64/66b encoding

Question

There are a few things I don't understand about 64/66bit encoding, and failed to find the answers to on the web. Any help/links would be greatly appreciated:

i) how is the start of a frame recognised? I don't think it can be by the initial 10/01 bits called the preamble on wikipedia because you cannot tell them apart (if an idle link is 0, then 0000 10 and 000 01 0 look rather similar). I expect the end of a frame is indicated by a control word, with the rest of the bits perhaps used for the CRC?

ii) how do the scramblers synchronise, and how do they avoid scrambling the same packet the same way? Or to put this another way, why is not possible for a malicious user to induce substantial packet loss by carefully choosing a bad message?

iii) this might have been answered in ii), but if a packet is sent to a switch, and then onto another host, is it scrambled the same way both times?

Once again, many thanks in advance

Answer 1

Layers

First of all the OSI model needs to be clear.

The ethernet frame is a data link layer, while the 64b/66b encoding is part of the physical layer (More precisely the PCS of the physical layer)

The physical layer doesn't know anything about the start of a frame. It sees only data. (The start of an ethernet frame are data bytes which contain the preamble.)

---

64b/66b encoding

Now let's assume that the link is up and running.

In this case the idle link is not full of '0'-s. (In that case the link wouldn't be self-synchronous) Idle messages (idle characters and/or synchronization blocks ie control information) are sent over the idle link. (The control information encoded with 0b10 preamble) (This is why the emitted spectrum and power dissipation don't depend on if the link is in idle state or not)

So a start of a new frame acts like following:

1. The link sends idle information. (with 0b10 preamble)
2. Upper layer (data link layer) sends the frame (in 64bit chunks of data) to physical layer.
3. The physical layer sends the data (with 0b01 preamble) over the link.

(Note that physical layer frequently inserts control (sync) symbols into the raw frame even during a data burst)

Synchronization

Before data transmission 64b/66b encoded lane must be initialized. This initialization includes the lane initialization which the block synchronization. Xilinx's Aurora's specification (P34) is an example of link initialization. Briefly receiver tries to match the sync character in different bit-position, and when it match multiple times it reports link-up.

Note, that the 64b/66b encoding uses self-synchronous scrambler. This is why the scrambler (itself) doesn't need to know anything about where we are in the data stream. If you run a self-synchronous (de-)scrambler long enough, it produces the decoded bit stream.

Maliciousness

Note, that 64b/66b encoding is not an encryption. This scrambling won't protect you from eavesdropping/tamper. (Encryption should placed at higher level of the OSI model)

Same packet multiple times

Because the scrambler is in different state/seed when you sending the same packet second time, the two encoded packet will differ. (Theoretically we can creates packets, which sets back the shift register of the scramble, but we need to consider the control symbols, so practically this is impossible.)