Unit 3 - Data Link Layer - Part B

Unit 3 : Data Link Layer
Chandan Gupta Bhagat
https://chandanbhagat.com.np/me

Data Link Layer
 Function of Data Link Layer (DLL)
 Data Link Control: Framing, Flow and Error Control
 Error Detection and Correction
 High Level Data Link Control (HDLC) & Point-to-Point Protocol (PPP)
 Channel Allocation Problem
 Multiple Access:
 Random Access (ALOHA, CSMA, CSMA/CD, CSMA/CA)
 Controlled Access (Reservation, Polling, Token Passing)
 Channelization (FDMA, TDMA, CDMA)
 Wired LAN: Ethernet Standards and FDDI
 Wireless LAN: IEEE 802.11x and Bluetooth Standards
 Token Bus, Token Ring and Virtual LAN

Channel Allocation Problem
 When there are more than one user who desire to access a shared network channel, an algorithm is deployed for channel
allocation among the competing users.
 The network channel may be a single cable or optical fiber connecting multiple nodes, or a portion of the wireless spectrum.
 Channel allocation algorithms allocate the wired channels and bandwidths to the users, who may be base stations, access
points or terminal equipment.
 Channel allocation is a process in which a single channel is divided and allotted to multiple users in order to carry user specific
tasks. There are user’s quantity may vary every time the process takes place.
 If there are N number of users and channel is divided into N equal-sized sub channels, Each user is assigned one portion. If the
number of users are small and don’t vary at times, than Frequency Division Multiplexing can be used as it is a simple and
efficient channel bandwidth allocating technique.
 Channel allocation problem can be solved by two schemes:
 Static Channel Allocation in LANs and MANs
 Dynamic Channel Allocation.

Static Channel Allocation
 In static channel allocation scheme, a fixed
portion of the frequency channel is allotted
to each user. For N competing users, the
bandwidth is divided into N channels using
frequency division multiplexing (FDM), and
each portion is assigned to one user.
 This scheme is also referred as fixed channel
allocation or fixed channel assignment.
 It is not efficient to divide into fixed number
of chunks.
 𝑇 =
1
𝜇(𝑐−𝜆)
 𝑇 𝐹𝐷𝑀 = 𝑁 × 𝑇 =
𝑁
𝜇𝑐−𝜆
=
1
𝜇𝑥−𝜆
𝑁
=
1
𝜇(𝑐
𝑁)−𝜆
𝑁
**In this allocation scheme, there is no interference between the
users since each user is assigned a fixed channel. However, it is not
suitable in case of a large number of users with variable bandwidth
requirements.**
 Where
 𝑇 = mean time delay
 𝐶 = capacity of channel
 𝜆 = Arrival rate of frames
 1
𝜇= bits/frame
 N = Number of sub channels
 T(FDM) = Frequency Division Multiplexing Time

Dynamic Channel Allocation
 In dynamic channel allocation scheme, frequency bands are not permanently assigned to the users. Instead channels are
allotted to users dynamically as needed, from a central pool. The allocation is done considering a number of parameters so
that transmission interference is minimized.
 This allocation scheme optimizes bandwidth usage and results is faster transmissions.
 Dynamic channel allocation is further divided into centralized and distributed allocation.
 Station Model: Assumes that each of N stations independently produce frames. The probability of producing a packet in
the interval IDt where I is the constant arrival rate of new frames.
 Single Channel Assumption: In this allocation all stations are equivalent and can send and receive on that channel.
 Collision Assumption: If two frames overlap in time-wise, then that’s collision. Any collision is an error, and both frames must re
transmitted. Collisions are only possible error.
 Time can be divided into Slotted or Continuous.
 Stations can sense a channel is busy before they try it.

Multiple Access Protocols : Aloha
 Aloha is one of the random access methods that was introduced in the early 70's. It was designed for
radio (wireless) LAN, but it can be used on any shared medium
 Aloha is basically of two types,
 Pure Aloha
 Slotted Aloha

Pure Aloha
 The original ALOHA protocol is called pure
ALOHA.
 The idea behind the protocol is that each
station sends a frame whenever it has a frame
to send.
 Since there is only one channel to share, there
is the possibility of collision between frames
from different stations.
 Figure here, shows an example of frame
collisions in pure ALOHA.

