SONET Packet-Oriented Data Framing

In my last legacy PSTN post I discussed how Synchronous Optical Network (SONET) is used to multiplex, transmit and then de-multiplex voice calls. Today, let’s take a look at how SONET  is being used to transmit packet-oriented data (in today’s world - basically Ethernet).

In that last SONET post we said the SONET international equivalent is called Synchronous Digital Hierarchy (SDH). Now, when we talk about data at the SONET/SDH level we’re talking frames (think layer 2 OSI model) and the base unit of framing for SDH is something called a Synchronous Transport Module, level 1 (STM-1) with operates at 155.52 Mbps. 

In the post I also said the base SONET standard bit rate is 51.84 Mbps and is referred to as Optical Carrier  (OC) -1 or Synchronous Transport Level  (STS) -1. Now, because we’re talking 3 times an STS-1 and it is concatenated (combined), the base SONET data framing unit (running at 155.52 Mbps)  is referred to as a STS-3c (Synchronous Transport Signal 3, concatenated) which is also referred to as an OC-3c (Optical Carrier - 3c). 

Now that I have you completely confused (!) lets’s talk a little more about packet frames. A typical packet frame consists of a header, payload (the actual data being sent) and some kind of trailer. I like to use a letter analogy to understand what is going on - someone writes a letter (think of the letter as the payload or data). It gets put on an envelope (think of the envelop as the header and trailer for now). At the sending end the letter gets a destination address, a return address, etc and gets delivered. At the receiving end the letter gets opened, the envelop discarded and the letter itself saved and used.

For an STS-3c framing unit, the payload rate is 149.76 Mbit/s and overhead is 5.76 Mbit/s.

If we look at an individual SONET STS-3c frame - it’s  2,430 octets long. SONET systems transmit nine octets of overhead and then 261 octets of payload in sequence. This transmission is  repeated nine times in 125 micro-seconds until 2,430 octets have been transmitted. 

Timing is critical here (that's why it's called synchronous) for communications across the entire network.

Ethernet To The Home Over Fiber - A PON Alternative?

Passive Optical Networks (PONs) have been getting a lot of attention here in the United States as companies like Verizon move to deliver voice, video and data services over optical fiber to homes. Mike Q and I even did a podcast detailing Verizon's Fiber to the Home (FTTH) PON configuration last May. In the United States we seem to believe PON technology is the best way to deliver signals over fiber to residences. This is not the case in Europe though. Lightwave.com has just posted a very interesting piece titled Active Ethernet FTTH offers PON alternative.

According to Lightwave.com, hundreds of networks around the world—and particularly in Europe—use Ethernet switches to deliver high-speed voice, data, and video services to single-family homes and apartment complexes. One of the reasons, according to Lightwave, it has not caught on is because proponents of the technology have not come up with a universal name for the technology. One name that has taken limited hold is Active Ethernet, which clearly differentiates itself from passive PON technology. This term works for some but not others - according to Lightwave.com, not all implementations place the switches in the field; the Ethernet equipment can reside in the central office (CO), with a fiber running directly from the CO to each subscriber.

As a result, some companies, like Alcatel-Lucent use the term Active Ethernet to describe implementations where there are Ethernet switches in the field and Point-to-Point to describe an implementation that is directly connected over fiber without switches in the field. Other companies are using other terms - Cisco just uses Ethernet FTTH for all Ethernet over fiber configurations.

I've always been a fan of Ethernet and have believed most connections would eventually become Ethernet. It scales well and is easy to implement, configure and maintain.What's likely hurt it most in this country as a long distance option are switch power needs in the field for active implementations and the cost of direct connecting a piece of fiber from a home directly to a switch in the CO. For these reasons I suspect companies like Verizon have gone with a PON delivery configuration. But, is this only temporary? Cost differences between Ethernet and PON actually shrink as bandwidth per subscriber increases - here's more from the Lightwave.com piece:

Bandwidth levels of 20 Mbits/sec and greater generally require that the number of splits per PON shrink. This translates into more fibers in the field and more ports in the CO to service the same number of subscribers, making PON infrastructures look more like point-to-point when it comes to fibers deployed and CO ports.

Here's an interesting quote from Ian Hood, senior marketing manager at Cisco:

"What I see in the marketplace is that providers are putting in PON for I'll call it the low end of the market—best effort, small customers, small businesses, the less than 15 Mbits per customer kind of speeds. For their high-value customers and some of their businesses, they're going with an Ethernet solution, be it building-oriented or dedicated from the CO to get beyond the 25-Mbit realm. So you're seeing a hybrid approach basically in a lot of the new large city deployments."

Will PONs go the route of DSL in the United States and be looked back on some day as a temporary technology? Perhaps - the one big advantage Ethernet currently has over PON is it has the potential to deliver the highest potential bandwidth per subscriber. Here's another quote from Hood:

"If we can get the optics costs down and embed them along with the multiplexer into our Ethernet switches, then you can go beyond 100 megabits to gigabits to whatever you can run on a lambda at that kind of cost for the next evolution of speed."

Which technology will eventually dominate? Will another alternative to PON and Ethernet come along that is superior to both? Time will tell. Be sure to read the entire Lightwave.com piece linked here.