> >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
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=2748&t=2525
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