RouterGod Celebrity Interview Series

Robert Downey Jr. on the IEEE 802.3 Ethernet Frame

With the possible exception of Todd Bridges, few celebrities love the jail lifestyle more than Robert Downey Jr.  Here in Los Angeles celebrities are treated very well and nowhere is that more true than at the L.A. County Jail.  During past incarcerations, Robert Downey Jr. has been chauffeured to movie sets by Sheriffs Deputies and was actually made an honorary Deputy by Sheriff Block himself.  Robert Downey or "Robbie D" as he is affectionately known in the slammer agreed to an interview with RouterGod Online Magazine but only if we paid him in cocaine.  We told Robbie D that that was out of the question so he then asked for marijuana and a bong and we refused.  We said "how about some booze?" to which he told us that the Celebrity Cell at L.A. County already has a fully stocked bar.  After hours of negotiations we agreed to bring him his favorite Russian caviar as the caviar provided by room service is not to his liking.  We sent RouterGod Field Reported John Lokie to visit Robert Downey Jr. at his beautiful celebrity skybox overlooking the ornate fountains and Japanese gardens at the VIP wing of the L.A. County Jail.   Let's join John as he talks to Robert Downey Jr. about the make up of the IEEE 802.3 Ethernet frame!


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RDJ explains that
Ethernet is a layer 2
standard.
John

Thank you Mr. Downey, our readers are anxious to hear what you have to say about Ethernet.

RDJ

Sure man, say, do you have any heroin on you?  I'm about out.

John

Sorry, but I wouldn't even know what heroin looks like.

RDJ

Oh well, it was worth a shot.  OK, let's get on with the show, I'll explain the Ethernet frame to you.  Do you mind if I get a massage while we do the interview?

John

No, I don't mind.

Robert Downey Jr. removes his $5,000 Bill Blass smoking jacket exposing his naked upper body.  A beautiful and voluptuous female Sheriff's Deputy enters carrying a towel and some lotions and follows Robert Downey Jr. to the massage table.  Robert Downey lays face down on the table, folds his arms in front of his face and rests his chin on his hands.  The Deputy proceeds to give RDJ his afternoon massage.

RDJ

Ahh, that's perfect.  Where do you want to start, John?

John

Why not start at the beginning...

RDJ

Swell, what we are going to talk about is the IEEE 802.3 Ethernet frame.  Ethernet frames are at layer 2 of the OSI model and what they do is encapsulate layer three data and carry it across the physical layer to a destination where it is decapsulated and the layer 3 data is processed. 

John

What is a 802.3 frame composed of?

RDJ

Well to begin with there is an 8 byte "preamble", the preamble is not technically part of the frame as it is added to the front of the frame by the NIC just before the frame is put out on the wire.  The preamble is a series of alternating "one" and "zero" bits.  Actually Manchester encoding is slightly more complicated than this but for our purposes think of the signaling as a simple on and off type of scheme.  These 64 bits in the preamble are for timing, the allow the receiving station to get synchronized with the incoming signal, this usually happens within the first 14 bit times.  The last 2 bits are switched on, so the last byte looks like: 10101011.  This last byte of the preamble is called the Start Of Frame delimiter and signals the start of the actual frame.

John

I see, since there is no clocking, the preamble allows the receiver to sync up and it signals the start of frame.  What is next?

RDJ

The next field is the destination MAC address, of course it is 6 bytes in length.  After that comes the source MAC address and it too is 48 bits in length.  MAC stands for Media Access Control, it is the actual layer 2 hardware address of a device.  Cisco sometimes refers to it as a BIA or Burned In Address.  Notice that the destination address is first, that saves time when you employ cut through switching.

John

Very nice, after the preamble comes the destination and source datalink addresses, what follows?

RDJ

The next field is the Length field, it is 2 bytes in length and indicates the length of the data or payload of the frame.  The length does not include the preamble, MAC addresses, the FCS (we'll talk about that in a minute) or the length field itself.  An Ethernet frame cannot be any smaller than 64 bytes in length or larger than 1518 bytes in total length, not including the preamble, which we don't count anyway.

John

So the length field just indicates the payload length, that's pretty simple.

RDJ

Well not so fast, one very important thing the length field does is communicate the end of the payload.  By knowing how long the payload is, we know exactly when it will end and when the Frame Check Sequence begins, that way we don't need any kind of "end of data delimiter" or some such thing in our frame.

John

Hey!  You're pretty good!  Keep going!

RDJ

Of course I'm good, that's why they pay me the big bucks!    Now we get into 3 cool little fields called the LLC header.  LLC means Logical Link Control.  Each of these is 1 byte in length and these 24 bits are counted in the Length field.


Robert strikes an introspective pose for
benefit of onlookers.  All Deputies in
Robert's employ are also members
 of the Screen Actors Guild and receive SAG scale in addition to ordinary wages.

Robert Downey Jr. arrives at the
Cannes Film Festival.
John

Well don't keep us in suspense!  Spill it man, spill it!

RDJ

OK, ok, the first octet in the LLC header is the DSAP or Destination Service Access Point.  This is a pointer to a location in the receivers memory.  It tells the receiving NIC card where in it's memory buffer to put this information.  The next 8 bit field is the SSAP or Source Service Access Point, this is the pointer to the buffer address from where this information came.  The DSAP and SSAP allow for multiple protocol stacks to use a single NIC card.  The next and last byte in the LLC header is the Control and it simply indicates the type off LLC that this is.

John

So far you have mentioned the preamble and six different fields, when do we get to the data?

RDJ

Right now my man!  The data field carries the packet that is constructed at layer 3 of the OSI model.  The data can be between 43 and 1497 bytes in length.  The last field is the Frame Check Sequence and that contains a number that the receiving station performs a calculation to determine if the frame is corrupt or not.

John

What if the receiving station does the FCS check and finds the frame to be corrupted, will the NIC card ask the the sender re-send the frame?

RDJ

No way, retransmission is a layer 4 function.  At layer 2 if a frame is bad, it is simply dropped or deleted from the input buffer of the interface.  The FCS field is 4 bytes in length so we have 21 bytes of overhead per frame.  We said that the maximum size of an Ethernet frame was 1518 bytes, so subtract the 21 bytes of overhead and the actual maximum payload is 1497 bytes of data.

John

Well that's not too bad, we get a maximum 1497 bytes of data for only 21 bytes of overhead, I'd say that's pretty efficient.

RDJ

Yeah, but don't forget that this is just the overhead at layer 2, layer 3 has a header as well as layer 4.  Since each of the upper layers of the OSI model gets encapsulated or wrapped in the layer below it, the actual, real data that is hiding in an Ethernet frame is less than the apparent payload of the frame.

John

Dog gone it!  Your right, I forgot all about the other layers.  You know Robert, for a crack smoking, drugged out freak of nature, you sure are pretty smart!  Have you ever given serious thought to quitting drugs?

RDJ

Never!

Editor's Note
Robert Downey Jr. is a trained, seasoned professional.
Do not attempt to consume illegal narcotics without
first consulting your theatrical agent!

 

   

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