Question $1$

$($a$)$ A $1,500-$byte block of data is transmitted between two computers using Idle Repeat

Request $($RQ$)$ protocol over three different links: Link$1$ a $10-$km of coaxial cable at a

transmission rate of $1$ Mb$/$s; Link$2$ a $200-$km leased line at a transmission rate of $5$ Mb$/$s;

Link$3$ a $30,000-$km free space satellite link at a transmission rate of $10$ Mb$/$s$.$

$($i$)$ Determine the transmission delay for each link, i$.$e$.$ the time taken by the

computer$$s transmitter to upload the frame onto each link.

$($ii$)$ Determine the propagation delay, i$.$e$.$ the minimum time required for the

transmitting computer to receive the transmission acknowledgement $($ACK or

NAK$)$ for each communication link. Assume that the velocity of propagation of the

signal in Links $1$ and $2$ is $2105$

km$/$s$,$ and in Link$3$ it is $3105$

km$/$s$.$

$($iii$)$ Find out which of the effects, the transmission delay, the propagation delay, or

both, determine the total round$-$trip delay associated with each link, and thus

explain and conclude for which link the Idle RQ protocol is suitable and for which

it is not.

$($b$)$ A Fast Ethernet transmits at $100$ Mb$/$s over a $1-$km optical fibre cable $($bus$)$ with a Carrier$-$

Sense Multiple$-$Access with Collision Detection $($CSMA$/$CD$)$ protocol in operation. Due to

very low level of losses, the optical fibre bus does not need repeaters. The speed of signal

in optical fibre is $200,000$ km$/$s$.$ The Ethernet frame is shown in Figure $1\mathrm{.}1$ below:

Figure $1\mathrm{.}1$

$($i$)$ Calculate the minimum frame size which is consistent with the CSMA$/$CD protocol,

and the minimum payload. Assume that $24$ margin bits should be added.

$($ii$)$ The station attempts to transmit $100$ bytes of data and is forced to use the truncated

binary exponential backoff algorithm after $5$ successive collisions were detected.

Calculate the minimum and maximum waiting times for the station in this case, and

the time required to transmit $100$ bytes of data.Question $1($continued$)$

$($c$)$ The characters A$,$ B$,$ FLAG, ESC are encoded by the Logical Link Control $($LLC$)$ protocol

as follows: A $10011111,$ B $11100001,$ FLAG $01111110,$ ESC $00110110.$ The

characters are framed into a single LLC frame and are transmitted as a continuous binary

bit$-$stream A B FLAG ESC. Show the binary bit sequence for the entire LLC frame when

the following framing method is used:

$($i$)$ Byte count

$($ii$)$ Flag bytes with byte stuffing

$($iii$)$ Flag bits with bit stuffing$($a$)$ A $1,500-$byte block of data is transmitted between two computers using Idle Repeat

Request $($RQ$)$ protocol over three different links: Link$1-$ a $10-$km of coaxial cable at a

transmission rate of $1M\frac{b}{s}$; Link$2-$ a $200-$km leased line at a transmission rate of $5M\frac{b}{s}$;

Link$3-$ a $30,000-$km free space satellite link at a transmission rate of $10M\frac{b}{s}.$

$($i$)$ Determine the transmission delay for each link, i$.$e$.$ the time taken by the

computer's transmitter to upload the frame onto each link.

$($ii$)$ Determine the propagation delay, i$.$e$.$ the minimum time required for the

transmitting computer to receive the transmission acknowledgement $($ACK or

NAK$)$ for each communication link. Assume that the velocity of propagation of the

signal in Links $1$ and $2$ is $2{10}^{5}k\frac{m}{s},$ and in Link $3$ it is $3{10}^{5}k\frac{m}{s}.$

$($iii$)$ Find out which of the effects, the transmission delay, the propagation delay, or

both, determine the total round$-$trip delay associated with each link, and thus

explain and conclude for which link the Idle RQ protocol is suitable and for which

it is not.

$($b$)$ A Fast Ethernet transmits at $100M\frac{b}{s}$ over a $1-$km optical fibre cable $($bus$)$ with a Carrier$-$

Sense Multiple$-$Access with Collision Detection $($CSMA$/$CD$)$ protocol in operation. Due to

very low level of losses, the optical fibre bus does not need repeaters. The speed of signal

in optical fibre is $200,000k\frac{m}{s}.$ The Ethernet frame is shown in Figure $1\mathrm{.}1$ below:

Figure $1\mathrm{.}1$

$($i$)$ Calculate the minimum frame size which is consistent with the CSMA$/$CD protocol,

and the minimum payload. Assume that $24$ margin bits should be added.

$($ii$)$ The station attempts to transmit $100$ bytes of data and is forced to use the truncated

binary exponential backoff algorithm after $5$ successive collisions were detected.

Calculate the minimum and maximum waiting times for the station in this case, and

the time required to transmit $100$ bytes of data.

Question $1($continued$)$

$($c$)$ The characters A$,$ B$,$ FLAG, ESC are encoded by the Logical Link Control $($LLC$)$ protocol

as follows: A $-10011111,$ B $-11100001,$ FLAG $-01111110,$ ESC $-00110110.$ The

characters are framed into a single LLC frame and are transmitted as a continuous binary

bit$-$stream A B FLAG ESC. Show the binary bit sequence for the entire LLC frame when

the following framing method is used:

$($i$)$ Byte count

$($ii$)$ Flag bytes with byte stuffing

$($iii$)$ Flag bits with bit stuffing