Pure Aloha
 Here the Aloha protocol comes in action.
 If all the sender sends the lost frames again together then there can be collisions again.
 Hence, aloha protocol says that the senders wait for a random amount of time.
 This randomness helps to avoid further collisions. We call this back-off time

Pure Aloha
t0
Collides with the end of
Shaded frame
Collides with the start of
Shaded frame
t0+t t0+2t t0+3t
Vulnerable
2*Tt
n

Pure Aloha
t0 - Tfr t t0 + Tfr
B
e
g
i
n
E
n
d
B
e
g
i
n
E
n
d
B
e
g
i
n
E
n
d
B
A
C
B’s end collides with
A’s beginning
A’s end collides with
C’s beginning
Time

Pure Aloha
 Vulnerable Time = 2 * Tt
 Efficiency of Pure ALOHA:
 Spure= G * e^-2G where G is number of stations wants to transmit in Tt slot.
 Maximum Efficiency: Maximum Efficiency will be obtained when G=1/2
 = 1/2 x e-2 x 1/2
 = 1 / 2e
 Which means, in Pure ALOHA, only about 18.4% of the time is used for successful transmissions.
 The maximum efficiency of Pure Aloha is very less due to large number of collisions.

Slotted Aloha
 Slotted Aloha divides the time of shared channel into discrete intervals called as time slots.
 Any station can transmit its data in any time slot.
 The only condition is that station must start its transmission from the beginning of the time slot.
 If the beginning of the slot is missed, then station has to wait until the beginning of the next time slot.
 A collision may occur if two or more stations try to transmit data at the beginning of the same time slot.

Slotted Aloha
Frame 1.1
Frame 2.1
Frame 3.1
Frame 1.2
Frame 2.2
Frame 4.1 Frame 4.2
Frame 3.2
Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6
Station 1
Station 2
Station 3
Station 4
Collision
Duration
Collision
Duration

Slotted Aloha
t
Vulnerable time = Tfr
B
e
g
i
n
E
n
d
B
e
g
i
n
E
n
d
B
e
g
i
n
E
n
d
B
A
C
B’s end collides with
A’s beginning
A Collides with C
Time
t0 - Tfr t0 + Tfr

Slotted Aloha
 Efficiency of Slotted Aloha (η) = G x e-G
 where G = Number of stations willing to transmit data at the beginning of the same time slot
 Maximum efficiency of Slotted Aloha, Maximum value of η occurs at G = 1
 = 1 x e-1
 = 1 / e
 = 0.368
 = 36.8%
 Maximum Efficiency of Slotted Aloha (η) = 36.8%
 The maximum efficiency of Slotted Aloha is high due to less number of collisions.

Pure Aloha Vs Slotted Aloha
Pure Aloha Slotted Aloha
Any station can transmit the data at any time. Any station can transmit the data at the beginning of any time slot.
The time is continuous and not globally synchronized. The time is discrete and globally synchronized.
Vulnerable time in which collision may occur = 2 x Tt Vulnerable time in which collision may occur = Tt
Probability of successful transmission of data packet = G x e-2G Probability of successful transmission of data packet = G x e-G
Maximum efficiency = 18.4% (Occurs at G = 1/2) Maximum efficiency = 36.8% ( Occurs at G = 1)
The main advantage of pure aloha is its simplicity in implementation.
The main advantage of slotted aloha is that it reduces the number of
collisions to half and doubles the efficiency of pure aloha.

Carrier Sense Multiple Access (CSMA)
 This method was developed to decrease the chances of collisions when two or more stations start sending their signals over
the datalink layer.
 Carrier Sense multiple access requires that each station first check the state of the medium before sending.
 The persistence methods can be applied to help the station take action when the channel is busy/idle.
A B C D
Time Time
Vulnerable Time = Propagation Time
C Senses Here
B Senses Here
t1
Propagation Time D Senses Here

Carrier Sense Multiple Access (CSMA)
 Persistence methods :- Methods for Sensing the channel (busy/ idle)
 3 Persistence methods are available:
 I-persistence
 Non-persistence
 P-persistence

I-persistence CSMA
 In this method, after the station finds
the line idle, it sends its frame
immediately (with probability I)
 This method has the highest chance
of collision because two or more
stations finds the line idle and send
their frames immediately
Busy Channel
Idle Channel
Time
Sense & Transmit
Continuously Sense
Channel?
Station can transmit
Idle
Busy

Non-persistence CSMA
 In the non-persistence method, a station that has a frame
to send senses the line
 If the line is idle, it sends immediately
 If the line is not idle, it waits a random amount of time and
then senses the line again
 The Non-persistence approach reduces the chance of
collision because it is unlikely that two or more stations
will wait the same amount of time and retry to send
simultaneously.
 This method reduces the efficiency of the network because
the medium remains idle when there may be stations with
frames to send.
Channel?
Idle
Busy
Wait Randomly
Busy Channel
Idle Channel
Time
Sense & Transmit
Sense Sense
Wait
(Random Time)
Wait
(Random Time)

P-persistence CSMA
 The P-Persistence Method is used if the
channel has time slot duration equal to
or greater than the maximum
propagation time
 The P-Persistence approach combines
the advantages of the two strategies
 It reduces the chance of collision and
improves efficiency
Busy Channel
Idle Channel
Time
Probability outcome
Does not allow transmission
Continuously Sense
Time slot Time slot Time slot
Channel?
Acts as through collision
has occurred and start again
Idle
Busy
Wait a slot
Probability
outcome?
Channel?
>p
<=p
Idle
Busy

