Layer 1 / Layer 2 Difference

Discussion in 'Cisco' started by newbie123, Aug 18, 2006.

  1. newbie123

    newbie123 Guest

    Hi, I was reading through some material and came across mpls vpn's. It
    said that Point-to-point Circuits (T1) etc were described as Layer 1
    Implementation and Frame Relay, ATM were layer 2 implementation. I am
    just trying to understand the difference. On a T1 circuit we still have
    HDLC or PPP encapsulation etc than how is it Layer 1. Doesn't the
    service provider dictate what kind of circuit it is. Does the customer
    dictate layer 2 and above on point-to-point circuits. thx
    newbie123, Aug 18, 2006
    #1
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  2. "newbie123" <> writes:
    >Hi, I was reading through some material and came across mpls vpn's. It
    >said that Point-to-point Circuits (T1) etc were described as Layer 1
    >Implementation and Frame Relay, ATM were layer 2 implementation. I am
    >just trying to understand the difference. On a T1 circuit we still have
    >HDLC or PPP encapsulation etc than how is it Layer 1. Doesn't the
    >service provider dictate what kind of circuit it is. Does the customer
    >dictate layer 2 and above on point-to-point circuits. thx


    T1 is Layer-1. It describes the physical electrical characteristics,
    as well as the framing and data encoding format.

    Frame Relay, HDLC and PPP implement Layer-2 protocols over the T1.

    ATM is an odd one, it doesn't quite fit things, but layer-2 is the
    best place for it, although there are some layer-1 ATM bits.

    The service provider doesn't do all that much with T1s, its more along
    the lines of pass the bits, and supply repeaters that look for
    something like a T1 and pass it on. Both customer ends do more to
    actually specifying what a T1 is.
    Doug McIntyre, Aug 18, 2006
    #2
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  3. newbie123

    newbie123 Guest

    Hi Doug,

    thanks for the reply. I am trying to understand that when service
    providers provide a Layer 1 Circuit for eg (T1, T3, OC-3 etc) than who
    provides the layer 2 in that case.

    When the service provider gives a Frame relay connection they are
    providing services upto Layer 2. But in the case of Layer 1 when they
    provide a physical circuit who drives the layer 2 portion.

    The only thing i can think of is the customer either specifying PPP or
    HDLC on the router for encapsulation. Is that what would constitute as
    layer 2 when the service provider just provides a physical circuit.

    In addition what would be considered layer 2 when the service provider
    provides a ethernet drop like say in the case of a metro ethernet.
    thanks for your help

    Doug McIntyre wrote:
    > "newbie123" <> writes:
    > >Hi, I was reading through some material and came across mpls vpn's. It
    > >said that Point-to-point Circuits (T1) etc were described as Layer 1
    > >Implementation and Frame Relay, ATM were layer 2 implementation. I am
    > >just trying to understand the difference. On a T1 circuit we still have
    > >HDLC or PPP encapsulation etc than how is it Layer 1. Doesn't the
    > >service provider dictate what kind of circuit it is. Does the customer
    > >dictate layer 2 and above on point-to-point circuits. thx

    >
    > T1 is Layer-1. It describes the physical electrical characteristics,
    > as well as the framing and data encoding format.
    >
    > Frame Relay, HDLC and PPP implement Layer-2 protocols over the T1.
    >
    > ATM is an odd one, it doesn't quite fit things, but layer-2 is the
    > best place for it, although there are some layer-1 ATM bits.
    >
    > The service provider doesn't do all that much with T1s, its more along
    > the lines of pass the bits, and supply repeaters that look for
    > something like a T1 and pass it on. Both customer ends do more to
    > actually specifying what a T1 is.
    newbie123, Aug 18, 2006
    #3
  4. newbie123

