data link layer.ppt

8/9/2019 data link layer.ppt

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COMPUTER NETWORKS

(DATA-LINK LAYER)

Data-Link Layer

BITS edu campus


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Computer

Networks

!uest"o# to $e %#swere& '

Why it is important to have DatalinkLayer?

What are Functions performedon Data-link layer?

Data-Link Layer


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Computer

Networks

te #ee& *or D%t%-+"#k +%,er

1) Communication circuits make errors

occasionally!) Communication circuits have a

limited date rate

") #here is a non$ero propa%ation delay&et'een the time a &it is sent and the

time it is received

Data-Link Layer


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Computer

Networks

.u#/t"o#s per*orme& o# D%t%-+"#k

+%,er %re0

1- .r%m"#2- .+ow Co#tro+

- Error Co#tro+

- Error Dete/t"o# 3 Corre/t"o#

Data-Link Layer


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Computer

Networks

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.r%m"#2 #he transmitted &it stream &y the physical

layer is not %uaranteed to &e error *ree

(t is up to the data-link layer to &ete/t andif necessary to /orre/t errors

#he data-link layer $re%ksthe &it stream

up into &"s/rete *r%mesand /omputethe/e/ksumfor each frame

When a frame arrives at the destination

the /e/ksum "s re/ompute&

Data-Link Layer


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Computer

Networks

5*elationship &et'een packets and frames

Data-Link Layer

6e%&er P%,+o%& *"e+& Tr%"+er

P%/ket

Se#&"#2 m%/"#e

6e%&er P%,+o%& *"e+& Tr%"+er

P%/ket

Re/e"7"#2 m%/"#e

.r%me


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SERVICES PROVIDED TO NETWORK

LAYER (2)

Placement of the data link protocol.

Data-Link Layer


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Networks

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.r%m"#2 #he easiest 'ay to achieve framin% is to

"#sert t"me 2%ps&et'een frames

+et'orks rarely make any %uaranteesa&out timin%

,ther .r%m"#2 Meto&sare

1) Character count

!) Fla% &ytes 'ith &yte stuffin%").tartin% and encodin% fla%s 'ith &it stuffin%

/) 0hysical layer codin% violations

Data-Link Layer


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Networks

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.r%m"#21) C%r%/ter /ou#tframin% method uses a

field in the header to specify the num&er of

characters in the frame

Data-Link Layer

5 1 1 4 1

C%r%/ter /ou#t

.r%me 1

C%r%/ters

.r%me

5 C%r%/ters

.r%me

C%r%/ters


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Computer

Networks

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.r%m"#2#he pro&lem 'ith C%r%/ter /ou#tal%orithm

is that the count can &e distorted &y a

transmission error

Data-Link Layer

: 1 1 4 1

Error

.r%me 1

C%r%/ters

.r%me (wro#2)

5 C%r%/ters

.r%me


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Networks

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.r%m"#2!) #he second method uses in each frame

st%rt %#& e#& spe/"%+ $,tes (.+%2 $,te)to

%et around the pro&lem of resynchronisation

after an error occurred- Fla% &ytes used as a delimiters

Data-Link Layer

.LA; 6e%&er P%,+o%& *"e+& Tr%"+er .LA;

Frame delimited &y fla% &ytes


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FRAMING (2)

(a)A frame delimited by flag bytes.

(b)Four examples of byte sequences before and afterstuffing.


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FRAMING (3)

Bit stuffing(a)The original data.

(b)The data as they appear on the line.

(c)The data as they are stored in receivers memory afterdestuffing.


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.r%m"#2- pro$+em o//urs'ith this method 'hen

&inary data are &ein% transmitted (t may

easily happen that the *+%2 $,te


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Networks

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.r%m"#2- #he &yte-stuffin% scheme 'ith a sli%ht

simplification is use& "# te PPP proto/o+=

- ma3or &"s%&7%#t%2eof this framin%method is that it is closely tied to the use of 5-

&it characters

Data-Link Layer


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Computer

Networks

1>

Error Co#tro+- 6rror Control is needed to make sure all

frames are eventually &e+"7ere&to the

net'ork layer at the destination and in the

proper or&er=

- #he techni4ue of %/k#ow+e&2me#tand

retr%#sm"ss"o#is used to ensure relia&le

delivery- #his can &e achieved &y usin% t"merand

%ss"2#"#2 se?ue#/e #um$erfor each

frame

Data-Link Layer


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Computer

Networks

1:

.+ow Co#tro+- Flo' Control is needed to deal 'ith the

situation 'hen the sender is ru##"#2 o# %

*%st /omputerand the receiver is ru##"#2

o# % s+ow m%/"#e=

- the sender keeps transmittin% the frames

out at a "2er r%teuntil the receiver is

completely sw%mpe& 7flooded)- t a certain point the receiver 'ill simply &e

u#%$+e to /ope w"tthe frames as they

arrive and 'ill start to +ose tem

Data-Link Layer


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Networks

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.+ow Co#tro+- #here are t'o approaches for flo' control

1) .ee&$%/k-$%se& *+ow /o#tro+ 'here the

receiver sends &ack information to thesender %ivin% it permission to send more

data

!) R%te-$%se& *+ow /o#tro+ 'here there is arate limit at 'hich senders may transmit

data 'ithout usin% feed&ack from the

receiver

Data-Link Layer


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Computer

Networks

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Error Dete/t"o# 3 Corre/t"o#Two $%s"/ str%te2"es to &e%+ w"t errors0

1) Error-/orre/t"#2 /o&es includes enou%h

redundant information 'ith each sent datato ena&le receiver to deduce 'hat the

transmitted data must have &een

)error-&ete/t"#2 /o&es includes only

enou%h information to allo' the receiver todeduce that an error occurred &ut not

'hich error and have it re4uest a

retransmission

Data-Link Layer


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Networks

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Error Dete/t"o# 3 Corre/t"o#- #he simplest error-detection scheme is a

P%r"t, Ce/k

- (n this scheme a sin%le parity &it is%ppe#&e&to the data

- #he parity &it is chosen so that te

#um$er o* 1&its in the code is e7e# (oro&&)=

Data-Link Layer


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Networks

Data-Link Layer

EDATA

E@*(DATA)

DATA

EDATA

E


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Networks

Error Dete/t"o# 3 Corre/t"o#- 6rror-detection and retransmission is

efficient 'here error rate is lo' 7over

copper 'ire or optical fi&er)

- With 'ireless links 'here unaccepta&le

delay caused &y repetition 6rror-correction

is more suita&le

Data-Link Layer

data link layer.ppt

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