Carrier Sense Multiple Access with Collision Detector
(CSMA/CD)
 In this method, a station monitors the medium after it sends a frame to see if the transmission was
successful. If successful, the station is finished, if not, the frame is sent again.
 The CSMA/CD derives from the following:
 CS means that stations first sense a carrier present on the media before transmitting their own signals.
 MA means that multiple stations can access the network media.
 CD means that if a collision is detected because of multiple simultaneous transmission of signals, the stations that
are transmitting signals stop, and then retransmit a short time later

CSMA/CD
 In the diagram, A starts send the first bit of its frame at t1 and since C sees the channel idle at t2, starts sending its frame at t2.
C detects A’s frame at t3 and aborts transmission. A detects C’s frame at t4 and aborts its transmission. Transmission time for
C’s frame is therefore t3 – t2 and for A’s frame is t4- t1.
 So, the frame transmission time (Tfr) should be at least twice the maximum propagation time (Tp). This can be deduced when
the two stations involved in collision are maximum distance apart.
A B C D
Transmission time
t1
t4
t2
t3
Transmission time
Time Time
Collision occurs here
A’s collision detection and
abortion
C’s collision detection and
abortion

CSMA/CD : Process Flowchart
Start
K = 0
Apply one of the persistence
methods
Transmission
done or collision
detected
Apply one of the persistence
methods
Collision detected
Success
Sending a jamming signal
K = K+1
K > Kmax
Abort
Choose a random number R
between 0 and 2K-1
Wait Tb time
(Tb= R * Tp) K = number of attempts
Tb = Backoff time
Tp = Max propagation time
No
Yes
No
Yes
Yes
No

CSMA/CD : Conclusion
 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) is a network protocol for carrier
transmission that operates in the Medium Access Control (MAC) layer. It senses or listens whether the
shared channel for transmission is busy or not, and defers transmissions until the channel is free. The
collision detection technology detects collisions by sensing transmissions from other stations. On
detection of a collision, the station stops transmitting, sends a jam signal, and then waits for a random
time interval before retransmission.

Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA)
 The basic idea behind CSMA/CA is that the station should be able to receive while transmitting to detect a
collision from different stations.
 In wired networks, if a collision has occurred then the energy of received signal almost doubles and the
station can sense the possibility of collision.
 In case of wireless networks, most of the energy is used for transmission and the energy of received signal
increases by only 5-10% if collision occurs.
 It can’t be used by station to sense collision.
 Therefore, CSMA/CA has been specially designed for wireless networks.

Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA) - Strategies
 InterFrame Space (IFS) – When a station finds the channel busy, it waits for a period of time called IFS time. IFS
can also be used to define the priority of a station or a frame. Higher the IFS lower is the priority.
 Contention Window – It is the amount of time divided into slots. A station which is ready to send frames
chooses random number of slots as wait time.
 Acknowledgements – The positive acknowledgements and time-out timer can help guarantee a successful
transmission of the frame.
Busy Send Frame Time-out Time
Found Idle
Continuously Sense
Size:
Binary Exponential
IFS
Contention Window

CSMA/CA : Process Flowchart
Start
K = 0
Idle Channel
Still Idle
Choose a random
number R between 0
and 2K - 1
K = K+1
K > Kmax
Abort
K = number of attempts
Tb = Backoff time
Tp = Max propagation time
No
Yes No
Yes
Yes
No
Wait IFS Time
Wait R Slots
Send Frame
Wait time-out
ACK
Received?
Success
Yes
No

CSMA/CD vs CSMA/CA
CSMA/CD CSMA/CA
CSMA / CD is effective after a collision. Whereas CSMA / CA is effective before a collision.
CSMA / CD is used in wired networks. Whereas CSMA / CA is commonly used in wireless networks.
It only reduces the recovery time. Whereas CSMA/ CA minimizes the possibility of collision.
CSMA / CD resend the data frame whenever a conflict occurs.
Whereas CSMA / CA will first transmit the intent to send for data
transmission.
CSMA / CD is used in 802.3 standard. While CSMA / CA is used in 802.11 standard.
It is more efficient than simple CSMA(Carrier Sense Multiple Access). While it is similar to simple CSMA(Carrier Sense Multiple Access).

Controlled Access
 In controlled access, the stations consults each other to find which station has right to send.
 Controlled access protocols grants permission to send only one node at a time, to avoid collision of
messages on the shared medium.
 A station cannot send data unless it is authorized by the other stations.
 Controlled Access Methods
 Reservation (e.g. Cable Modem)
 Polling (e.g. HDLC (Normal Response Mode))
 Token Passing (e.g. Token Ring, Token Bus)

Controlled Access : Reservation
 In the reservation method, a station needs to make a reservation before sending data.
 Time is divided into intervals.
 Reservation Interval of fixed time length
 Data Transmission period of variable frames
 In each interval, a reservation frame precedes the data frames sent in that interval
 If there are N stations in the system, there are exactly N reservation mini slots in the reservation frame.
 Each mini slot belongs to a station. When a station needs to send a data frame, it makes a reservation in its own
mini slot.
 The stations that have made reservations can send their data frames after the reservation frame a situation with
five stations and a five minislot reservation frame.
 a situation with five stations and a five minislot reservation frame.

