone system that is managed by one jurisdictional entity. In these cases, there is typically a main PBX at a central site, with the facilities of other jurisdictions connecting as remote sites. In order to guarantee that real-time voice conversations will stay connected, and ensure that necessary bandwidth will always be available to link the remote sites to the main PBX with a Quality of Service (QoS) that will always facilitate such connections, these jurisdictions are choosing to utilize a circuit emulated T-1 over ATM platform because of its proven capabilities for facilitating traditional voice architectures.
Similarly, ATM has been chosen by several states as the backbone transport for large statewide networks that are or will be used for a variety of applications, including H.323-based video for videoconferencing and distance learning purposes. In some of these cases, institutional organizations at the regional level that have developed private MAN infrastructure have, or are looking at, ATM as the primary backbone technology in the private MAN because of its ease of interfacing with the statewide network.
ATM is also popular in such implementations because of its general availability in the public network where it can enable users without private MAN connections to employ a similar T-1 over ATM architecture. Besides the aforementioned H.323 video, these networks are used for a variety of applications, including Internet access services and state-enabled voice services. In some cases, the push for ATM in the private MAN may even come from the state itself because of the ability to easily integrate those who are using leased public network services and those who would be using ATM transport over private MAN networks.
In the past, ATM has also been an especially popular choice with some school districts because they have been able to develop a wide range of applications at an affordable cost because of the way such ATM services have been offered by ILECs (incumbent local exchange carriers). Specifically, as it relates to K through 12 school districts, ILECs have drastically reduced the cost of these circuits for those that qualify for federal E-Rate funding.
For implementations that are data centric, gigabit ethernet (GbE) (and its "just-around-the-corner" cousin, 10x GbE) is increasingly the private WAN technology of choice because it:
- bridges building backbone LANs easily into the WAN, keeping all data traffic in a native ethernet environment;
- is, and will continue to be, eminently scalable in how traffic enters the network (10 Mbps, 100 Mbps, 1 Gbps, etc.);
- is extremely well-suited to a traditional star topology for the WAN where a significant amount of intelligence can be placed at a central switch, and cost-effective uplink modules can be employed in existing or upgraded routers and switches at remote sites; and,
- with the carriage of more and more services in an IP (Internet Protocol) format, may be the best investment going forward to ensure delivery of advanced voice and video, as well as data services.
Concerning its implementation in the MAN, ATM supporters continue to contend that GbE does not have the proper QoS characteristics to provide necessary guarantees to multiple users that all of their services will "get through." Current GbE supporters and users, though, indicate that the transfer rate and throughput capabilities are so large that they haven't yet seen any debilitating contention in the network, even though some GbE users in a MAN environment are now running IP-based voice and video services over the network, as well as their traditional data traffic. It remains to be seen if this will continue to be the case as networks may become dominated by streaming video and other high-capacity traffic.
One of the other knocks on GbE is that it presents more difficulties in taking advantage of the capabilities afforded by ring architectures that are typically found in the MAN environment. As an alternative to straight GbE implementation, some vendors suggest utilizing GbE in a star topology WAN, with IP over Sonet transport around the MAN ring. The advantages here are that: an efficient interface can be made between the core WAN and the MAN; the focus on IP services is maintained throughout; and Sonet was developed to be an efficient, effective, bi-directional, fault tolerant ring-based transport system.
Some say that multi-user WAN/MAN network design is like a detective story where the pieces of the puzzle are all there; they just have to be found and put together through methodical investigation in order to solve the mystery. Well, to again paraphrase that old radio staple, "Who knows what evil lurks if you make the wrong network choice?" I think both the Shadow and I know the answer to that question.