I stand duly correct by both of you. :-)
Please forgive the SNA goof. Its not something that I've ever worked with so
it doesn't stick in my mind all that well.
As for the HDLC/LAPB... I know better than that, I just wasn't thinking. All
I can say in my defense was that I had just gotten home from a dinner with
this fellow from Thrupoint. Too many caphirinas (spelling?).
Never mind what I think about the "Cisco way" argument . Cisco isn't the
be-all end-all of the universe, there are other fish in the pond and lets
leave it at that. :-)
Take care,
Karen
*********** REPLY SEPARATOR ***********
On 5/1/2001 at 10:55 AM Howard C. Berkowitz wrote:
>> >b. HDLC --> The job of HDLC is to ensure that data passed up to the next
>>>layer has been received exactly as transmitted (i.e error free, without
>loss
>>>and in the correct order).
>>
>>Except Cisco's HDLC doesn't do all that, and for the CCIE test you should
>>know the Cisco way. Cisco's HDLC does error detection only. It does not
>>retransmit.
>>
>>Great answer otherwise!
>>
>>Priscilla
>
>Not sure if this answer is pedantic or nuanced, but HDLC does have a
>role in ensuring that the receiver only passes correct data to its
>upper layer: the error detection causes bad data to be dropped.
>
>Reliable and "unreliable" protocols either have error detection
>mechanisms, or have expectations that a layer below them will have
>such mechanisms. The question is what to do when an error is
>detected:
>
> -- don't pass the data to the upper layer and, if there is a need
> for retransmission, let the sender detect the need and act on it
> (This, in general, is the IP way)
> -- don't pass the data to the upper layer and send a request for
> retransmission
>
>(Forward error control and redundant transmission (e.g., SSCOP) are
>beyond the scope of this discussion)
>
>>
>>
>>>c. SDLC --> Less error control than HDLC in that it only provides link
>>>integrity. Frames received correctly are acknowledged by the receiver
while
>>>erroneous frames are ignored.
>>>
>>>d. Frame-relay --> There is no guarantee of data integrity at all. Error
>>>control is sacrificed in the interest of speed. Its assumed that higher
>>>layer protocols will handle all necessary error control. The network
>>>delivers frames, whether the CRC check matches or not. It does not even
>>>necessarily deliver all frames, discarding frames whenever there is
network
>>>congestion.
>>>
>>>e. DLSw+ --> Error control is provided by SNA, not DLSw. DLSw only handles
>>>link control. Most bridging protocols (as far as I know anyway) deal
>>>strictly with getting the data from point A to point B and let the layer
>3/4
>>>protocols handle error control/correction.
>
>In most cases, retransmission comes from TCP, not SNA or DLSw.
>
>> >
>>>f. ATM --> ATM does not have error control functionality (think about what
>>>goes into an ATM cell - and what doesn't). Any error control has to be
>>>performed by the protocols that are encapsulated in the ATM cells.
>>>
>>>g. T1, E1 --> These are layer 1 protocols. T1 and E1 deals with signaling
>>>and encoding. At this level, its just bits, not frames or packets. Error
>>>control is generally handled at layers 2 through 4 (when its done at all).
>>>
>>>Hope this helps,
>>> Karen
>>>
>>>*********** REPLY SEPARATOR ***********
>>>
>>>On 4/30/2001 at 12:28 AM Kuldip Singh wrote:
>>>
>>> >Hi,
>>> >
>>> >Can someone help me find out who (router or host) will
>>> >retransmit the packets if they were lost in the cloud
>>> >while using:
>>> >a. X.25 --> guarantees data integrity by using HDLC to handle error
>>control.
>>> >b. HDLC --> the purpose of this protocol is to provide an error free
>link
>>>between two connected devices.
>>> >c. SDLC --> provides link integrity. Frames received correctly are
>>>acknowledged by the receiver while erroneous frames are ignored.
>>> >d. Frame-relay --> Think about what makes Frame-Relay different from
>X-25.
>>>There is no guarantee of data integrity at all.
>>> >e. DLSw+ --> Error control is provided by SNA, not DLSw.
>>> >f. ATM --> ATM does not have error control functionality (think about
>what
>>>goes into an ATM cell - and what doesn't).
>>> >g. T1, E1 --> These are layer 1 protocols. Error control is generally
>>>handled at layers 2 through 4.
>>> >to connect from one router to another.
>>> >
>>> >HostRoutercloudRouterHost
>>> >
>> > >Thanks
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