Controlled Access : Reservation
 The following figure shows a situation with five stations and a five slot reservation frame. In the first interval, only stations 1, 3, and
4 have made reservations. In the second interval, only station 1 has made a reservation.
 Suppose if station 1 has a frame to send, it transmits 1 bit during the slot 1. No other station is allowed to transmit during this slot.
 In general, ith station may announce that it has a frame to send by inserting a 1 bit into ith slot.
 After all N slots have been checked, each station knows which stations wish to transmit.
 The stations which have reserved their slots transfer their frames in that order.
 After data transmission period, next reservation interval begins.
 Since everyone agrees on who goes next, there will never be any collisions.
0 0 0 0 0 1 0 0 0 0
Data
Station 1
Data
Station 4
Data
Station 3
Data
Station 1
1 0 1 1 0
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

Controlled Access : Polling
 To impose order on a network of independent users and to establish one station in the network as a
controller that periodically polls all other stations which is called Polling.
 There are two general polling policies:
 Round Robin Order
 Priority Order
 It works with topologies in which one device is designated as a Primary Station and the other devices are
Secondary Stations.
 The Primary device controls the link, where as the secondary follows it’s instructions.
 Exchange of data must be made through the primary device even though the final destination is
secondary.

 Polling process is similar to the roll-call performed in class. Just like the teacher, a controller sends a message to
each node in turn.
 In this, one acts as a primary station(controller) and the others are secondary stations. All data exchanges must
be made through the controller.
 The message sent by the controller contains the address of the node being selected for granting access.
 Although all nodes receive the message but the addressed one responds to it and sends data, if any. If there is
no data, usually a “poll reject”(NAK) message is sent back.
 Problems include high overhead of the polling messages and high dependence on the reliability of the
controller.
 Efficiency : Let Tpoll be the time for polling and Tt be the time required for transmission of data. Then,
Efficiency = Tt/(Tt + Tpoll)

 SELECT FUNCTION:
 Whenever primary has something to send, it sends
the message to each node.
 Before Sending the data, it creates and transmits a
Select(SEL) frame, one field of it includes the address
of the intended secondary.
 While sending, the primary should know whether the
target device is ready to receive or not.
 Hence, it alerts the secondary for the upcoming
transmission and wait for an acknowledgement (ACK)
of secondary’s status.

 POLL FUNCTION:
 When the primary is ready to receive data, it must ask
(poll) each device if it has anything to send.
 If the secondary has data to transmit, it sends the data
frame. Otherwise, it sends a negative
acknowledgement(NAK) .
 The primary then polls the next secondary. When the
response is positive (a data frame), the primary reads the
frame and returns an acknowledgment (ACK).
 There are two possibilities to terminate the transmission:
either the secondary sends all data, finishing with an EOT
frame, or the primary says timer is up.

 Advantages:
 Priorities can be assigned to ensure faster access from some secondary .
 Maximum and minimum access times and data rates on the channel are predictable and fixed. Drawbacks:
 High dependence on the reliability of the controller.
 Increase in turn around time reduces the channel data rate under low loads and it’s throughput.

Controlled Access : Token Passing
 A Station is authorized to send data when it receives a
special frame called a Token.
 Stations are arranged around a ring (physically or logically)
 A Token circulates around a ring
 If a station needs to send data ,it waits for the token
 The Station captures the token and sends one or more
frames as long as the allocated time has not expired
 It releases the token to be used by the successor station.
Token

 Station Interface is in two states
 Listen state: Listen to the arriving bits and check the
destination address to see if it is its own address. If yes the
frame is copied to the station otherwise it is passed
through the output port to the next station.
 Transmit state: station captures a special frame called free
token and transmits its frames. Sending station is
responsible for reinserting the free token into the ring
medium and for removing the transmitted frame from the
medium.
1 bit delay
Listen Mode
Input
From
Ring
to station from station
Output
To
Ring
Delay
Transmit Mode
to station from station

 In token passing scheme, the stations are connected logically to each other in form of ring and access of stations is governed by
tokens.
 A token is a special bit pattern or a small message, which circulate from one station to the next in some predefined order.
 In Token ring, token is passed from one station to another adjacent station in the ring whereas in case of Token bus, each station
uses the bus to send the token to the next station in some predefined order.
 In both cases, token represents permission to send. If a station has a frame queued for transmission when it receives the token, it
can send that frame before it passes the token to the next station. If it has no queued frame, it passes the token simply.
 After sending a frame, each station must wait for all N stations (including itself) to send the token to their neighbors and the
other N – 1 stations to send a frame, if they have one.
 There exists problems like duplication of token or token is lost or insertion of new station, removal of a station, which need be
tackled for correct and reliable operation of this scheme.