    Guest

    > Doug McIntyre wrote:
    > > "newbie123" <> writes:
    > > >Hi, I was reading through some material and came across mpls vpn's. It
    > > >said that Point-to-point Circuits (T1) etc were described as Layer 1
    > > >Implementation and Frame Relay, ATM were layer 2 implementation. I am
    > > >just trying to understand the difference. On a T1 circuit we still have
    > > >HDLC or PPP encapsulation etc than how is it Layer 1. Doesn't the
    > > >service provider dictate what kind of circuit it is. Does the customer
    > > >dictate layer 2 and above on point-to-point circuits. thx

    > >
    > > T1 is Layer-1. It describes the physical electrical characteristics,
    > > as well as the framing and data encoding format.
    > >
    > > Frame Relay, HDLC and PPP implement Layer-2 protocols over the T1.
    > >
    > > ATM is an odd one, it doesn't quite fit things, but layer-2 is the
    > > best place for it, although there are some layer-1 ATM bits.
    > >
    > > The service provider doesn't do all that much with T1s, its more along
    > > the lines of pass the bits, and supply repeaters that look for
    > > something like a T1 and pass it on. Both customer ends do more to
    > > actually specifying what a T1 is.


    newbie123 wrote:
    > Hi Doug,
    >
    > thanks for the reply. I am trying to understand that when service
    > providers provide a Layer 1 Circuit for eg (T1, T3, OC-3 etc) than who
    > provides the layer 2 in that case.
    >
    > When the service provider gives a Frame relay connection they are
    > providing services upto Layer 2. But in the case of Layer 1 when they
    > provide a physical circuit who drives the layer 2 portion.
    >
    > The only thing i can think of is the customer either specifying PPP or
    > HDLC on the router for encapsulation. Is that what would constitute as
    > layer 2 when the service provider just provides a physical circuit.
    >
    > In addition what would be considered layer 2 when the service provider
    > provides a ethernet drop like say in the case of a metro ethernet.
    > thanks for your help
    >


    It's all relative.

    The layered network models are tools that need to
    be customised and applied to your needs.

    The easiest cases are, typically for us, IP over Ethernet. In
    that case you know that the cable and the data signalling
    are at layer 1 etc.

    Things get confusing as more sophisticated systems are considered.
    For example ISDN has a phy, bit signalling, encoding, addressing,
    routing ... if you do a show "isdn status" is says "xx layer 3
    calls active" A typical IP engineer though sees ISDN as a
    Layer 1 technoloy (well I do anyway) which he then uses
    to send his bits over.


    Here is an analogy I just made up.
    In the air transport business (well this one anyway)

    Carrier Pigeons are placed in Layer 1 of the
    Open Air Transport (OAT) Reference Model.
    Crated goods are in Layer 2
    A managed end to end delivery service in Layer 3.

    Jim calls up a carrier pigeon to deliver a letter
    and a few days later gets confirmation that
    his letter has arrived.

    What Jim doesn't know and indeed //doesn't care about//
    is that Ms Pidgeon used her Gold amex card and travelled
    in a luxury box on a private jet to the destination spending
    the balance of the time resting up on the beach eyeing up
    the peacocks.

    All Jim //does// care about is that his Pigeon interface
    works as expected and that the information arrives.

    MPLS for examle can be used to provide an apparent
    Layer 1 service (i.e. it looks EXACTLY like a wire
    to the two end parties if they do not look too closely)
    but in reality the data is carried over a routed IP network.
    [Looking closely would involve for instance sending in
    bad Ethernet frames (L2) and observing that perhaps they did
    not come out the other end. If it was really a wire the bad
    frames would come out the other end.]

    I am on shakier ground here but here goes:-
    Similarly a working ISDN interface can be considered
    like a layer 1 bit carrier for PPP. When the ISDN is broken
    and you want to fix it, then a different approach may be helpful.
    You may need to know that ISDN has L3 addressing (phone
    numbers) etc.

    ATM is a fully featured network with addressing,
    resilient routing, and many other features however
    as an IP engineer ATM is largely used
    to simulate a wire, which is indeed one of
    its design goals.

