Network Computingwww.networkcomputing.com

You will likely need multiple technologies to gain any benefits. For example, if you are running IPX and IP simultaneously, you will probably require a traffic-management service at the data-link or physical layers to balance the load between these different protocols, in addition to whatever services you have for specific application protocols.

Even on IP-only networks it isn't entirely feasible to rely on a single technology. For example, your switch vendor may support only data-link prioritization, while your router vendor supports network- or transport-layer services. In this article, we will help you pinpoint the most appropriate technologies for enterprisewide traffic management.

Physical and Link-Layer Services
The most fundamental traffic management comes in the form of physical-layer and data-link-layer access services, with the network determining which stations get the highest access levels. Token ring and FDDI have led this area, each featuring a managed-access topology and a set of control services that use an access token. If a node with high-priority data got the token, it would send as much data as possible before other devices with lower priority levels would be able to transmit.

Neither of these features was found in the original Ethernet design. Over the past few years, however, Ethernet has moved from shared access (with access based on who recovered from a collision first) to managed access (switching). Now an Ethernet switch can make access decisions based on the MAC (Media Access Control) address of the attached device, or by the physical port number of the switch itself.

Control services were added to Ethernet with last year's IEEE adoption of the 802.1Q VLAN (virtual LAN) tagging specification and the 802.1p extension to the 802.1D bridging specification. Essentially, the 802.1Q VLAN tag adds 32 new bits of header data to Ethernet frames, with three of those bits used to provide eight levels of prioritization tagging for each frame. The 802.1p extension to the 802.1D bridging specification provides a topology-neutral mechanism for priority enforcement within the infrastructure devices, letting the switches make queuing decisions based on content instead of identity. In addition, per-frame tags let the data be carried through the network across switch boundaries, while location-specific elements typically do not. For a more detailed discussion of these technologies, see "Bringing Prioritization Services to Ethernet," at www.networkcomputing.com/914/914ws1.html.

Many other technologies work at the data-link layer (including those found in frame relay and ATM). Almost all of them rely on a managed-access physical layer and do not work on a shared-access medium, such as CD/ CSMA Ethernet, because that model offers no means for queuing data. It's interesting that AIM Engineering and other companies are working to deliver prioritization to shared-access media using software within the end point devices, which essentially requires that devices ask for permission before transmitting data. This approach may be particularly useful with wireless or SOHO (small office/home office) networks that lack managed-access topologies.