 Performance:
 Delay, which is a measure of time between when a packet is ready and when it is delivered. So, the average
time (delay) required to send a token to the next station = a/N.
 Throughput, which is a measure of the successful traffic.
 Throughput, s =
1
1+
𝑎
𝑁
𝑓𝑜𝑟 𝑎 < 1
 𝑠 =
1
𝑎(1+
1
𝑁
)
𝑓𝑜𝑟 𝑎 > 1
 𝑊ℎ𝑒𝑟𝑒 𝑁 = 𝑛𝑢𝑚𝑏𝑒𝑟𝑠 𝑜𝑓 𝑠𝑡𝑎𝑡𝑖𝑜𝑛𝑠
 𝑎 =
𝑇𝑝
𝑇𝑡
, 𝑇𝑝 = 𝑃𝑟𝑜𝑝𝑎𝑔𝑎𝑡𝑖𝑜𝑛 𝐷𝑒𝑙𝑎𝑦 𝑎𝑛𝑑 𝑇𝑡 = 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛 𝐷𝑒𝑙𝑎𝑦

Controlled Access : Token Passing Flow Chart
Start
Wait for the token
Data frame to
send?
Allocated
time
expired?
No
Yes
Wait IFS Time
Stop
Yes
Capture the Token
Send the frame
No

 We need token management , if there is a loss of token or it is destroyed when a station fails
 We can assign priorities as which station can receive the token. Network Topology : o The way in which
different systems and nodes are connected and communicate with each other is determined by topology of
the network.

Controlled Access : Types of Logical Rings
Physical Ring Topology

Controlled Access : Types of Logical Rings
Dual Ring Topology

Channelization
 The available bandwidth of the link is shared in time, frequency and code to multiple stations to access
channel simultaneously.
 Frequency Division Multiple Access (FDMA)
 Time Division Multiple Access (TDMA)
 Code Division Multiple Access (CDMA)

Channelization : Frequency Division Multiple Access (FDMA)
 The available bandwidth is divided into equal bands so that each station can be allocated its own band.
Guard bands are also added so that no to bands overlap to avoid crosstalk and noise.
 Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a
link (in hertz) is greater than the combined bandwidths of the signals to be transmitted.
 In this illustration, the transmission path is divided into three parts, each representing a channel that carries
one transmission.

 Multiplexing Process:
The following figure is a conceptual illustration of the multiplexing process. Each source generates a signal of a similar frequency range. Inside
the multiplexer, these similar signals modulates different carrier frequencies (f1, f2 and f3). The resulting modulated signals are then combined
into a single composite signal that is sent out over a media link that has enough bandwidth to accommodate it.
 Demultiplexing Process:
The demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals. The individual signals
are then passed to a demodulator that separates them from their carriers and passes them to the output lines.
Input lines Output lines
Channel 1
Channel 2
Channel 3
M
U
X
D
E
M
U
X

 Applications of FDM:
 To maximize the efficiency of their infrastructure, telephone companies have traditionally multiplexed signals from lower-
bandwidth lines onto higher-bandwidth lines.
 A very common application of FDM is AM and FM radio broadcasting.
 The first generation of cellular telephones (still in operation) also uses FDM.
 Implementation:
FDM can be implemented very easily. In many cases, such as radio and television broadcasting, there is no need
for a physical multiplexer or demultiplexer. As long as the stations agree to send their broadcasts to the air using
different carrier frequencies, multiplexing is achieved. In other cases, such as the cellular telephone system, a base
station needs to assign a carrier frequency to the telephone user. There is not enough bandwidth in a cell to
permanently assign a bandwidth range to every telephone user. When a user hangs up, her or his bandwidth is
assigned to another caller.

Channelization : Time Division Multiple Access (TDMA)
 In this, the bandwidth is shared between multiple stations. To avoid collision time is divided into slots and
stations are allotted these slots to transmit data. However there is a overhead of synchronization as each
station needs to know its time slot. This is resolved by adding synchronization bits to each slot. Another
issue with TDMA is propagation delay which is resolved by addition of guard bands.
 Time-division multiplexing (TDM) is a digital process that allows several connections to share the high
bandwidth of a linle Instead of sharing a portion of the bandwidth as in FDM, time is shared. Each
connection occupies a portion of time in the link.

