POS-PHY
All through the digging I have done on the web about data transfer over fibre, one of the expressions that popped up from time to time was POS.
- sometimes it was just POS on its own
- sometimes it was POS-PHY
There was rarely any explanation of what they were talking about, so it was a bit time consuming to try and work out what POS and POS-PHY are.
I`ve already done a page about POS - Packet-over-Sonet/SDH.
The various versions of POS-PHY need a different page, because they are actually quite different from POS - they are actually an industry standard bus connection - they are designed to allow integrated circuits to talk to each other over a standard kind of bus.
However they are still very much connected with data transmission over fibre - they were originally written by the Saturn Development Group, and then taken over by the Optical Internetworking Forum, who are a fairly large group of users, as well as equipment designers who develop hardware and systems for the fibre industry.
POS-PHY specifically relates to the type of connection for the packet transfers between the Link Layer devices and the physical devices - usually, but not exclusively, the physical layer is assumed to be Packet-over-SONET.
POS-PHY was the original name for the standards that are now called SPI - System Packet Interface - by the OIF.
There are 4 SPI`s that I am aware of, there maybe more. The different SPI`s are known as levels, and the different levels can transfer data at different rates. The ones that I have seen reference to are :-
- SPI level 3 - supports up to 2.4Gb/s, and STS-1, STS-3/STM-1, STS-12/STM-4, STS-48/STM-16
- SPI Level 4 - phase 1 - supports OC-192
- SPI Level 4 - phase 2 - supports 10Gb/s ethernet and STM-64/STS-192
- SPI Level 5 - supports 40Gb/s, and OC-768, as well as Packet-over-SONET
Like most parallel bus systems, as well as a number of data signals, there are various control signals as well - the different levels of SPI`s are somewhat different in the number of data signal and control signals they have, they are not just faster versions of the same thing.
It is noticeable that the faster the standard, the fewer the types of control signals are used.
All the SPI`s are tens of pages long, so there is little possibility of putting all the information about them on this single page, but here are some of the key characteristics.
SPI Level 3
- supports data rates up to 2.4Gb/s
- supports several of the lower speed SONET and SDH frame rates - STS-1, STS-3/STM-1, STS-12/STM-4, STS-48/STM-16
- runs with a clock frequency of up to 104Mhz
- specified for use with either 8 or 32 data signals
- full duplex operation - there is a complete set of data and control signals for each direction of data flow
SPI Level 4 phase 1
- supports data rates of 10Gb/s - OC-192
- runs with a clock frequency of up to 200Mhz
- bus data rates up to 12.8Gb/s
- specified for use with either 64 data signals or four groups of 16 data signals
- I can`t see if it specifies bi-directional or full duplex operation
SPI Level 4 phase 2
- it looks like SPI-4 phase 2 is very different from SPI-4 phase 1
- SPI-4 Phase 2 specifies a higher speed interface than what is specified in phase 1
- supports 10Gb/s ethernet, OC-192, and STM-64/STS-192
- it seems to be more aware of Packet-over-SONET/SDH than phase 1
- bus frequencies seem to go up to about 650Mhz, but seem to depend on what the interface is doing - 622Mhz is the minimum allowed
- it has just 16 data signals
- it seems to be full duplex operation
- it has very few control signals
The signals that are specified for the transmit path - ie, from higher level Link layer to lower pseudo physical layer - are
- TSCLK
- TSTAT - 2 of
- TDCLK
- TDAT - 16 of
- TCTL
The signals that are specified for the receive path - ie, data transfer from the fibre up the stack to the Link layer - are
- RDCLK
- RDAT - 16 of
- RCTL
- RSTAT - 2 of
- RSCLK
SPI Level 5
- supports data rates up to 40Gb/s
- supports OC-768 and Packet-over-SONET/SDH
- minimum data rate on the data signals is 2.488Gb/s
- specified for use with 16 data signals
- full duplex operation - there is a complete set of data and control signals for each direction of data flow
- uses scrambling on each bit lane
The signals that are specified for the transmit path - ie, from higher level Link layer to lower pseudo physical layer - are
- TDCLK
- TDAT - 16 of
- TCTL
- TSTAT
The signals that are specified for the receive path - ie, data transfer from the fibre up the stack to the Link layer - are
- RDCLK
- RDAT - 16 of
- RCTL
- RSTAT
Finally .......
As already said, the documents are quite large, so all I can present here is an overview. If you want more information, they are available from the Optical Internetworking Forum website. They are presented as pdf`s, and I had trouble opening some of them. Normally I don`t have any problems with pdf`s, some of the IEEE 802.3 standards are also pdf`s, are nearly 10 times the size, and they worked fine. So I don`t know why the OIF ones cause problems.
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