The OC3 - A Great Transport And Access Solution

Optical Carrier 3 Services

OC3 stands for Optical Carrier level 3 and is part of the SONET hierarchy. It is a great solution that can be used as a core transport medium in remote locations and as a full fledged customer service line in more dense and populated urban and suburban areas. OC3 is highly flexible and extremely versatile transport link.

Before we look at Optical Carrier level 3 in details, let us first look at the predecessor to the SONET technology and the drawbacks in the same. The first technology to incrementally grow the network capacity was called the Plesiochronous Digital Hierarchy or PDH. With the PDH solution, we have regular voice call as a DS0 and this was combined into DS1 carrying 24 voice calls. The DS1 was further aggregated into DS3 with 28 DS1.

While this was a great solution in terms of traffic aggregation, it had a major inherent drawback. The PDH networks were not universally synchronized. As a result there had to be some synchronization information that needed to be added to the frames. This meant that the DS1 was not just 24 voice calls, but had some overheads too.

The problem with the overheads in PDH networks was that to drop a DS0 and add another one, the entire DS1 frame had to be broken and rebuilt. This was highly inefficient, especially with the need for bandwidth increasing. This is where SONET or Synchronous Optical NETwork became prominent.

SONET also has its share of overheads, but they are factored into a synchronous frame. It also has pointers in the overhead that allow the PDH signals to be directly added or dropped from the frame without having to rebuild it. This meant that signal transport can be carried out at a very fast rate and it ushered many SONET levels; each level supporting much higher speeds than the lower one.

The most basic level in SONET is the OC3 or the Optical Carrier 3. The bit rate for Optical Carrier level 3 line is 155.52 Mbps. While this is the fundamental SONET network link, it inherently consists of three STS-1s. Each of these is around 51.84 Mbps and can carry a complete DS3 by itself.

The OC3 link has been one of the most important link rates in SONET networks. In the regions on outskirts of the major cities, the Optical Carrier level 3 links are quite commonly used as the backbone transport. With the capacity of 3 DS3 it can actually support over 2000 voice calls simultaneously which makes it a perfect core link. In urban areas it is fast becoming a link that connects to the customer location itself, especially in commercial buildings with multiple enterprises.


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