When and why you should move to Carrier Ethernet?

First things first, I would suggest the Communication Service Provider (CSP) should critically analyze the book value of the existing legacy system, spare capacity availability, vendor support availability, Next Generation Network (NGN)/Ethernet transport feature availability, upgrade costs and operational costs like space and power. As long as the existing system addresses the requirement cost effectively in short term and mid term, I would suggest that the CSP continue with the legacy system with a clear replacement/migration plan for long term.

I'll now list 2 important highlights below to show you why I would replace an existing legacy (ex:- SDH) network with a Carrier Ethernet (CE) network.

(1) *Native* support of cost effective and ubiquitous Ethernet interface/service/transport

As you know the current and future traffic on CSPs are more towards IP/Ethernet and this requires that CSP
1. provide "Ethernet interfaces" to customers,
2. transport/provide "Ethernet Services" like MEF E-LINE, E-LAN, E-TREE and E-Access,
3. build "Ethernet based transport networks" like Carrier/Metro Ethernet MANs/WANs.

If you are to transport Ethernet over SDH, you require EoSDH capabilities on your SDH platforms. On the other hand, for you to transport TDM over Ethernet, you require Circuit Emulation Service over Packet Switched Networks (CESoPSN)/Structure Agnostic Transport over Packet (SAToP). Since the trend is now to move from legacy (TDM) to NGN (IP/Ethernet), you will see more IP/Ethernet than TDM in the future. So it's logical to have CE with TDMoEthernet, rather than having SDH with EoSDH. Finally all will converge to CE.

(2) Vendor support

As these trends happen in the CSP ecosystem (CSPs, OEMs, SDOs etc.), the vendors are also discontinuing legacy systems and move to NGN based technologies like CE. This will make the cost of legacy systems high and cost of NGN systems low due to the economies of scale. This will also made the support (AMC- Annual Maintenance Contract) charges for legacy systems be also high.

E-LAN vs. E-TREE

Metro Ethernet Forum (MEF) has defined 4 types of service, namely;

1. E-LINE (Ethernet Line) - Point to Point
2. E-LAN (Ethernet Local Area Network (LAN)) - Multipoint to Multipoint
3. E-TREE (Ethernet Tree) - Point to Multipoint
4. E-Access (Ethernet Access)

The lack of E-TREE implementations/support on Communication Service Providers (CSP) networks today make the customers buy Virtual Private LAN Service (VPLS) (Multi Protocol Label Switching (MPLS) bases E-LAN implementation) services for point-to-multipoint applications such as 3G back-hauling to Radio Network Controller (RNC), Head Quarters (HQ) to branch communication on corporates. 

If the resources required to E-LAN and E-TREE are compared, when the E-TREE requires N number of Pseudo Wires (PW)s for N locations, E-LAN requires N*(N-1)/2=> N^2. If the Media Access Control (MAC) learning is considered, in E-TREE leafs learn only the root MACs and root learns all the root and leaf MACs (2 if there's only one root exists). In E-LAN, all the points in the VPLS (Virtual Switching Instance (VSI)) will learn all the MACs (N). 

Note that MAC table size is a limited resource for a Network Element (NE). Number of PWs, number of Label Switch Path (LSP)s, number of VSI instances and number of Virtual LAN (VLAN)s are also limited resources.