POS

This is a fourth page about data transmission over fibre. It looks at a mechanism for carrying IP packets over SONET/SDH networks.

POS is an acronym for Packet-over-SONET/SDH.

The basis of POS is that it uses another protocol - PPP - Point-to-Point Protocol - to wrap the IP packet into a PPP frame, instead of wrapping it up into an ethernet MAC frame. So it replaces the ethernet frame.

PPP transports multi protocol datagrams over point-to-point links ( like SONET/SDH ), so it can carry not just IP, but other protocols like IPX, IPv6, NetBIOS, and Appletalk.

There are two standards for POS -

RFC 1619 - this was the first one, and there were found to be security weaknesses with it
RFC 2615 - this was the second one, and it uses a more rigorous form of scrambling, amongst the changes

The standards include the various containers that are part of the SONET/SDH frames that POS is specified for.

STS-3c-SPE VC-4
STS-12c-SPE VC-4-4c
STS-48c-SPE VC-4-16c
STS-192c-SPE VC-4-64c

The PPP frame

As said above, PPP wraps the incoming IP packet up into a PPP frame, instead of into the ethernet MAC frame.

The PPP frame has its own format, in fact there are several different PPP frame formats, depending on what kind of information that particular frame is carrying.

However they all have the same general frame format, and the general frame format of PPP has its roots in HDLC framing. Some of the fields in HDLC frames aren`t required in PPP, however they are still in the frame structure, and so are filled with dummy values.

Start flag - 1 byte - 0x7e
Address - 1 byte - this isn`t really used in PPP, so is always set to 0xff
Control - 1 byte - this isn`t really used in PPP, so is always set to 0x03
Protocol - 2 bytes - identifies the type of protocol being carried in the payload section
Payload - variable in length - this section can be used for control information instead of data
Padding - not always used, variable in length
FCS - Frame Check Sequence - usually 2 bytes, can be 4 - similar to the CRC check in the ethernet frame
End flag - 1 byte - 0x7e

The PPP frame produces an octet interface to SONET/SDH

The processes involved in wrapping up the IP packet and delivering it to SONET/SDH are :-

the IP packet is wrapped into the PPP frame structure
the PPP frame headers are added
the Frame Check Sequence information is added
any required byte stuffing is added
the frame is scrambled using an X**43 + 1 polynomial
the final PPP frame is passed to the SONET/SDH framing mechanism

The successive PPP frames fill up the rows within the payload container within the SONET/SDH frames. PPP frames are variable in length, and can cross SPE / higher order VC boundaries.

POS and scrambling

As suggested above, during the creation of the PPP frame, before it passed into the SPE / VC, a significant part of the PPP frame is scrambled.

As discussed on my previous web page about SDH, both SONET and SDH frames are scrambled.

So it looks as if the PPP frames are scrambled twice :-

during its creation, the PPP frame is scrambled using a "1 + x43" polynomial
during the creation of the SONET / SDH frame, everything in the frame except the first line of the overhead section is scrambled again, using a "1 + x6 + x7" polynomial

I believe that in some cases, the "1 + x43" scrambling can be switched off, but this is not recommended, and can lead to the nodes at either end of the fibre link not speaking to each other.

The SONET/SDH "1 + x6 + x7" scrambling is a fundamental part of SONET / SDH, and cannot be switched off.

Disadvantage of POS

POS has got a significant disadvantage when transporting IP frames between ethernet networks - all of the ethernet MAC frame has to be stripped off at the network edge, and then put back on again at the other network edge. This causes at least three problems -

it requires additional hardare / software to do the stripping off and re-assembly of the MAC frame
this additional signal processing will increase latency
all the additional information that is carried in the MAC frame is lost - for example - priority tagging and VLAN tagging

For ethernet speeds up to 1Gb/s, there are or were ways to get round this by using EOS - Ethernet over SONET/SDH - where the whole MAC frame is encapsulated and sent over SONET/SDH.

10Gb/s ethernet has got round it by having the WAN-PHY, with the lower speed, and the WIS layer that directly creates SONET/SDH frames, so doesn`t need POS.

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