1. Wi-Fi, Li-Fi Data Link layer
Done By:
Reema Ahmad
Supervisor:
Dr.Mohammad Saleh

2. Outlines Data Link Layer Wi-Fi data Link Layer - Sub-Layers
- CSMA/CD and CSMA/CA
Li-Fi data Link Layer
- CSMA
- Modified CSMA/CA (fairness)

3. OSI Layers
Data Link Layer (Layer 2)
Physical Layer (Layer 1)

4. Data Link Layer
this layer responsible for data transmission in LAN or WAN, it focus on how to transmit this data between the network entities by providing the  functional and procedural means.
It controls the channel and corrects the physical layer’s errors.
It provide a flexible channel for the entities communication

5. Data Link Layer(Cont.)
Data Link Layer protocols to prevent the collision:
1. Ethernet (for the local networks (multi-nodes)).
2. Point-Point Protocol (PPP)
3. HDLC and ADCCP ( for the Point-Point(dual-nodes)connection).

6. Data Link Layer(Cont.) In TCP/IP
Data Link Layer (As the lowest layer) within the link layer

7. Data Link Layer(Cont.)
This layer alone doesn’t have the Acknowledgment for the receiving process.
There are some protocols or standards such as IEEE 802( as ).
It has two sub layers:
1. Logical Link Control(LLC)
2. Media Access Control (MAC)

8. Data Link Layer(Cont.) Furthermore, it supports: 1. check errors.
2. Acknowledgement.
3. re-transmission.
Data link layer is divided into: LLC, MAC and application protocol convergence.

9. OSI architecture
Network link layer send service requests to  data link layer  this layer perform these requests by issuing them to  PHY layer.

10. Sub-layers 1. LLC - It is used for:
- check the errors and ensuring the message is received successfully by using an acknowledgment packet.
- Determining how the data transmission and reception are done.
- Control the message between the sender and receiver.

11. Sub-layers)cont.)
2. MAC
It determines who will access the channel, when the channel is accessible and what is the procedure by using Carrier Sense Multiple Access with collision avoidance (CSMA/CD
frame based on the MAC addressing.
Example:
Talker_1
Talker_2

12. Sub-layers)cont.)
In this example, the MAC determine which talker will talk and which will hear.
In the case that both talk at the same time, period of time is created and the process is repeated.
The MAC also determine the “Frame synchronization”( when one will end to start the others).

13. Sub-layers)cont.) The frame synchronization types: 1. time based.
2. character counting.
3. byte stuffing.
4. bit stuffing.

14. Frame synchronization types
Time based
It gives a period of time to complete the procedure.
Its challenge: The long and short space of time based on external environments that effects the network.
Character counting
It uses the counter to count remained character in the frame header
Its challenge when an system’s error occurs.

15. Frame synchronization types(Cont.)
Byte stuffing
It uses before the frame a sequence of bytes(such as DLE STX with DLE ETX) .
Bit stuffing
It uses before frame with sequence of bits (such as ) and at the receiving the inserted 0 will be deleted. This distinguish the bit stuffing bit with the normal bits to ensure the correct received data.

16. Error detection
Compute a math equation(i.e. the meta-data) and the received message and make a matching comparison to insure the correct message.
example: the received message is “HELLO”, it will translated as numbers based on the alphabet’s order as follows:

17. Error detection (Cont.)H E L O 8 5 12 15
when computing the message ( = 52) and the meta-data(2+5=7).
in the case that the message received as “ ”, the computing will be = 51 and the meta-data as 1+5 = 6 which doe not match and the message is not correct.

18. Sub-layers extra
1. LLC:
A) provides services to the network layer above it and hides the detail information for allowing the other technologies to use it seamlessly.
B) Most of the local area networking technologies use LLC control.

19. Error detection example
CRC (Cyclic Redundancy Check) is used to ensure that the received data is correctly received.

20. Wi-Fi data Link layer Application Layer Presentation Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer

21. How this Layer works There are two sub-layers: 1. LLC 2. MAC
Network Layer send IP packet to  LLC (encapsulate the data into addressing and control information that called frame and sent to) MAC(modifies these information and sent to ) PHY layer that transmit as RF. At the receiving side this process is reversed.

