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.

Next Generation Optical Transport Networks

When we say Optical Transport Networks (OTN), it could mean two things;

1. OTN wrapper capability 
2. OTN switching capability

To implement an OTN, there are many technological options available. From least cost (in general) to highest, the options are as follows;
Note: All options need to support OTN wrapper

1. Fixed Optical Add Drop Multiplexers (FOADM)
2. Re-configurable OADM (ROADM)
3. Tunable ROADM (TROADM) (Wave-length Selective Switching (WSS)): supports Color-Less and Direction-less)
4. FOADM with Automatically Switched Optical Networks (ASON)/ Generalized Multi-Protocol Label Switching (GMPLS) control plane
5. ROADM with ASON/GMPLS control plane
6. TROADM with ASON/GMPLS control plane
7. FOADM with ASON/GMPLS control plane and OTN switching
8. ROADM with ASON/GMPLS control plane and OTN switching
9. TROADM with ASON/GMPLS control plane and OTN switching

Other than Color-Less and Direction-less, Contention-less is also a good feature to have on WSS systems. 

Cost of adding OTN switching capability vs. loosing sub-lambda grooming at intermediate sites need to be properly analyzed based on your current and future traffic matrix. 

To tell you the truth, OTN switching is more a hype than a reality. This is quite evident from the low number of OTN switching deployments currently in the world.

The prime advantage of OTN switching is the sub-lambda grooming at intermediate sites. The industry trend(both suppliers and operators) is to start without OTN switching and go for OTN switching in the future if all the lambdas run out/close to run out (aka Wave-length blocking). This requires that you select a vendor who's capable of OTN switching but you need not purchase OTN switching components (cards) on day one.

You do not need OTN switching to achieve mesh protection. What is then required is ASON/GMPLS.

A good approach,adopted by many operators when publishing Request For Proposal (RFP)s for Optical Transport Networks is keeping the RFP open for all the options given above. It's required to give the fiber characteristics, locations and the traffic matrix (current and future). Based ion these inputs the vendors can come out with the least TCO option. The evaluation should also be based on lowest TCO (this covers both initial Capital Expenditure (CAPEX), future expansion CAPEX and the running Operational Expenditure (OPEX) such as site rentals, power etc.).  

When you want to do sub-lambda grooming at intermediate sites, you'll have to have OTN switching (CAPEX!).

When you have OTN switching, the earlier Point-to-Point lambda passed through several intermediate nodes at the optical domain (OOO) now need to go to electrical domain to do grooming (OEO) making it multi-segment. This requires several OTN ports (CAPEX!). However, you use only one lambda. Some call the latter as Layer 1-ASON and former as Layer 0-ASON.

If you do not do sub-lambda grooming at the intermediate site, you will have to have a separate lambda (CAPEX!) at the intermediate site, though the traffic goes to the same destination.

The above 3 CAPEX components need to be properly analyzed for the current and future traffic matrix. Then only the most optimized design and then the most optimized cost can be calculated.

For the comparison, following options are recommended to be used as ASON/GMPLS is better to have. This will also make sure that the comparison is more balanced.

1. FOADM with ASON/GMPLS control plane
2. ROADM with ASON/GMPLS control plane
3. TROADM with ASON/GMPLS control plane
4. FOADM with ASON/GMPLS control plane and OTN switching
5. ROADM with ASON/GMPLS control plane and OTN switching
6. TROADM with ASON/GMPLS control plane and OTN switching 

"Digital Optical Networking (DON)" technology claimed to be available with some vendors can address the sub-lambda grooming at optical level without using OTN switching (i.e using OOO as opposed to OEO) giving cost advantages.The component used is known as "Digital ROADM".

 

Future of wire-line access networks

Introduction

Communication Service Provider (CSP) networks can be divided into many categories. If divided based on the media used for the transport, wired and wireless are the widely known. Both types can be again sub divided based on the functional hierarchy; core, aggregation and access. If wired networks are concerned, optical fiber is used in almost all the core and aggregation networks because of its ability to carry/transport large amounts of data.

Wire line CSP access networks can be built using cable, copper or optical fiber. The cable based access networks are common in North America. Countries like Sri Lanka are more used to copper and fiber based access networks.

Access networks can again be classified based on the customer segment. While it is simple to have a single access network for both residential and business customers, some CSPs build different access networks for these 2 segments. The requirements and expectations of the 2 segments are different.

Design Criteria

A best practice in any access network design is to start with some forecasted marketing data. The networks that we build today should be future proof as far as possible in terms of service demand and technology used. For an example, for a residential triple play access network, it is better to have some figure on the number of customers per area, type of services required by those customers, the bandwidths consumed for each service and the bandwidth per house hold. Based on this information, we can find out the immediate bandwidth requirement per customer and the future demand. While there are many differences between fiber and copper based access networks, the fundamental difference is the ability of fiber to carry large amounts of data. Optical fiber networks in that sense are very much future proof.

It is always a good idea to design the networks for your real requirement rather than going behind various technologies. A technology used in United States (US) or Europe may not be able to directly deployed in countries like Sri Lanka.

The main decision factors when deciding on an access network technology/media are cost per user, bandwidth demand and demographic criteria. The current trend is to deliver more bandwidth per user in a secure way at lower cost.

Copper or fiber

If copper is already laid and available to the households, then it will be a good idea to use them if the required bandwidths can be delivered using them. The fundamental problem of copper based access is the decaying of speed with the distance. Most of today’s copper based access networks start from the Central Office (CO) of the CSP and goes to the customer. The access network is fully passive, consisting of, but not limited to, Main Distribution Frame (MDF), primary cable, Cabinet, secondary cable, Distribution Point (DP) and overhead cable. The popular Digital Subscriber Line (DSL) technologies are struggling to deliver high bandwidths when the distance between the CO and the customer increases.

