Telecom Networking

Many applications require high quality of service (QoS) transmission to make the service operate properly. Audio and video streams suffer from loss of packets that translate into audible blips and picture frame loss, reducing the end user’s quality of experience (QoE). Ethernet is a ubiquitous, cost effective technology that enables delivery of multiple services with proven efficiency and simplicity. However, with carrier Ethernet (CE) proliferating into the transport network and replace the legacy SONET/SDH networks, we should take a deeper look into it, making sure it can live up to its promise.
Ethernet technology has evolves from its enterprise days to cater for the needs of carriers. The VLAN tags used in the enterprise to separate between departments have been enhanced with “double tags” defined in the Q-in-Q (IEEE 802.1ad) standard. This step was perceived by many as a step that was sufficient to make CE a “carrier-grade” technology. A further enhancement aimed at increasing the scalability of the solution was added (PBB, 802.1ah), and the migration to packet networks seemed smoother than ever… or so it would seem.
Classical Ethernet terminology is misleading, at least for transport engineers. Whether a Q-in-Q or a PBB technology is used, Ethernet services use a parameter called: “Committed Information Rate” (CIR). This CIR value is used to mark packets that are sent through and are not exceeding the CIR rate as Green frames, meaning they get priority in the network queues over excess traffic of the same class. However, when such a term is used in the transport market, it is translated to an implied message that the traffic flow is indeed guaranteed. The service provider and its customers think they can rest assured that all packets will get to their destination just like in the old SONET/SDH days, right? – Dead wrong!!!
The use of CIR to mark Green frames is good to ensure priority of the “committed” traffic (for which the customer paid a lot) over the “excess” traffic (i.e. the extra bandwidth for which the customer paid less). However when we look at the traffic flow in the network, “committed” traffic from many users can concentrate on a specific network resource, e.g. a link, and exceed the capacity of this resource. In such case, some frames must be dropped, meaning losing part of the so called “committed” traffic.

For example, in the above figure, assuming the traffic from all 3 customers flows into switch F through switch D, the link between switches D & F becomes congested and therefore over 10% of the “committed” traffic will be dropped. To avoid this congestion, the traffic from switch C could be routed to go directly from switch E to switch F, but unfortunately the service provider has no control over it. The routes are determined automatically by the spanning tree protocol (STP) and could change due to network rearrangements without service provider’s involvement.

Carrier Ethernet 2.0 has evolved to overcome this issue, and make CE as a real alternative to SONET/SDH. It has introduced the concept of “connection-oriented” Ethernet (CoE), so that the service provider can have full control on the paths taken by the services and reserve bandwidth for committed services. CoE has been recently standardized and ratified by IEEE as 802.1Qay, a.k.a. PBB-TE. With CoE, the control plane functionality is implemented by an external management system, provisioning the trunks each service must follow and ensuring bandwidth allocation meets the requirements. There could not be any contention between committed traffic since the management system verifies there is sufficient bandwidth for all committed traffic throughout the whole path before the service is configured and approved.
Carrier Ethernet is now ready for prime time. The combination of connectionless Ethernet flexibility and connection-oriented service guarantee, combined with connectivity fault management for service assurance deliver on their promise. Now there is a viable, cost-effective replacement for legacy TDM.