22. How this Layer works(Cont.)
This process is done in 500 milliseconds.
In case of including the QoS, this time is reduced.
However, in the case of the dense networks: this will increase the medium control, the latency and the transaction time.

23. Sub-Layers 1. LLC (802.2)  The same as the LLC data link layer.
2. MAC:
- Which is unique in WLAN.
- It allows for multiple users to communicate via share medium by having the sender senses before accessing it to avoid the collision occur.

24. Sub-Layers(Cont.)
In the case of the collision, there are two techniques are used:
1. CSMA/CD
2. CSMA/CA

25. CSMA/CD It is Carrier Sense Multiple Access with Collision Detection.
The collision is caused when two or more of the devices transmit their data at the same time.
This technique is not possible here, because the collision detection requires transmitting and listening to the channel at the same time , in radio systems this ability is not possible for hearing the channel. It is solved by the next step(CSMA/CA).

26. CSMA/CA
It is a Carrier Sense Multiple Access with Collision Avoidance.
It avoids the collision by explicit Acknowledgment packet to ensure the received data successfully.

27. How CSMA/CA works
The transmitter senses the channel, wait period of time then transmit the data after this time is finished and the channel is still free.
The receiving side replies with Acknowledgment packet that the data is received successfully and end the process

28. CSMA/CA challenge
In the case of the ACK packet does not received due to:
- the original data did not received or
- a Ack problem occur.
The re-transmision is done in this case.

29. Wi-Fi challenges
It does not have the Mac as same as the Ethernet, this will make it as slower performance than the Ethernet is.
The hidden nodes problem when two nodes transmit at the same time without knowing about each other. It is solved by Handshaking (using RTS/CTS)

30. RTS/CTS
They are the Request to Send(RTS) and Clear To Send(CTS) packets to avoid the collision.
The sender transmits RTS to the receiver node which reply with CTS to inform the other nodes that there is a sending and receiving process in this sides and can’t transmit their data until they are finished.

31. References links https://en.wikipedia.org/wiki/Data_link_layer

32. Li-Fi Data Link layer Data link layer SSCS Logical Link control (LLC)
Media Access Control(MAC

33. Li-Fi Data Link layer
The Service specific Convergence sub layer(SSCS) is used for the LLC and MAC communication that is used in ATM(Asynchronous Transfer Mode)and a secure data as in the WPAN.
MAC handling issues such as addressing, collision avoidance and data acknowledgment(that ensure that the data is received successfully).

34. Li-Fi Data Link layer(Cont.)
The MAC are shared with IEEE specifications like:
- beacon and guaranteed time slot
management.
- channel access.
- frame validation.
- association and dissociation.

35. Visible Light communication
According that the Li-Fi based on Visible Light communication, there some points must be taken into account when dealing with this communication such as:
- Visibility.
- flicker.
- mitigation(reduce the overhead).
- dimming support.

36. MAC layer
This layer focus on how to control the medium for the transmitting/ receiving process by using :
- Carrier Sense Multiple Access with Collision
Avoidance(CSMA).
- Distributed CSMA.

37. Carrier Sensing
This is used when the VLC device wants to transmit data, it sense before the channel to inform that it either idle or busy.
It is related to the first MAC’s techniques (i.e CSMA/CA).
It has two types, the basic and the fast senses.

38. Carrier Sensing types
The basic Sense, to determine by the PHY’s reports that the channel is either idle or not.
The fast Sense, when the channel is idle, it is used to deal with the data as low and high and it is especially used in low state.
these types is used for the CSMA/CD(Carrier Sense Multiple Access with Collision Detection) and CSMA/ CA to deal with collision state.

39. CSMA/CA
It is similar to a way for the communication between the VLC device and the channel.
The channel is sensed by th VLC device to decide if it Is idle or busy for transmitting its data and how it is handled in the both cases.
in this communication, there are two types of data(i.e. the transmitted data and the acknowledgement).

40. CSMA/CA This process is done as follows:
1. The channel is sensed by the VLC device.
2. the channel contains the waiting time and the contention window (CW), each one of 16 slots.
16 slots(time waiting)
16 slots(CW)

41. CSMA/CA (Cont.)
The first part(i.e. the time waiting) to ensure that the channel is really idle and not exist another device wants to transmit at the this time(to avoid the collision).
The Second part(i.e. the CW) to ensure that this is the transmitted data and not include any acknowledgment packet. In this part the transmission process is begun.

