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  • PPPoE Vs PPPoA

    Hi All,

    Just thought the below information might be usefull for you all....

    Regards, Brian.
    __________________________________________________ ____

    PPPoA vs PPPoE

    There are two main methods used in the DSL world to authenticate users - PPPoA and PPPoE. PPPoA stands for Point-to-Point protocol over ATM, and PPPoE stands for Point-to-Point protocol over Ethernet. In fact PPPoE should be called PPPoEoA as it is really PPP over ethernet over ATM but that's a moot point.

    One note is that I'm going to give a 'medium' level discussion here - I'm not going to go into LCP requests, or PADI PADO requests etc... So no complaints please of leaving out detail! [If people are interested I can go into more detail]

    OK, first you need to realise that DSL makes use of ATM technology. ATM is an asynchronous transfer technology that delivers data in payloads called cells. ATM is well suited to 'delay intolerant' applications such as voice and video due to the fact that these cells are quite small (53 bytes). ATM networks are connection oriented - that is you must have a configured connection between devices for them to talk.

    OK, so when you plug your CPE (modem) into the phone line, the modem communicates with the aggregator and creates a connection. This connection is called a PVC (permanent virtual circuit).
    Each different device you connect to requires a separate PVC - so for normal internet access you will get only one.

    So, you've plugged your modem in and created your PVC to the aggregator. So what does this give you? Not much. This is basically the equivalent of getting a link light on your hub.

    So the next step in the process is to create a PPP session with the aggregator so that you can get an IP address and start tunnelling your IP traffic across this ATM PVC.

    This is where the processes start to differ so I'll have to separate the process's here.

    The next stage in a PPPoA session is authentication. The CPE device sends out a authentication request to the aggregator. The aggregator looks at the request and inspects the username. It inspects the domain part (the @bigpond for example) and then forwards the request to the domain aggregator, if there is no domain part it tries to authenticate locally. The authenticating aggregator will assign an IP address to the device, and a PPP session is established.
    Note that in this scenario the PPP session is terminated at the CPE!

    OK, In PPPoE the CPE at this point becomes an ethernet bridge (rfc1483). As you know a bridge is basically a way to transparently join two networks - any ethernet traffic that is seen by the modem is forwarded over the PVC. PPPoE sessions are initiated by software on the client PC rather than the CPE (although there are now CPE's , generally routers, that include a PPPoE client!). PPPoE software works somewhat similar to DHCP. The client sends a broadcast request (PADI) that is bridged by the modem and sent down the PVC. When an aggregator receives this request it answers with a PADO unicast packet back to the origin. Because the origin request was a broadcast, the client can receive more that one response - it's choice can be based on criteria such as service offerings (i.e special sessions such as video on demand!) The client then sends another request (PADR) back to the concentrator it has chosen. This concentrator then creates a uniqe session id, starts the PPP initation and sends back the id to the client (PADS).

    So what are the scenarios and pros /cons of each solution?

    PPPoA sessions are terminated on the CPE. On Windows machines this results in loading a device specific driver and using either DUN or a proprietary dialer app. for modems anyway, generally USB
    Many routers only support PPPoA as it is an easier protocol to implement - when you think of the fact that PPPoE is actually PPPoEoA why bother to add another layer?
    PPPoA also preserves IP address's as there is only one IP assigned.

    What are the downsides? Well for providers PPPoA is a pain for support and install purposes - rather that have one client, each device has it's own specific drivers. Devices can appear either as RAS devices, or NIC's.

    You also cannot use PPPoA with ethernet modems - there are a couple of exceptions here but once again they include loading device specific software on the clients PC.
    Each PPP session requires a separate PVC. This requires configuration of the CPE and the aggregator, so no dynamic PPP sessions.

    PPPoE has several advantages. It can be used for both Ethernet and USB modems. It allows for multiple PPP sessions over one PVC - you can have one PPP session for Internet, one for gaming etc etc
    There is also only one bit of software that the provider has to support!

    Disadvantages include that fact that you DO have to install software on the clients PC, rather than relying on DUN.
    And as PPPoE relies on rfc1483 bridging, it is susceptible to broadcast storms and DoS attacks.

    Phew! never written a post this long before !
    [The following section I've addedsince posting]

    IMHO providers choose PPPoA for Business use as
    1. They tend to use routers rather than modems and more routers support PPPoA
    2. Business's services tend to be pure traffic - no need for multiple PPP sessions
    3. Business can be charged to add seperate PVC's for inter-office traffic etc
    4. Provider provides the CPE

    and PPPoE for home users cause
    1. They want the ability to offer multiple connections [and therefore charge separately]
    2. They want to be able to open up the CPE choice without increasing support overheads
    3. PPPoE software provides them a way to put their 'presence' on the users desktop.
    4. Make Self-install kits easier

  • #2
    Great info! Thanks!

    Daniel Petri
    Microsoft Most Valuable Professional - Active Directory Directory Services