M
U
X
D
E
M
U
X
4
1
2
3
4
1
2
3
4
3 2 1 4 3 2 1 4 3 2 1 4 3 2 1

 2 Schemes of TDM
 Synchronous
 Each input connection has an allotment in
the output even if it is not sending data. In
synchronous TDM, the data flow of each
input connection is divided into units, where
each input occupies one input time slot.
 Time slots are grouped into frames. A frame
consists of one complete cycle of time slots,
with one slot dedicated to each sending
device. In a system with n input lines, each
frame has n slots, with each slot allocated to
carrying data from a specific input line.
A1
A2
A3
B1
B2
B3
C1
C2
C3
T T T
A1 B1 C1 A2 B2 C2 A3 B3 C3
M
U
X Each frame is 3 time slots
Each time slots duration is T/3 secs
Data are taken from each line
every T s

 Statistical
 In statistical time-division multiplexing,
slots are dynamically allocated to
improve bandwidth efficiency. Only when
an input line has a slot's worth of data to
send is it given a slot in the output frame.
 In statistical multiplexing, the number of
slots in each frame is less than the
number of input lines. The multiplexer
checks each input line in round robin
fashion.
A1
A2
A3
B1
B2
B3
C1
C2
C3
T T T
A1 B1 A2 C2 A3 B3
M
U
X

Channelization : Code Division Multiple Access (CDMA)
 One channel carries all transmissions simultaneously. There is neither division of bandwidth nor division of
time.
 For example, if there are many people in a room all speaking at the same time, then also perfect reception
of data is possible if only two person speak the same language.
 Similarly, data from different stations can be transmitted simultaneously in different code languages.

Wired LAN: Ethernet Standards and FDDI

Ethernet Standards
 The Ethernet standards come under the IEEE 802 section which deal with local area networks and
metropolitan area networks. In particular, IEEE 802.3 defines Ethernet.
 The different IEEE 802.3 standards define different aspects of Ethernet covering the physical layer and data
link layer's media access control (MAC) of wired Ethernet.
 Some of the individual standards may introduce new versions or flavours of Ethernet to keep pace with the
growing requirements for speed and performance, whereas other standards may define aspects like the
data frames used.

Ethernet Standards
 Standard Ethernet Code
10
at the beginning means the network operates
at 10Mbps.
BASE means the type of signaling used is baseband.
2 or 5
at the end indicates the maximum cable length
in meters.
T the end stands for twisted-pair cable.
X at the end stands for full duplex-capable cable.
FL at the end stands for fiber optic cable.
 Some of Ethernet version numbering:
 10BASE5: 10 Mb/s over coaxial cable (ThickWire)
 10BROAD36: 10 Mb/s over broadband cable, 3600 m max
segments
 10BASE5: 1 Mb/s over 2 pairs of UTP
 10BASE2: 10 Mb/s over thin RG58 coaxial cable (ThinWire),
185 m max segments
 10BASE-T: 10 Mb/s over 2 pairs of UTP
 10BASE-FL: 10 Mb/s fiber optic point-to-point link
 10BASE-FB: 10 Mb/s fiber optic backbone (between
repeaters). Also, known as synchronous Ethernet.

Ethernet Frame : Basic Frame Format
 PREAMBLE – Ethernet frame starts (PRE).
 Start frame delimiter (SFD). This field (1 byte: 10101011) signals the beginning of the frame.
 Type. This field defines the upper-layer protocol whose packet is encapsulated in the frame.
 Data. This field carries data encapsulated from the upper-layer protocols. For example, a datagram has a field
that defines the length(padding) of the data.
 Cyclic Redundancy Check (CRC): The last field contains error detection information
Preamble
S
F
D
Destination
Address
Source
Address
Type Data and Padding CRC
7 bytes 1 byte 6 bytes 6 bytes 2 bytes
Minimum payload length : 46 bytes
Maximum payload length : 1500 bytes
4 bytes
Physical layer
header Minimum frame length : 512 bits or 64 bytes
Maximum frame length : 12,144 bits or 1518 bytes

Fiber Distributed Data Interface (FDDI)
 A set of ANSI and ISO standards for transmission of data in local area network (LAN) over fiber optic cables. It is
applicable in large LANs that can extend up to 200 kilometers in diameter.
 Features
 FDDI uses optical fiber as its physical medium.
 It operates in the physical and medium access control (MAC layer) of the Open Systems Interconnection (OSI) network
model.
 It provides high data rate of 100 Mbps and can support thousands of users.
 It is used in LANs up to 200 kilometers for long distance voice and multimedia communication.
 It uses ring based token passing mechanism and is derived from IEEE 802.4 token bus standard.
 It contains two token rings, a primary ring for data and token transmission and a secondary ring that provides backup if the
primary ring fails.
 FDDI technology can also be used as a backbone for a wide area network (WAN).