42. CSMA/CA (Data transmission)
When the VLC device wants to transmit its data, it selects one of the 16 CW slots. The CW gives a probability of 1/16 and the channel is busy at the time.
The CW waits a period of time to ensure that not other device will transmit at this time then allow the VLC to transmit.

43. CSMA/CA (Data transmission)
When the data is completely received, the acknowledgement is returned from the destination.
One of the major issue that there is a four slots as a time waiting between the data and the acknowledgment can’t be considered as an idle channel for transmitting for any device.

44. CSMA/CA (The collision)
The collision when two VLC devices select the same CW at the same time.
- in this case the collision can’t be detected or handled directly.
The Acknowledgment plays a significance role in term of detecting the collision by detecting that it is not received after its period of time.

45. CSMA/CA (The collision)
In case that the collision occurs and the Ack is not received, this data is retransmit again.
The re-transmission during three different times. If it still not received the data will be discarded and new transmission will be begun at the same steps.

46. Modified CSMA/CA
It is called the fairness CSMA for the VLC communication via the channel.
It is similar to the original CSMA/CA, however, it focuses on how to make this process as fair as possible.
It is used three parameters(i.e. NB, BE, NB_max).

47. Modified CSMA/CA(Cont.)
Where:
- NB is the number of the backoff times(the waiting for the VLC device before it transmits its data via the channel).
- BE is the backoff exponent(for increasing the backoff time).
- NB_max is the maximum value of the number of backoff time that the device can transmit by.

48. Modified CSMA/CA(Cont.)
BE is derived from the Binary exponential Backoff (BEB).
It is represented as a graph of vertices and an edge(vertices as the sender and receiver while the edge for their relationship and determine who will transmit and the other will stop to avoid the collision occurs).

49. Modified CSMA/CA(cont.)
BEB determines two ranges(i.e. the transmission (R_tx) and the interference (R_I) ranges), the sender and the receiver distance (d) and the Signal to Interference and Noise Ratio(SINR).
these parameters play a role in avoiding the collision by two comparison ratios as follows:
- if d<= SINR then R_I<R_x the device can transmit, otherwise it will stop since the channel is busy.

50. How Modified CSMA/CA works
It is done as follows:
1. The parameters are initiated
- NB with zero, BE as one and the NB as the initial BE.
2. The backoff time is set to one and the NB is increased by 1.
3. using the BE equation(i.e. 2^BE[1] -1).

51. How Modified CSMA/CA works(Cont.)
4. executing the carrier Collision Avoidance(CCA) by the physical layer to determine that the channel is idle or not.
5. if idle, the device transmits successfully.
ohterwise:
- another backoff time is set, after a period of time, the NB is checked either it is larger of NB_max or not, if it is the data is dropped.

52. How Modified CSMA/CA works (Flowchart)

53. Modified CSMA/CA (Results)

54. Discussion
As a result, we can infer that the goal is achieved and the fairness is occurs.
As the first result shows a comparison of different devices at different levels of high and low states and the throughput is increased only at the high state.
As a second one, after the BE is used, the relationship is appeared nearly the same which is the fairness concept.

55. Fairness MAC (cont.)
To distinguish between the data and the Ack frame :
- The Ack Is received directly after the succeful received data is done while the data waits two period of times:
1. The Inter Frame Space(IFS).
2. The Backoff time (The random time).

56. Contention Window(CW)
CW has a counter for counts the number of successful transmission:
- When the channel is busy, this counter stops until the channel become idle then resumes again, instead of building a new one.
it is transferred between the min and max state and in low and high using Pin.

57. Contention Window(CW)(Cont.)
When there is an error, The CW is doubled between the min and the max and the Pin will get in the dark state instead of going in high then low again in the successful transmission.

58. Research points
How this data is stores in the case of the light interruption?
Can we integrate the Li-Fi with Wi-Fi to achieve the best result? And how this will be benefit in term of the Li-Fi since the wi-Fi has several challenges that Li-Fi attempts to solve them(like bandwidth, speed and the health)?