HDLC help

[blackeagle]

i really need help understanding this as fast as possible

a machine A wants to send 4 frames using
HDLC LAPB - to a machine B , it is assumed that for
now the state of exchange between A and B is given in the following figure



you are asked to complete it to exchange
these four frames. (assuming that the exchange is completed without error
and with a window of 2 ) .

this is the answer in the pic below



so basicly what i know is that
n(S) is number of the frame sent
n® the receive sequence number
i have no idea whats the P=1 or P=0 and the F=1

I,N(s)=5,N®=2,P=1 here what does the n® means and why its value is 2 ?
RR N®=6,F=1 ok here i know what does the N®=6 do the next information sent will be N(s)=6 but what about f=1 ?

why after N(s)=7 RR N®=0 why didn't we continue going up N®=8 ?

[HTH]

In hldc (not extended hldc) the sequence number is defined as three bits.
2^3 - 1 = 7

It restarts at 0 because thats what happens when sequence numbers roll over, same for resecieved seq numbers as well.

P and f designate the poll final bit.

That combined with Wikipedia will answer all your questions.

[blackeagle]

ok HTH i got the 1st part i did understand why it resets to 0 after the counter is 7 .
but still im having troubles understanding I,N(s)=5,N(R)=2,P=1 here what does the n(R) means and why its value is 2 ?
PS: i know why RR N(R) =6,F=1 in here N(R) is equal 6 because its calling for the next frame to be sent which is 6 in this case .
about the P and F if i understood right and correct me pls if i'm wrong it's a 1 bit p is 1 when its the last msg before the second pc reply and F is 1 when that second pc reply without error and 0 if an error happens  ?

[HTH]

ok HTH i got the 1st part i did understand why it resets to 0 after the counter is 7 .
but still im having troubles understanding I,N(s)=5,N(R)=2,P=1 here what does the n(R) means and why its value is 2 ?
PS: i know why RR N(R) =6,F=1 in here N(R) is equal 6 because its calling for the next frame to be sent which is 6 in this case

N(R) indicates the receive sequence, RR dont count as frames so the n(R) going that way wont increase.

[blackeagle]

things are getting more clear so basicly that N(R) counter wont go up unless the pc to th right sends an information frame to the pc to the left ?

[HTH]

yes exactly that, N(R) indicates the last useful frame received and since it doesnt receive information just RR frames it wont go up

[blackeagle]

i have last two questions
1.about the P and F if i understood right and correct me pls if i'm wrong it's a 1 bit p is 1 when its the last msg before the second pc reply and F is 1 when that second pc reply without error and 0 if an error happens  ?
you didn't tell me if this is how it works or i got it wrong

2.should i always start with                   SABM
                                                                      UA

 and end with                                           DISC
                                                                      UA

[HTH]

i have last two questions
1.about the P and F if i understood right and correct me pls if i'm wrong it's a 1 bit p is 1 when its the last msg before the second pc reply and F is 1 when that second pc reply without error and 0 if an error happens  ?
you didn't tell me if this is how it works or i got it wrong

2.should i always start with                   SABM
                                                                      UA

 and end with                                           DISC
                                                                      UA

you had the P half correct, it is 1 when indicating an end of transmission (f=1).

and you only start with SABM if thats the mode you want :| there are... 6 or 7 other modes.

and yes you should end with a DISC and then UA

[blackeagle]

that was really helpful tyvm HTH i really appreciate it

[blackeagle]

first am sorry to re-post on this thread but while studying for my exam i found some thing weird .

In a network HDLCILAPB two machines "A" and "B" wish to exchange information .

"A" owns 9 frames to send , it is assumed that the window size is 2 , and that possessing frames as N ( s) = 2 and N ( s) = 5

arrive with error for the first time sent out, and then after a retransmission had come without error.

build the diagram of exchange of information between  "A" and "B" indicating for each case the value of N (R ) and N ( S).




this diagram isnt full i just showed the part where the weird thing was.
well the window size is 2 from the start they been sending 2 information frames at the same time but suddenly n(s)=7 and n(s)=0 are sent 1 by one can any1 explain this ? is it an error in the solution,or for some reason it should be sent one by one