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The only way to overcome this problem is to find a way of treating real-time voice and video packets differently from data packets in the network, and to give them preferential treatment when transient data overloads cause queues to form on busy network links. In practice, this means that LAN packets must be tagged with some kind of priority information that enables switches to identify which packets need to jump the queue

The IEEE 802, which oversees standards for LAN technologies, has initiated a project identified as 802.1p, which is concerned with “Traffic Class Expediting” in LAN switches.

The principal problem faced by 802.1p is that there is no spare information field in the standard Ethernet packet format that could carry the required priority tag. As a result, it has been necessary to propose a new Ethernet packet format with an additional 4 bytes of information in the packet header that can contain a 3-bit priority tag field (offering 8 levels of priority), together with some other information concerned with Virtual LANs.

With the new Ethernet packet format containing a priority tag, end station applications can identify real-time voice or video packets by assigning them a high priority value in the tag. LAN switches that have been enhanced to process the priority tags can separate high and low priority traffic in the switching fabric and place them in separate queues at outgoing switch ports. The LAN switches need to implement a queue scheduling algorithm that gives preference to the higher priority queues on outgoing ports, and by this means it is hoped that real-time voice and video can be carried over the LAN without incurring unacceptable delays during periods of heavy data traffic.

As of July 1997, the 802.1p standard was still in draft form and the standard is not expected to be completed until 1998. Ethernet switches that support the 802.1p priority tags with multiple internal queuing structures will require a new generation of switching silicon, and the earliest we could expect to see products that conform to the standard would be late 1998 or into 1999. Surprisingly, we may see Token Ring switches that handle multiple priority levels before that time, leveraging the capabilities of the existing Token Ring standard that supports 8 levels of priority.

Hybrid ATM Networks

The discussion of ATM described how it offers guaranteed Quality of Service for real-time voice and video streams. Today, ATM is increasingly used as a LAN backbone for pure data applications, because it offers greater scalability and fault tolerance than other LAN technologies. Ethernet and Token Ring LANs are connected to ATM via “edge switches” equipped with ATM uplinks, typically supporting the ATM Forum standard for carrying LAN traffic over ATM, know as LAN Emulation.

It is possible to enhance ATM edge switches to enable desktops connected via Ethernet or Token Ring to enjoy the benefits of ATM Quality of Service across the LAN backbone. Two techniques have been proposed to achieve this.

The first technique, known as “Cell-in-Frame,” extends the native ATM signaling protocols over dedicated Ethernet connections from the edge switch to the end station. The voice or video application in the end station places the voice or video stream in ATM cells using the ATM Forum standards for native ATM transport, and then encapsulates the ATM cells in Ethernet packets for transport to the edge switch for onward transmission onto the ATM network. Effectively, this is ATM to the desktop, but using physical Ethernet with standard Ethernet adapter cards as a kind of physical transport layer for ATM traffic.

The second technique makes use of an emerging standard protocol for end stations to request Quality of Service for IP-based voice or video applications, known as the Resource Reservation Protocol, or RSVP. The enhanced edge switch intercepts RSVP requests originated by end stations and converts them into ATM signaling to request the set-up of connections across the ATM backbone with the appropriate Quality of Service. The edge switch then distinguishes between IP packets containing data and those containing voice or video, using the information provided by RSVP, and steers voice and video packets onto ATM connections that have Quality of Service.

At the time of writing, the technique described here for RSVP-to-ATM mapping enjoys somewhat broader industry support than Cell-in-Frame, perhaps because of its relationship with Internet technology.

Until LAN switches supporting 802.1p priority tagging have proved themselves capable of meeting the very stringent end-to-end delay requirements for real-time voice and video communications, hybrid approaches based on ATM in the backbone and switched Ethernet or Token Ring to the desktop are likely to find acceptance as the solution of choice for voice and video over the LAN.


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