    The absolute key thing is that there are layers and that the
    layers operate independently (mostly/ideally:) As an IP
    designer/troubleshooter this independence can facilitate
    simpler and more managable network designs and easier
    behaviour isolation.


    Finally - Jim's Boss may care about the details of the
    service implementation since such details can affect the
    reliability of the service. We are back again to that
    flexibility thing. Apply the //model you need// to
    //get your job// done.

    I am sure that this is very confusing but I hope that
    I have helped a little. Stick with it, I find the Layered
    model very useful in my work. I find it assists very
    much in creating the right questions to ask. Then
    it is only a matter of determining the answers.

    --
    Me -- CCEI #TBA
    I hope no one minds my joke here. I have just begun
    on the trail and hope that reminding myself of it here
    will help keep the nose to the grindstone.
    , Aug 18, 2006
    #4
  5. "newbie123" <> writes:
    >thanks for the reply. I am trying to understand that when service
    >providers provide a Layer 1 Circuit for eg (T1, T3, OC-3 etc) than who
    >provides the layer 2 in that case.


    Depends on who does what, and what kind of service.

    The telco really hands off a connection at two sites, and repeats a
    signal that looks like its structured like the one you've purchased
    from them with enough bandwidth to handle what you've paid.

    The Layer-2 really starts with your router one one side going to the
    remote site, and then the remote site back to you.

    >When the service provider gives a Frame relay connection they are
    >providing services upto Layer 2. But in the case of Layer 1 when they
    >provide a physical circuit who drives the layer 2 portion.


    The telco typically will have two groups. One that runs the WAN
    switch, and one that provides the pipe. You could consider the group
    that runs the WAN switch to be providing the layer-2, but really its
    just a piece of gear like you have, but its centeralized to probably
    save you money by combining it all done to your connection.

    Both sides "drive" the layer-2 portion.


    >The only thing i can think of is the customer either specifying PPP or
    >HDLC on the router for encapsulation. Is that what would constitute as
    >layer 2 when the service provider just provides a physical circuit.


    Yes, the service provider configures their side to match the
    requirements, and you setup your side to match the requirements.

    >In addition what would be considered layer 2 when the service provider
    >provides a ethernet drop like say in the case of a metro ethernet.
    >thanks for your help


    Ethernet protocol is layer-2. The fiber would be layer-1 in MOE.
    Again both sides really "drive" it. Typically the network provider
    will implement ethernet switches at both ends that they control to
    monitor/shape your access into the MOE network.
    Doug McIntyre, Aug 19, 2006
    #5
  6. newbie123

    Scooby Guest

    "newbie123" <> wrote in message
    news:...
    > Hi Doug,
    >
    > thanks for the reply. I am trying to understand that when service
    > providers provide a Layer 1 Circuit for eg (T1, T3, OC-3 etc) than who
    > provides the layer 2 in that case.
    >


    Depends on the type of service. Point to point circuits need layer 2
    provided by the customer. In the case of frame relay, both the customer and
    the telco provide layer 2 services.

    > When the service provider gives a Frame relay connection they are
    > providing services upto Layer 2. But in the case of Layer 1 when they
    > provide a physical circuit who drives the layer 2 portion.
    >


    As mentioned above, layer 2 is provided by both the customer and the telco.
    The customer will speak layer 2 (lmi) with the telco at each location. Then
    the telco creates virtual circuits to connect each site as needed.

    > The only thing i can think of is the customer either specifying PPP or
    > HDLC on the router for encapsulation. Is that what would constitute as
    > layer 2 when the service provider just provides a physical circuit.
    >


    Yep.

    > In addition what would be considered layer 2 when the service provider
    > provides a ethernet drop like say in the case of a metro ethernet.
    > thanks for your help
    >


    A metro ethernet service does offer layer 2 from the telco. These services
    are atm and mpls based within the telco network and are passed to the
    customer via ethernet.

    Jim
    Scooby, Aug 20, 2006
    #6
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