Fiber Distributed Data Interface (FDDI)
 Frame Format
 Preamble: 1 byte for synchronization.
 Start Delimiter: 1 byte that marks the beginning of the frame.
 Frame Control: 1 byte that specifies whether this is a data frame or control frame.
 Destination Address: 2-6 bytes that specifies address of destination station.
 Source Address: 2-6 bytes that specifies address of source station.
 Payload: A variable length field that carries the data from the network layer.
 Checksum: 4 bytes frame check sequence for error detection.
 End Delimiter: 1 byte that marks the end of the frame.
Preamble
S
F
D
Destination
Address
Source
Address
Data and Padding
End
Delimiter
1 byte 1 byte 2-6 bytes 2-6 bytes 1 byte
Checksum
4 bytes
Variable Length
Frame
Control
1 byte

Wireless LAN: IEEE 802.11x and Bluetooth Standards

802.11X
 A generic term to refer to the IEEE 802.11 standard for defining communication over a wireless LAN (WLAN).
 802.11, commonly known as Wi -Fi, specifies an over-the-air interface between a wireless client and a base station
or between two wireless clients
 It refers to the common flavors of Wi-Fi, most notably 802.11a, 802.11b, 802.11g, and 802.11n.
 IEEE 802.11 defines two MAC sub-layers
 Distributed Coordination Function (DCF)
DCF uses CSMA/CD as access method as wireless LAN can’t implement CSMA/CD. It only offers asynchronous service.
 Point Coordination Function (PCF)
PCP is implemented on top of DCF and mostly used for time-service transmission. It uses a centralized, contention-free
polling access method. It offers both asynchronous and time-bounded service.

802.11X : MAC layer frame
Frame Control
2 bytes
Duration
2 bytes
Address 1
6 bytes
Address 2
6 bytes
Address 3
6 bytes
SC
2 bytes
Address 4
6 bytes
Data
0-2312 bytes
CRC
4 bytes
Protocol
Version
2 bits
Type
2 bits
Subtype
4 bits
To
DS
1 bit
From
DS
1 bit
More
Frag
1 bit
Retry
1 bit
Power
Mgmt
1 bit
More Data
1 bit
WEP
1 bit
Order
1 bit
IEEE 802.11 MAC Frame Structure

802.11X
 Frame Control (FC). This is 2 byte field and defines the type of frame and some control information. This field
contains several different subfields.
 D . It stands for duration and is of 2 bytes. This field defines the duration for which the frame and its
acknowledgement will occupy the channel. It is also used to set the value of NA V for other stations.
 Addresses. There are 4 address fields of 6 bytes length. These four addresses represent source, destination, source
base station and destination base station.
 Sequence Control (SC). This 2 byte field defines the sequence number of frame to be used in flow control.
 Frame body. This field can be between 0 and 2312 bytes. It contains the information.
 Frame Check Sequence (FCS). This field is 4 bytes long and contains error detection sequence.

802.11X : Bluetooth
 It is a wireless LAN technology using short-range radio links, intended to replace the cable(s) connecting
portable and/or fixed electronic devices.
 It is an ad hoc network where devices can automatically find each other, establish connections, and discover
what they can do for each other.
 Range 10-100 mtrs.
 Features are robustness, low complexity, low power and low cost.
 Uses a 2.4-GHz ISM band divided into 79 channels of 1 MHz each
 A Bluetooth device has a built-in short-range radio transmitter.
 It uses Frequency Hop Spread Spectrum (FHSS) to avoid any interference.

802.11X : Bluetooth
 Applications
 Automatic synchronization between mobile and stationary devices
 Connecting mobile users to the internet using Bluetooth-enabled wire-bound connection ports
 Dynamic creation of private networks
 Types of Bluetooth Wireless Technology : Depending on the power consumption and range of the device,
there are 3 Bluetooth Classes as:
 Class 1: Max Power – 100mW ; Range – 100 m
 Class 2: Max Power – 2.5mW ; Range – 10 m
 Class 3: Max Power – 1mW ; Range – 1 m

802.11X : Bluetooth : Protocol Architecture
 Bluetooth is a layered protocol architecture
 Core protocols
 Cable replacement and telephony control protocols
 Adopted protocols
 Core protocols
 Radio
 Baseband
 Link manager protocol (LMP)
 Logical link control and adaptation protocol (L2CAP)
 Service discovery protocol (SDP)
 Cable replacement protocol
 RFCOMM
 Telephony control protocol
 Telephony control specification – binary (TCS BIN)
 Adopted protocols
 TCP/UDP/IP
 OBEX
 WAE/WAP

802.11X : Bluetooth : Network Topology - Piconet
 Known as small net, have up to 8 stations
 One primary, rest are secondary
 Communication mode : One-to-One or One-to-Many
 Each of the active slaves has an assigned 3-bit active
member address
 An additional eight secondary’s can be in the parked
state
 A secondary in a parked state is synchronized with the
primary but cannot take part in the communication until
it is moved in the parked state
Primary
Secondary Secondary Secondary Secondary

