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The SONET and SDH Signal Hierarchy: How many T-1s are in an OC-1, OC-3, OC-12, or OC-48? May 10, 2009

Posted by HubTechInsider in Definitions, Fiber Optics, Telecommunications, Uncategorized.
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I have found that there exists out there in the wide world a touch of confusion when it comes to recognizing the different signal levels and transmission speeds associated with what is referred to in the telecom industry as digital hierarchies, the two most common of which are, in North America, the PDH and SDH, or SONET, hierarchies.

Throughout my work as a telecommunications enthusiast, a pastime of discovery which has kept me occupied ever since my teen years, and on through many of my professional pursuits, I have always served as a point of reference for others in regards to the various telecommunications signal levels as well as the transmission speeds that these levels in the hierarchies represent. The following is my rough attempt to put this information into one place that can serve as a reference for me and others:

SONET was developed to aggregate, or multiplex, circuit switched traffic such as T-1, (E-1 in Europe) T-3, and slower rates of data traffic from multiple sources on fiber-optic networks. SONET transports traffic at high speeds called OC (Optical Carrier). The international version of SONET is called the synchronous digital hierarchy (SDH). SDH carries traffic at synchronous transport mode speeds. Equipment interfaces make SONET and SDH speeds compatible with each other, so the same SONET switching equipment can be used for both OC and SDH speeds.

OC-1 operates at 52 Mbps and is equivalent to 28 DS-1s (same as a T-1) or 1 DS-3 (same as a T-3). OC-1 is generally used as customer access lines. Early-adopter types of customers such as universities, airports, financial institutions, large government agencies, and ISPs – use OC-1.

OC-3 operates at 155 Mbps and is equivalent to 84 DS-1s (same as a T-1) or 3 DS-3s (same as a T-3). OC-3 speeds are required by end users such as companies in the aerospace industry and high-tier ISPs.

OC-12 operates at 622 Mbps and is equivalent to 336 DS-1s (same as T-1) or 12 DS-3s. This is another capacity towards which high-tier ISPs are moving. It was originally deployed for the metropolitan area fiber rings built out across cities worldwide, although those rings are now moving to OC-48.

OC-48 operates at 2,488 Mbps and is equivalent to 1,344 DS-1s (same as a T-1) or 48 DS-3s (same as a T-3). This capacity has been deployed for backbone, or core, networks. Today the metropolitan area rings are moving from OC-48 to OC-192.

OC-192 operates at 9,953 Mbps and is equivalent to 5,376 DS-1s (same as a T-1) or 192 DS-3s (same as a T-3). OC-192 is in use for backbone networks.

OC-768 operates at 39,812 Mbps and is equivalent to 21,504 DS-1s (same as a T-1) or 768 DS-3s (same as a T-3). Use of OC-768 is very rare outside of testing or research networks due to the great expense of this transmission speed level.

At times, you may see OC levels such as OC-1c, OC-3c, OC-12c, etc. This is called concatenation, and it puts streams of data into one fat, or high-bandwidth, contiguous stream. For example, OC-1 speeds of 52 Mbps may be used to carry broadcast video. In this case, OC-1c, or concatenated OC-1, carries OC-1 streams back-to-back. These streams travel contiguously through the network as long as capacity is available. Most applications for concatenation are high-speed data and broadcast-quality video.

As far as the DS, or Digital Signal Levels, of the older PDH, or Plesiochronous Digital Hierarchy (plesiochronous means “minute variations in timing”), they follow what is known as the T-carrier signal levels. Technically, the DS-x and CEPT-x terminology (DS-1, DS-3, CEPT-1, CEPT-3, and so on) indicates a specific signal level (and thus usable bandwidth), as well as the electrical interface specification. T-x and E-x terminology (T-1, T-3, E-1, E-3, and so on) indicates the type of carrier – a specific implementation of a DS-x/CEPT-x. More often than not these days, however, the terms DS-x and T-x are used interchangeably. So some people might use the term DS-1 and T-1 to refer to the same thing – a digital transport that can carry 1.544 Mpbs over a total of 24 voice channels. In Europe, the same is true: E-1 is the same as CEPT-1, and so forth.

A DS-0 (T-0) has a bit rate of 64 Kbps and carries 1 voice-grade channel.

A DS-1 is equivalent to a T-1 and has a bit rate of 1.544 Mpbs and carries 24 voice channels.

A DS-2 has a bit rate of 6.312 Mpbs and carries 96 voice channels, equivalent to 4 T-1s. This is also sometimes referred to as a T-2, or T2.

A DS-3 has a bit rate of 44.736 Mpbs and carries 672 voice channels, equivalent to 28 T-1s. This is also sometimes referred to as a T-3, or T3.

A DS-4 has a bit rate of 274.176 Mpbs and carries 4,032 voice channels, equivalent to 168 T-1s. This is also sometimes referred to as a T-4, or T4.

Want to know more?

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About the author.

I’m Paul Seibert, Editor of Boston’s Hub Tech Insider, a Boston focused technology blog. You can connect with me on LinkedIn, follow me on Twitter, even friend me on Facebook if you’re cool. I own and am trying to sell a dual-zoned, residential & commercial Office Building in Natick, MA. I have a background in entrepreneurship, ecommerce, telecommunications and software development, I’m the Senior Technical Project Manager at eSpendWise, I’m a serial entrepreneur and the co-founder of Tshirtnow.net.

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