10G ethernet and SDH

 

This is a third page about data transmission over fibre, it is having a look at 10Gb/s ethernet and SDH.

When I started to look at 10G ethernet for this page, I didn`t realise how much I would be side-tracked by the different versions of 10G ethernet - but I think I have got there in the end.

There are two ways to go with 10G ethernet and fibre -

I guess we need to cover both of these, here is some information I have dug up so far.

 

10G is different !

10Gb/s ethernet has a number of differences that separate it from other ethernet types -

 

Ethernet coding

Most ethernet technologies up to and including 1Gb/s use a form of coding that means for each 8 bits of data, 10 bits are actually put on the line. This is known as 8B/10B coding.

So 1Gb/s ethernet has a data bit rate of 1Gb/s. However on the line the actual bit rate is 1.25Gb/s. The additional bits are used for housekeeping. At lower speeds, this additional speed requirement isn`t too much of a problem.

However for 10G ethernet, it was reckoned that this would be a significant problem, so a different coding mechanism was used - it is called 64B/66B, and it increases the data bit rate by a very much smaller amount - I think it is also used on Fibre Channel.

So 10G ethernet running at a data rate of 10.0Gb/s has a line rate of 10.3125Gb/s, rather than 12.5 Gb/s that would have happened with 8B/10B coding.

Just to confuse the issue, there are types of 10G ethernet that use WWDM in LAN`s, and use 8B/10B coding - it is specified within IEEE 802.3-2008 for some particular types of connections.

 

Different speeds

Amongst the numerous different standards for 10G ethernet, there are two broad classes -

The line rate of 9.95328Gb/s is the same rate as OC192 or the STM-64 frame in SDH. However this doesn`t mean that 10G ethernet will just plug into an STM-64 frame - there are other differences.

The data rate of 10Gb/s is the core data rate for 10G ethernet, and the core line rate is 10.3125Gb/s. The lower rate is only used when it is actually required for a particular WAN.

The line rate of 9.95328Gb/s for STM-64 produces a data capacity rate of 9.621504Gb/s, which is just greater than the 9.58464Gb/s of the WAN PHY data rate.

 

10G ethernet frame

The 10G ethernet frame used on the higher speed 10G ethernet is just the same as the frame used in lower speeds of ethernet.

This is a deliberate choice, so that 10G ethernet fits straight into existing lower speed ethernet networks.

The lower speed 10G ethernet uses a different frame structure, which is produced out of the above frame structure.

 

Different types of 10G ethernet

The IEEE 802.3-2008 standard specifies a number of different sub-standards for different types of fibre, and copper - there are four main divisions, and three of these are subdivided again. The four main divisions are -

Expanding on these various sub-divisions :-

You can get an idea what a particular type of 10G ethernet is for by the letters in the name -

I`m not certain about this, but I believe that 10GBase-LX4 was the first standard for 10G ethernet. Once 1G ethernet was up and running, it was reckoned that the next development would be 2.5Gb/s. So it was a small step to think about using four of 2.5Gb/s streams running in parallel to produce 10Gb/s ethernet. The standard for it was produced around 2002. I think it was originally known as 10GbE. However 10GBase-LX4 is still in the IEEE 802.3 standard, and is still used.

It was followed in 2004 by an equivalent standard for carrying 10Gb/s over copper - 10GBase-CX4. This uses 4 pairs of cable to carry the four 2.5Gb/s streams.

I think that this history explains why they still use 8B/10B coding.

About the same time, the standards for 10GBase-SR and 10GBase-LR were produced, so they presumably were the first versions of 10G ethernet to use 64B/66B coding, and a single serial line bit rate of 10.3125Gb/s, instead of four streams in parallel.

Subsequently the lower speed 9.58Gb/s versions were produced, in response to campaigning by the WAN fibre backbone providers who already had a considerable investment in SDH based lit fibre.

It is possible to carry 10GBase-T on good quality cat 5 and cat 6 cable, but distances are quite limited. Cat 7 is better. Fully screened cat 6 and cat 7 will support 10GBase-T up to distances of 100 metres.

There is another two versions of 10G ethernet not shown above, which are not included in the IEEE 802.3 standard. They cover distances up to 80kms -

They are being developed by various manufacturers. I wonder if the IEEE will eventually adopt them. Further, I believe they are based on the specification for SDH for fibre lengths up to 80kms. But as well as 80kms, there is an SDH specification for fibre lengths up to 120kms - so will 10G ethernet specifications emerge for 120kms as well ?

IEEE 802.3 also contains another two 10G ethernet standards, but not for fibre - they are specified for backplanes, and are 10GBase-KX4 and 10GBase-KR. Looking at the letters in the name, I guess that -

 

10G ethernet and SDH

As stated above, the core data rate for 10G ethernet is 10.0Gb/s, and the lower rate is 9.58Gb/s.

The MAC layer operates at the 10Gb/s rate for both speeds, however when the MAC layer creates the frames for the lower rate, it increases the interframe gap.

The interframe gap is an obligitory space between frames, so equipment can re-initialise itself for each frame. Increasing the interframe gap is a well known method of decreasing the effective bit rate of a data stream. The interframe gap is specified as 96 bit periods in nearly all version of ethernet. For normal 10G ethernet, the interframe gap lasts for 6.4ns.

For the lower speed 10G ethernet, the interframe gap is increased to 104 bits. It took me a long time to find that information, I eventually found it in IEEE 802.3-2008 section 4.4.2 note 5.

I`m not sure about other versions of ethernet, but in 10G ethernet, the interframe gap is not empty, but is composed of a series of idle control characters. I still haven`t found out what an idle control character is !

The frames with the increased interframe gap are passed down through an intermediate software layer, to an additional software layer called WIS - WAN Interface Sublayer. This software layer only exists in the software stack for the lower speed 10G ethernet versions.

The first thing the WIS does is to discard the idle control characters. ( IEEE 802.3-2008 section 46.1.3 )

The WIS layer then maps the frame produced by the MAC layer into a frame which is compatible with an STM-64 frame

In doing this, the WIS has to take account of the fact that the bit numbering is different between ethernet and SDH :-

More on this in IEEE 802.3-2008 section 50.1.7

Along with the bit mapping, the WIS layer adds fixed stuff bits, and path overhead octets. This creates an SPE - Synchronous Payload Envelope. An SPE is the SONET equivalent of a virtual container in an STM frame.

Line and Section Overhead objects are added.

The whole frame is then scrambled just as STM frames are scrambled in SDH.

This produces a WIS frame that is compatible with STM-64. The frame period is 125µs, and there are are no spaces between the frames, again, as per SDH.

IEEE 802.3-2008 section 50.3 is the place to go for this.

So I think I have finally found how 10G ethernet is carried over SDH. Obviously, the whole process is reversed when the ethernet data stream leaves the SDH network.

 

Finally ...

When I started digging into how 10G ethernet is carried over SDH, I had no idea that I would find :-

So it has all taken a lot longer than I thought it would !

 

Update -

Thinking a bit more about this, it occurred to me that to be technically correct, to say that WIS produces an STM-64 frame is wrong.

The IEEE are American, so in fact IEEE 802.3-2008 refers to SONET, which is the American version of SDH.

So WIS actually produces a SONET STS-192 frame - and a SONET STS-192 frame is not the same as an SDH STM-64 frame. There are differences in the structure of the payload section, and there are differences in the structure of the overhead section.

I`m not sure how this will affect the carriage of 10G ethernet over SDH - there must be plenty of equipment available that can cross-connect SONET and SDH, as every trans-atlantic fibre must have this same problem. But it is an extra layer of technology.

 

 

 

 

 

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