Sunday, September 16, 2012

Why the need of Ethernet in Back-Haul???

Ethernet, what on earth it is doing in a back-haul environment?

Many of my transport friends feel “why do we, after-all need Ethernet in the transport back-haul environment? Why on earth everything is changing to Ethernet? We were well off with the traditional TDM and were well versed with it. All we needed to do was to provision some trails, create some cross connects and then boom the switch guy used to do things. “

The transmission planning used to plan the fiber routes and the trails and the NOC used to provision the same. In some cases of troubleshooting, well just give a loop to check. Loop – break….loop-break……loop-break and find the problem. So why today we have to have this complicated thing called ehternet?

Well, my dear readers, every child has to grow and so will your network. As the customers keep on increasing their demands also increase. As they see more things happening outside our country so they also want the same thing. This ushers a kind of requirement for heavy BW and of course a very clever way to engineer it.

TDM, classically works on a point to point model without any BW sharing. So as to say if you have 10 sites in a ring each having a drop of 10 Mb/s  then  in case some site is not using the BW the others cannot use up momentarily the same BW. Which means there is no sharing of BW, the access is hard coded to 10 Mb/s and even though there is a need of BW at some other place while one site is not using it this cannot be provided.

Please look at the figure below for complete understanding.

As we can see in this figure that there are 5 locations in the STM-1 ring that are parented to the aggregate locations. All these locations are Mux or Multiplexer locations.  For each location using the traditional TDM deployment there is a mapping of 5XVC-12 to each of the location.  So what is the limiting factor.

1.       Suppose Site -1 is having a heavy requirement say around 14 Mb/s in some instance and Site 3 is only using 4 Mb/s then the 6 Mb/s form Site-3 cannot be dynamically allocated to Site -1 as a timely lease. This has only to be done by manual provisioning.
2.       As the committed BW increases in the entire requirement the number of physical interfaces also increase to a great extent.  Hence if tomorrow the need is 22 Mb/s per site then it has to be clubbed with 11 E-1s and that too not in a shareable mode. The scalability is actually limited. This is because of the interface limitation.

Due to these two major factors there was a need of Ethernet in the network. There needed to be a kind of a device connected to these first mile wireless location which are actually able to handle huge BW. And this my friends can only be at present possible by an Ethernet device.  Hence the access interface had to be an Ethernet interface which can scale upto 10Gb/s base band. This suffices the second requirement of the above two.

What about BW sharing then? How does Ethernet help in doing this?

Well the basis of Data services are the fact that not every one is using the same BW at the same time. So at any point of time the entire capacity of the ring can be floated across. In Ethernet there is not hard coding of BW provisioning like there is in TDM. What we call trails in TDM actually boils down to a service in Ethernet.  However this service is having a special characteristic. This characteristic is that it can actually have dynamic BW sharing. So as to say that the entire BW of the ring is actually available to all.

Typical Ethernet configuration is shown in the figure below.

In this figure as you can see there are no hard coded VC-12 mapping for the traffic. There is definitely a service entity but that is not the trail. This is called a Ethernet service and this is having two parameters over here.

CIR: Committed Information Rate, meaning the amount of BW that the site is always honored to get and should get irrespective of whatever happens.

EIR: Extra information Rate, meaning the amount of BW that the site can achieve extra provided the resources in the network are not being utilized.

This means that if we make services like this for all the 5 sites each of them may get 100Mb/s peak. However, in case all the sites are using BW at the same time then at least 10Mb/s is guaranteed.  So there is no need of frequent BW grooming.

In high end data requirements one thing is to be taken as a postulate and that is “Not every one is going to peak at the same time”.

This basic assumption actually forms the basis of deploying Ethernet in back-hauls that are more data rich vis a vis 3G, LTE, HSIA, EvDO, Retail Broadband etc etc.

The main thing to remember:

The main thing to remember about classical TDM is that in classical TDM one deals with Multiplexers. The work of a multiplexer is actually to add and drop traffic from a lower tributary to higher rates in a framed manner. Hence, a TDM provisioning of transport is not essentially concerned about the utilization of the traffic. So in a classical TDM environment the parameters most essential for the transmission guys are actually concerned about health of the path with respect to errors and fault.

However, with the onset of Data services and Ethernet deployment, the multiplexing environment takes a shift and moves to switching environment. The transport guy is also responsible for switching of traffic, which was previously a responsibility of the Switch guy. This is because the elements in the transport are no longer multiplexers, they are actually switches, which measure, meter and also show the transmission guys the utilization of the BW.

This helps the transmission guy, especially the planner to actually monitor the traffic and provision as per requirement thus judiciously saving resources and delivering high BW services at the same time.

This main thing is the only important thing for a TDM transmission guy to understand while he and his company is moving towards a Ethernet back-haul. 

So friends, do not be paranoid about this change.....Just accept it as a new technology......