SONET/SDH

Prior to the 1980s, ATT operated a monopoly on telecommunications within the US. AT&T was ordered to breakup, and shortly after new long-distance carriers stepped into the market. The problem these new carriers were faced with, was that local-loops were all operated by AT&T and terminated to AT&T equipment. The infrastructure work required to re-do this was prohibitive, so these local loops were kept as they were. Therefore, the long-distance carriers needed to interact with the local AT&T equipment in each CO (central office). This equipment was by its nature propreitary to AT&T, so the new carriers would have to purchase AT&T equipment in order to interface with AT&T. It seems a little ridiculous that to enter the market, a company needs to purchase the equipment of the company that previously held a monopoly.

To solve this issue, the open standards called SONET (Synchronous Optical Network) and SDH (Synchronous Digital Heirarchy) were created. These are time-division based protocols which operate over fiber. SONET is used in the US and Canada, and SDH is used everywhere else in the world.

TDM Circuits

To understand where SONET came from, we’ll first look at TDM circuits.

Time-Division Multiplexing (TDM) is the process of allowing a single channel to use the entire bandwidth of a circuit, but only part of the time. A comparison to WDM (wavelength division multiplexing) may help explain this. In WDM the multiplex is the “color” of the light. Multiple p2p circuits can be created out of a single fiber medium because each connection uses a different color. In TDM, multiple channels use a single circuit, but they each only get a partial period of time to transmit. The T-Carrier system uses 24 channels for a circuit, and E-Carrier uses 32 channels.

A T1 takes 24 input sources (called DS0) at 8bits per sample and puts them altogther into a single frame, with a frame bit added. This results in 193 bits. 8000 frames are sent per second, resulting in 1.544 Mbps line rate. The initial use was to multiplex individual 64Kbps voice calls onto a single circuit.

An E1, used outside the US and Canada, uses 32 channels and therefore has a line rate of 2.048 Mbps.

Digital Signal (DS) System

A DS1 is an interchangable term for a T1. Four DS1s can multiplex into a DS2 to provide a throughput of 6.312Mbps. Seven DS2s can multiplex into a DS3 to provide a throughput of 44.736Mbps.

When going from DS1 to DS2, the first bit of each DS1 is placed one behind the other, then the second bit of each DS2, etc. This ensures that each DS1 is still effectively being sent at a T1 speed, because the DS2 rate can send four times as fast as T1. When this is done, control bits have to be added to the data, creating an overall DS2 frame. As DS2 signals are multiplexed into DS3, this process repeats and complexity is added.

The problem is that, if you need to remove a DS1 signal out of this DS3 and place it at its destination, you need to completely de-multiplex the DS3 into DS2, then from DS2 into DS1, pull out the single DS1, and then rebuild back into the DS3 and continue the data along its path. This is a very complex problem, and the very issue that SONET solves.

Circuit	Composed of	Rate
T1 / DS1	24 DS0	1.544 Mbps
DS2	4 DS1	6.312 Mbps
T3 / DS3	28 DS1 (or 7 DS2)	44.736 Mbps
E1	30 DS0	2.048 Mbps
E3	16 E1	34.368 Mbps

SONET

SONET adds a lot of overhead to the carried payload, which results in robust OAM tools. Included in this overhead is Section Overhead, Line Overhead, and Path Overload. Many argue that this bloats SONET and makes it inefficient, however the resulting tools that this enabels are extremely useful.

Like a T1, SONET frames are timed and sent at a rate of 8000 frames per second. However, a SONET frame is 810 bytes, which yeilds a throughput of 51.84 Mbps. The base unit of a SONET frame is called an STS-1 or OC1. STS is the frame format, while OC is the optical carrier level, which describes the optical signal.

Just like a DS2 and DS3, SONET frames can be bit interleaved and combined to produce higher bitrates. Three STS-1s can combine to produce an STS-3 frame with a line rate of 155.52 Mbps. Just like a DS2, the STS-3 takes one bit from each STS-1, one at a time, and interleaves them together.

OC	STS	Line rate
OC-1	STS-1	51.84 Mbps
OC-3	STS-3	155.52 Mbps
OC-12	STS-12	622.08 Mbps
OC-24	STS-24	1.244 Gbps
OC-48	STS-48	2.488 Gbps
OC-192	STS-192	9.953 Gbps
OC-768	STS-768	39.813 Gbps

Part of the SONET overhead is used for automatic protection switching (APS). This is a built-in ring protection mechanism which allows for detection and failure recovery in a ring topology within 50msec. This is where the 50msec Ethernet FRR repair target time comes from. In order for Ethernet to compete with WAN technologies like SONET, Ethernet needed support for OAM, ring protection, fast failover, etc. SONET is commonly deployed in a ring topology, because you get redundancy for limited cost. (A full mesh is highly redundant but very expensive to implement).

SONET requires synchronized timing among devices, due to its time division multiplexing nature. Typically a device in a CO (central office) obtains time using GPS and is therefore a stratum 1 device. This device provides a clock source to all other devices in the CO.

SONET is actually not as strictly timed as it may seem, though. A SONET device can obtain timing from the line input timing signal instead of a stratum 1 clock source. Additionally, a device can just be “free running” in which it simply uses its own internal timing ability.

In a SONET ring, all devices must either use an external timing source, or one device must be “free running” and all devices determine their timing via the line rate which is dictacted by the “free running” device.

SONET is a layer1 and loosely a layer2 techonology. It doesn’t have the same concept of broadcast that Ethernet does. Therefore you don’t need to worry about spanning-tree and loop prevention in a ring, because SONET cannot cause a “broadcast loop” in the first place.

SONET can carry traffic from other protocols, such as PPP (PoS), Ethernet (EoS) and ATM.

Further Reading

SONET/SDH Demystified, Steven Shepard, Chapters 1 and 2

http://pld.cs.luc.edu/telecom/mnotes/tcarrier_sonet.html

https://en.wikipedia.org/wiki/Synchronous_optical_networking