Just two weeks ago Zahid Ghadialy uploaded a post on Carrier Ethernet Transport (CET). It was a basic introduction which had to be supplemented by my own research to understand what CET actually meant. Part of the difficulty is the “collision” of terms. For example, the interview of a Nokia-Siemens manager mentions the term “transport layer”. This can be confusing if it is taken in relation to the OSI model. Transport layer within the context of CET is quite different from the transport layer that is TCP or UDP. We will revisit this later.
So what is CET and how does it fit into the world of mobile and wireless? Meriton Networks defines CET as,
an architectural approach for building scalable transport infrastructure for supporting Ethernet and the evolution to NGNs.
This is a good definition because we can learn much about CET from the definition alone. The following points can be made:
- CET is not Ethernet itself but an architecture that enables wide scale deployment of Ethernet from its so far traditional use.
- Current transport networks are somehow not scalable and CET with the use of Ethernet can provide that scalability.
- NGNs require QoS support, high bandwidth, transport efficiency and scalability. These can be met with CET.
I got interested in CET because of a recent press release from India’s cellular market leader Airtel [Shockingly Fast Broadband]. The article claims that Airtel can now provide 8 Mbps broadband to its customers, the first provider in India to reach such a high bandwidth. Their transport architecture that makes this possible is CET which has been deployed in Bangalore, Chennai and Delhi. In other areas, CET will be introduced slowly. Meanwhile SDH has been upgraded to support 8 Mbps. Airtel claims that its infrastructure is now IPTV ready.
Ethernet is defined in IEEE 802.3 and relates to Data Link layer of the OSI model. The physical media can be anything. Traditionally, coaxial cables were used. Today, most networks use twisted pairs. Those with high bandwidth requirements such as Gigabit Ethernet may use optical fibres. Ethernet has been used in LANs widely but it do not scale to MANs and WANs. For example, spanning trees do not scale to large networks. Thus Ethernet have rarely been considered for carrier transport. Moreover, SONET has consistently offered higher bandwith than Ethernet and sub-50 ms resilience, so that most transport networks today use SDH/SONET architecture. In a wireless environment, data link layers are usually different and do not conform to the IEEE 802.3 Mac data frame format.
The advantage that Ethernet brings is its ease of implementation and low-cost when compared to SONET. Ethernet is essentially a connection-less protocol. It enables multiple access but has no functionality for providing QoS. Yet CET is attempting to get more out of Ethernet by using it with other technologies/layers.
There was a time when end-to-end packet service was essentially connection-less using IP. Routing was performed at IP (Layer 3). With MPLS, label switching was done (Layer 2). MPLS introduced a connection-oriented virtual paths based on labels. This enabled traffic engineering and QoS provisioning. It was widely deployed in transport networks. With the higher bandwith requirements and greater flexibility demanded by NGNs, this task is being pushed from Layer 2 to Layer 1. Ethernet enables a simple and uniform interface while the underlying transport could be any suitable physical layer that has some switching functions as well. CET, for the sake of system performance, blurs some of the traditional boundaries between layers as defined in the OSI model.
Ethernet data frames can be carried across the transport network. Switching is likely to happen at the optical domain. Wavelength switching, sub-wavelength switching or Ethernet tunnel switching are possible. Using just enough Layer 2 functionality at the optical layer, CET enables a clear separation between service layer and transport layer. The latter refers to the mechanism by which data is transported within the network. It does not refer to the transport layer of OSI model. Switching happens within the transport layer. Carriers can concentrate on their services rather than the transport mechanisms because these two are no longer closely coupled. This is represented in Figure 1 (from Meriton Networks).
Figure 1: CET Separation of Service and Transport Layer
Issues of scalability and QoS are addressed using Provider Backbone Bridging with Traffic Engineering (often called Provider Backbone Transport) PBB-TE/PBT. Spanning trees have been replaced with GMPLS to create bridging tables. It will be apparent from these considerations that Carrier Ethernet is a lot more than just Ethernet. Carrier Ethernet is defined in relation to services on provider networks. It is partly about the transport infrastructure but also about the service delivery aspects. Thus, we have two key aspects of Carrier Ethernet – Carrier Ethernet Service and Carrier Ethernet Solution/Transport.
Many organizations are involved in the standardization of Carrier Ethernet (IEEE, IETF, ITU, MEF). Among these Metro Ethernet Forum (MEF) is the primary organization. The standard is by no means complete. Carrier Ethernet is the future and it is still in its infancy.
For further reading:
- Tom Nolle’s Blog
- Comparing Alternative Approaches for Ethernet-Optical Transport Networks
- Ethernet Technologies, Cisco documentation.
- Abdul Kasim, Delivering Carrier Ethernet: Extending Ethernet Beyond the LAN, McGrawHill, 2007.