802.11X : Bluetooth : Network Topology - Scatternet
 Formed by the combination of piconet
 A secondary station in one piconet can be the primary in
another piconet
 This station can receive messages from primary in the
first piconet where it was secondary and delivers them to
the secondary in the second piconet.
Secondary Secondary Secondary Secondary
Primary
Secondary Secondary Secondary
Primary
Secondary

802.11X : Bluetooth : Link Security
 Elements:
 Authentication – verify claimed identity
 Encryption – privacy
 Key management and usage
 Security algorithm parameters:
 Unit address
 Secret authentication key (128 bits key)
 Secret privacy key (4-128 bits secret key)
 Random numbe

Token Bus, Token Ring and Virtual LAN

Token Bus (IEEE 802.4)
 The 802.4 IEEE standard defines the Token Bus protocol for a token-passing access method on a bus topology.
 In a token-passing access method, a special packet called a token is passed from station to station and only the
token holder is permitted to transmit packets onto the LAN.
 No collisions can occur with this protocol (Only One Station can transfer)
 When a station is done transmitting its packets, it passes the token to the "next" station.
 The next station does not need to be physically closest to this one on the bus, just the next logical station.
 A station can hold the token for only a certain amount of time before it must pass it on -even if it has not
completed transmitting all of its data
 This assures access to all stations on the bus within a specified period of time

Token Bus (IEEE 802.4)
Token Bus Network ( Blue Arrow Indicates Token Passing Sequence )

Token Ring (IEEE 802.5)
 A Station is authorized to send data when it receives a
special frame called a Token.
 Stations are arranged around a ring (physically or logically)
- A Token circulates around a ring
 If a station needs to send data ,it waits for the token
 The Station captures the token and sends one or more
frames as long as the allocated time has not expired
 It releases the token to be used by the successor station.
Stations in LAN
Direction of token
passing Direction of token
passing

Token Ring (IEEE 802.5)
 The 802.5 IEEE standard defines the Token Ring protocol which, like Token Bus, is another token passing
access method, but for a ring topology
 A ring topology consists of a series of individual point-to-point links that form a circle
 A token is passed from station to station in one direction around the ring, and only the station holding the
token can transmit packets onto the ring
 Data packets travel in only one direction around the ring
 When a station receives a packet addressed to it, it copies the packet and puts it back on the ring
 When the originating station receives the packet, it removes the packet.

Virtual LANs
 A VLAN is a switched network that is logically segmented by functions, project teams, or applications without
regard to the physical location of users.
 For example, several end stations might be grouped as a department, such as engineering or accounting.
 When the end stations are physically located close to one another, you can group them into a LAN segment.
 If any of the end stations are in different buildings (not the same physical LAN segment), you can then group them
into a VLAN.
 Two types of VLANs
 Static VLAN
 Dynamic VLAN

Virtual LANs : Static
 They are called port-based and port-centric membership VLANs.
 Ports on a switch are manually assigned to a VLAN.
 This is the most common method of assigning ports to VLANs.
 As a device enters the network, it automatically assumes the VLAN membership of the port to which it is
attached

Virtual LANs : Dynamic
 They allow membership based on the MAC address of the device connected to the switch port.
 As a device enters the network, it queries a database within the switch for a VLAN membership.
 Membership is configured using a special server called a VLAN Membership Policy Server (VMPS).

Virtual LANs : Features
 Simplification of end-station moves, adds and changes
 When an end station is physically moved to a new location, its attributes can be reassigned from a network management
station through Simple Network Management Protocol (SNMP) or through the user interface menus.
 When an end station is moved within the same VLAN, it retains its previously assigned attributes in its new location. When an
end station is moved to a different VLAN, the attributes of the new VLAN are applied to the end station.
 Controlled traffic activity
 VLANs allow ports on the same or different switches to be grouped so that traffic is confined to members of only that group.
 This feature restricts broadcast, unicast, and multicast traffic (flooding) only to ports included in a certain VLAN.
 The management domain is a group of VLANs that are managed by a single administrative authority.

Virtual LANs : Features
 Workgroup and network security
 You can increase security by segmenting the network into distinct broadcast domains.
 To this end, VLANs can restrict the number of users in a broadcast domain.
 You can also control the size and composition of the broadcast domain by controlling the size and composition of a
VLAN.
 Components : Networks that have VLANs contain one or more of the following components:
 Switches that logically segment connected end stations
 Routers that provide VLAN communications between workgroups
 Transport protocols that carry VLAN traffic across shared LAN and ATM backbones
 Interoperability with previously installed LAN system

Virtual LANs : Advantages and Disadvantages
 Advantages:
More Security
Ease of administration
Broadcast control
Reduction in network traffic
Disadvantage:
Costly
Software based
Human labor to program
Depending on variety
switches
Management complexity

Thank you
https://chandanbhagat.com.np/me

Unit 3 - Data Link Layer - Part B

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Unit 3 - Data Link Layer - Part B