Extending Ethernet into the access arena

IP QoS, 802.1q VLANs enhance Ethernet bandwidth capabilities.

By Karen Livoli
Network World, 03/04/02

A great deal of interest surrounds the deployment of Ethernet in metropolitan-area networks as a replacement for SONET. At the same time, Ethernet advocates are trying to elevate Ethernet to become a broadband alternative to xDSL and cable access services.

To accomplish this goal and to support IP services such as videoconferencing, video on demand, interactive gaming, and bundled voice, video and data services, the Ethernet-based infrastructure must be able to deliver individualized services to customers.

Subscriber-based IP quality of service (QoS) and 802.1q virtual LANs (VLAN) are two technologies that can enhance the bandwidth capabilities of Ethernet to meet the requirements for deploying next-generation services such as voice and video.

Most Ethernet services offer class-based services that put all customers into a handful of predefined service classes. These users share bandwidth. But service providers can use IP QoS to divide traffic into four distinct classes, each with its own priority-level segment of bandwidth:

The low-latency and low-jitter class provides support for real-time, and delay- and jitter-sensitive applications such as voice over IP. Packets in this class are scheduled to achieve low latency first. In addition, they are shaped to achieve low jitter.

The low-latency class provides that packets are scheduled to achieve low latency first. This traffic does not require shaping, but it should be rate-limited so that non-low-latency traffic is not completely bandwidth starved.

These two classes conform to the Expedited Forwarding traffic class that the Internet Engineering Task Force (IETF) established in RFC 2598.

Expedited Forwarding requires that the egress rate exceed the ingress rate and works like a virtual leased line. The Expedited Forwarding treatment polices and drops packets on network ingress, and shapes traffic on egress to make sure that the connection to the next provider is at the same priority level.

The low-loss class provides that packets will be given a higher weight in the scheduler, meaning more bandwidth will be given, and also will be given a higher threshold, or more buffering. This class is often used for mission-critical applications. It is equivalent to the IETF Assured Forwarding model (RFC 2597).

The best-effort class means that packets will be scheduled as soon as possible. This class gets the leftover bandwidth; it should be the least expensive.

Like Ethernet, VLAN capabilities are familiar to enterprise customers and offer a valuable service when extending Ethernet into the access arena.

IEEE 802.1q VLAN support offers the ability to logically segregate subscribers and their traffic on a shared interface, such as Gigabit Ethernet or Fast Ethernet. IEEE 802.1q describes two key Ethernet fields that have been added to the Ethernet frame to identify and prioritize traffic: VLAN identifier and user priority values (802.1p).

The VLAN ID field is 12 bits, allowingfor 4,096 VLAN IDs. Tagged frames are identified by the value 0x8100 in the Ethernet protocol type field; the actual protocol type of the frame then follows the tag. Using VLANs, service providers can offer per-subscriber services by assigning each subscriber a unique VLAN tag.

On the last-mile access side, this architecture promotes true subscriber-based IP services such as QoS, bandwidth shaping, filtering and VPN membership, which ensures that users can purchase the exact services they require.

On the metropolitan access side, Ethernet has three qualities that make it appealing as a replacement for SONET in MANs:

Speed: 100M bit/sec, 1G bit/sec or even higher.

Cost: can be up to 10 times less expensive than SONET technology.

Familiarity: A standards-based technology, Ethernet is widely deployed in corporations.

Extending Ethernet into the access arena

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