OSI Model Question

I see the flames a comming.....

Based on your remark, I figured this must have been a topic of (possibly heated) discussion in the past. So I did a search for "layer 1 header" on google groups for this newsgroup asuming that a ton of stuff would come up. Not much did.

I did find that sonet has a header which identifies the contents of the payload. So I guess this answers my question.

No, there is no such thing as a layer 1 header.

Data is encapsulated (which includes a header), as stated in CCNA material, as follows: - data - segment - packet - frame - bits Framing is a layer 2 encapsulation and is dependant on the layer 2 technology. This can be tricky because some layer 2 and layer 1 technologies are closely bound, but not exclusive to one another. Therefore, ethernet framing for transmission over copper cabling is the same as ethernet framing over fiber optic cabling. This ethernet framing is different than the layer 2 framing that token ring uses with copper cabling and layer 2 framing that ATM uses over fiber optic cabling. The point is that framing, which happens at layer 2, is not dependant on the layer 1 medium used.

It is understandble that framing may be considered dependant on the layer 1 technology because of the close relation of layer 2 data link and layer 1 physical technologies. T-1 lines, modem communication, and ethernet are commonly related to copper cabling while FDDI and ATM are commonly related to fiber optic cabling. With all of these relationships being thought of, it is conceivable that one may consider framing to be dependant on the layer

1 physical technology when in fact it is not.

As I sometimes say, "Give me two copper wires and I can signal like a modem for up to 56kb/s or like ISDN for up to 128kb/s. Give me two more copper wires (for a total of 4) and now I can signal like a T-1 line, Ethernet, FastEthernet, or GigabitEthernet at respective speeds of 1.5mb/s, 10mb/s,

100mb/s, or 1000mb/s. Replace my 4 copper wires with 2 fiber optic lines and now I can signal like a T-3, Ethernet, FastEthernet, and GigabitEthernet. Layer 2 data link technologies/protocols are all about HOW you signal over the layer 1 physical medium."

----- Scott Perry Indianapolis, IN

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Scott, Thank you for the excellent explanation. The key to this seems to be signalling info which is contained in the header.

When you use frame relay to transport ethernet, are you encapsulating a L2 protocol with another L2 protocol? I haven't seen that the OSI model allows for that. Bob

it is worth remembering the OSI model is exactly that - a model designed to make conversations about networking easier to understand in a consistent way.

And a useful model usually involves abstrations to simplify the discussions, or a way to ignore some detail that doesnt impact whatever you do want to talk about.

Anyhow my take is that layer 1 can be more than just a lump of bits, as that layer may need some structure to make it work.

Some plumbing needs clock info and other "layer 1" stuff when there is no packet to maintain clock, sync and so on - think HDB3 on an E1 type

2 Mbps link for example, where the format maintains the clock transitions.

Or maybe stuctured E1, where some 64k timeslots are used for a L2 frame and some are there for other reasons - maybe this example has 2 different "layer 2" aspects?

So, what is on the wire when there is no frame present? no layer 2 info as no layer 2 frame, so it cannot be "layer 2" ?

The reality is there are several ways in which OSI does not map exactly into some real networks, and where there are in practice sub layers, or the model layer boundaries do not match up exactly to an implemention.

And all of those examples are not limited to the copper or fibre transports suggested (ok - you can argue that T1 is defined to use copper, but all the others have at least UTP & fibre varients).

Or 24 Mbps for ADSL 2+ (but not as far) :)

Give me two more copper

Um - 4 pairs for Gige on UTP?

Replace my 4 copper wires with 2 fiber optic lines

And take the fibre away, use free space optics and these work without any medium........

Layer 2 data link technologies/protocols are all about HOW

SONET is a good example - one could 'argue' that the FP sequence on DSx signals and subrate data multiplexers also constitute L1 'headers'............

-ja

.

Frame-relay framing does not encapsulate or get encapsulated by ethernet framing, they are replacements for each other.

Use the example from your question. Consider an ethernet LAN which crosses a frame-relay WAN to get to an ethernet LAN on the other side. * The originating host creates an IP packet destined for the receiving host on the other end. The IP packet is encapsulated in an ethernet frame destined for the next-hop router. * The router receives the ethernet frame sent to it, removes the ethernet framing, evaluates the IP packet to determine the detination, and encapsulates the IP packet in a frame-relay frame destined for the next-hop router. * The next router receives the frame-relay frame sent to it, removes the ethernet framing, evaluates the IP packet to determine the destination, and encapsulated the IP pakcet in an ethernet frame destined for the receiving host. * The receiving host receives the ethernet frame sent to it, removes the ethernet framing, finds that the IP packet is intended for itself, and then retrieves the data from the original and unaltered IP packet. Total: 3 data-link frames, 1 network packet

----- Scott Perry Indianapolis, IN

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actually - it can do. You describe routing IP over frame relay which is not what was asked.

there is an encap defined for Ethernet over frame as well, which keeps the Ethernet MAC frame.

Long ago i used it for LAT based terminal servers.......

Stephen, You may be correct but Scott answered my boneheaded question. I knew (and forgot) that the router strips the incoming L2 header and encapsulates with the outgoing frame type. But one sometimes needs to be reminded of the basics again. Bob

If stupidity even strikes educated people from time to time, what would you expect from those who are not? BS

If one is bridging, frame relay will encapsulate ethernet frames.

John Agosta wrote:

:) Hey John, long time!

Bob, Others already chimed in, but the fact is, DOD's TCP/IP model won. ISO's OSI model is dead dead dead dead. It's not even a useful training aid since the four layers of TCP/IP is all that matters.

If one were being pedantic, one could argue that concepts like manchester encoding mean that L1 header exists.

One could also argue that one is pedantic just by using the word pedantic! :)

"Bob Simon" ha scritto nel messaggio news: snipped-for-privacy@4ax.com... | Is there any such thing as a layer 1 (physical layer) header?

Hi,

The last PDU in the ISO OSI model is at Layer-2 (L2PDU), so based on the OSI encapsulation model Layer-1 does not have an header or a trailer.

Regards, Gabriele

I never liked that word, "pedantic."

This is a totally valid question. There is layer 1 framing (T1 ESF, SONET, etc).

And with respect to some of these protocols, particularly ISDN, there is even a layer3 in ISDN which indeed maps to layer 3 on the *OSI* model. You are really transporting one protocol stack over another in this case.

Technically speaking, there is a layer1/2 in DSL, since most DSL uses ATM encapsulation with VPI/VCI numbers.

Which may be confusing to some people who use PPPoE for their DSL links.

Its really PPP over Ethernet over ATM... so you have three layer 2 technologies there...

They're coming up the rack And I ain't seen the sunshine since I don't know when I'm stuck in the data centre and time keeps draggin' on But those flames keeps a-burnin' on up to the backbone When I was just a baby, my mama told me, man Always be a good boy, don't ever play with LANs But I patched a rack in Reno, just to watch it bridge When I hear that aircon blowin', I clean my AUI.

Sorry, couldn't resist.

P.