One solution is to use optical fiber, instead of copper. But the main problem of copper, as identified earlier, is the decaying of speed with distance. While it is accepted that the ideal solution is to have fiber, we can also try to shorten the copper length and deliver high bandwidths. This will save lot of cost as fiber is not freshly drawn.  This introduces a new term called FTTX, Fiber To The “place you want”. X could be building, home, curb/cabinet, node or even desk. So now the issue is between FTTH and FTTC/B.

Optical access networks

Optical access networks could be active or passive. An example of an Active Optical Network (AON) is Metro Ethernet or Carrier Ethernet. Here, active Ethernet switches are deployed in the network to deliver services, mainly to business customers. The networks mostly take the form of rings assuring high availability required by business critical applications.

The other category is Passive Optical Networks (PON).  PON works by delivering an end to end fiber access to the building or home. Though PON can be used for FTTC/N applications, most PON applications are based on FTTH/B architecture. Unlike AON, PONs have limited network protection in the last mile, because of its passive nature. PON works by dividing an optical signal into multiple fibers using a passive optical splitter. After the splitter, the network is liner and does not provide any direct protection. This type of a solution is mainly suitable for a residential rather than business.

The Table 1 below gives a snapshot of various PON technologies.

TDM PON			WDM PON	Hybrid TDM / WDM PON
ATM PON (APON) =Broadband PON(BPON) =A/B PON	Ethernet PON (EPON) = Gigabit Ethernet PON (GEPON )	Gigabit PON (GPON)
ITU-T G.983 standard	IEEE 802.3ah standard	ITU-T G.984 standard		Still at research stage
Developed from Telco side	Developed from Internet side	Evolved from A/B PON	Evolution from TDM PON
Layer 2 encapsulations are Ethernet and Asynchronous Transfer Mode (ATM)	Layer 2 encapsulation is Ethernet	L2 encapsulations are GEM(GPON Encapsulation Method) for Ethernet and ATM
Maximum up stream is 155Mbps, Maximum downstream is 622Mbps	Maximum up stream and downstream is 1.25Gbps	Maximum up stream and downstream is 2.5Gbps	Maximum up stream and downstream is 10 Gbps
Deployments: US (ex: Verizon FiOS)	Deployments: Japan (ex: NTT, KDDI), Korea (ex: KT), China, India	Deployments: US (ex: AT&T), Europe	Deployments: Korea (ex: KT), China

Table 1. PON technology comparison.

TDM PON is the current choice because of its low cost. Out of the available TDM PON technologies, GPON has much better multi-service capabilities and carrier grade management capabilities and therefore the winning technology. It is also future proof, because of its high bandwidth support.

Metro Ethernet & Carrier Ethernet

In the Communication Service Provider (CSP) industry today, there are many confusions with terms. One such example is Metro Ethernet (ME) and Carrier Ethernet (CE).  Many people have asked me the difference between the two. Therefore, I just though of writing this small blog, just to clarify the doubts.

ME was 1^st to come. Hence the Metro Ethernet Forum (MEF). Carrier Ethernet (CE) came later and MEF adopted it as well. So, MEF the one defines the generic standards for ME and now CE.

Both ME and CE, though technical terms, are used as commercial words these days by vendors.

ME is a service. CE can provide ME services too. ME and CE demarcation is now becoming blurred and the future term will be CE.

In fact CE and ME refers to the same thing, i.e well known Ethernet technology/interface used in the Local Area Network (LAN) being used in the Metropolitan Area Network (MAN) and Wide Area Network (WAN) with hardened capabilities, defined as 5 attribute by MEF.

New strategies for Telcos

The Telcos (more correctly, Communication Service Providers (CSPs)) face a severe competition and a pressure today to keep their noses above the water. They can no more sell voice and bandwidth and make that a viable business proposition. Both voice and bandwidth has become a given. Most of the customers granted that as given. Recently the United Nations (UN) has accepted Internet access as a basic human right.

CSPs over the years have built their transport networks (Core, Aggregation and Access). In developing countries, the expansion of wired access networks seems to be prohibitive with the associated cost and the demography. The incumbents specially, have started copper based access networks from the urban areas and started expanding toward the sub-urban and rural. Most of the cases we find that the  already invested infrastructure delivers Plain Old Telephone Service (POTS) services to the customer and there's a huge potential that most of them can be enabled with broadband. Even with that, the total wire based coverage will still be less than 50% of the households. As you reach the rural areas the demand is more scattered and for a developing country, the only viable option is wireless.

If the balance households are reached using wireless, and an acceptable pipe is delivered to the household or individual, just by selling voice and Internet will not make the CSP win. The Average Revenue Per User (ARPU) has saturated and CSP have to think how they can make the customer spend more. The old form of merely satisfying  their communication, information and entertainment requirements is not just enough. We have to think selling the "convenience"  to the user. One example of selling the "convenience" is the e-banking introduced by the banks. People are prepared to pay a premium to this facility as they can make all their bill payments electronically.

The CSPs need to this considering the total Information Communication Technology (ICT) ecosystem, making sure all stakeholders are benefited. Just take an example. The CSPs are good at building networks (providing the transport), but they may not be good at  producing applications to run on all three screens (computer, mobile and TV). On the other hand, the software companies can produce applications (of course after a good socioeconomic study ), but they can not build communication networks. Both CSPs and software companies can get together in a win-win arrangement to provide a service to the customer. The applications so developed need to be based on the principle of "convenience".