1. Net 221D : Computer Networks Fundamentals
Lecture 4:
Data Link Layer
Net 221D : Computer Networks Fundamentals
Behrouz A. Forouzan” Data communications and Networking

2. Functions of the Data Link Layer
Data link layer is divided into:
data link control (DLC) sublayer: deals with the design and procedures for communication between two adjacent nodes; node-to-node communication. DLC functions include:
Framing
Flow control (Regulating the flow of data so that slow receivers are not swamped )
Error control (Dealing with transmission errors )
media access control (MAC) sublayer: coordinate access to the link when multiple noes are connected and shared a common link. This is called medium access control.
Data Link layer as a whole is responsible for Addressing ( MAC / physical/ link address) .
Computer networks / Andrew S. Tanenbaum-- 5th ed

3. DLC and MAC sublayers
Computer networks / Andrew S. Tanenbaum-- 5th ed

4. 1. addressing Three types of addresses:
Unicast MAC Address: each host or each interface of a router is assigned a unicast address. It allows each single transmitting device to send a packet to another single destination device. Unicasting means one-to-one communication. In the structure of Multicast MAC address the second digit should be even. Example: A2:34:45:11:92:F1.
Multicast MAC Address: it allows a source device to send a packet to a group of devices. Multicast means one-to-many communication. In the structure of Multicast MAC address the second digit should be odd. Example: A3:34:45:11:92:F1.
Broadcast MAC Address: A frame with a destination broadcast address is sent to all entities in the link. Broadcasting means one-to all communication. Example: FF:FF:FF:FF:FF:FF.
Computer networks / Andrew S. Tanenbaum-- 5th ed

5. 2. Framing
The data link layer needs to pack bits into frames, so that each frame is distinguishable from another.
Example :
Our postal system practices a type of framing.
The simple act of inserting a letter into an envelope separates one piece of information from another; the envelope serves as the delimiter.
Behrouz A. Forouzan” Data communications and Networking

6. 2. Framing
A good design (for Breaking up the bit stream into frames) must make it easy for a receiver to find the start of new frames while using little of the channel bandwidth.
Two approaches are used for Framing:
1. Character-oriented approach
2. Bit-oriented approach

7. Character-oriented (or byte-oriented) approach
To separate one frame from the next, an 8-bit (I-byte) flag is added at the beginning and the end of a frame. The flag, composed of protocol-dependent special characters, signals the start or end of a frame.
However, there is a still a problem we have to solve; It may happen that the flag byte occurs in the data, especially when binary data such as photographs or songs are being transmitted.
Computer networks / Andrew S. Tanenbaum-- 5th ed

8. Byte Stuffing
One way to solve this problem is to have the sender’s data link layer insert a special escape byte (ESC) just before each ‘‘accidental’’ flag byte in the data.
The data link layer on the receiving end removes the escape bytes before giving the data to the network layer.
This technique is called byte stuffing.
Byte stuffing is the process of adding 1 extra byte whenever there is a flag or escape character in the text.
Computer networks / Andrew S. Tanenbaum-- 5th ed

9. Byte Stuffing
Computer networks / Andrew S. Tanenbaum-- 5th ed

10. Bit-oriented approach
Problem in Character oriented approach: The universal coding systems in use today, such as Unicode, have 16-bit and 32-bit characters that conflict with 8-bit characters. We can say that in general, the tendency is
For this reason; framing can be also done at the bit level, so frames can contain an arbitrary number of bits made up of units of any size.
Each frame begins and ends with a special bit pattern, or 0x7E in hexadecimal.
Computer networks / Andrew S. Tanenbaum-- 5th ed

11. Bit-oriented approach
Whenever the sender’s data link layer encounters five consecutive 1s in the data, it automatically stuffs a 0 bit into the outgoing bit stream (bit stuffing).
When the receiver sees five consecutive incoming 1 bits, followed by a 0 bit, it automatically de-stuffs (removes) the 0 bit.
Computer networks / Andrew S. Tanenbaum-- 5th ed

12. Note
Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data, so that the receiver does not mistake
the pattern for a flag.

13. 3. Error control
How to make sure all frames are eventually delivered to the network layer at the destination and in the proper order.
Assume for the moment that the receiver can tell whether a frame that it receives contains correct or faulty information (using error detection and correction techniques).
The usual way to ensure reliable delivery is to provide the sender with some feedback about what is happening at the other end of the line.

14. Typically, the protocol calls for the receiver to send back special control frames bearing positive or negative acknowledgements about the incoming frames.
If the sender receives a positive acknowledgement about a frame, it knows the frame has arrived safely.
On the other hand, a negative acknowledgement means that something has gone wrong and the frame must be retransmitted again.

15. 10.#
Redundancy
The central concept in detecting or correcting errors is redundancy. To be able to detect or correct errors, we need to send some extra bits with our data. These redundant bits are added by the sender and removed by the receiver. Their presence allows the receiver to detect or correct corrupted bits.
10.15

16. Error detection and correction
The correction of errors is more difficult than the detection. In error detection, we are looking only to see if any error has occurred. The answer is a simple yes or no.
In error correction, we need to know the exact number of bits that are corrupted and more importantly, their location in the message.
Behrouz A. Forouzan” Data communications and Networking

17. 1. Parity Check Parity check:
Simple parity check; detects single errors. Fig.10.1
Two dimensional check; detects burst errors. Fig.10.2
Figure Single-bit error
Figure Burst error of length 8
Behrouz A. Forouzan” Data communications and Networking

18. Figure 10.10 Encoder and decoder for simple parity-check code

19. 1. Parity Check: simple parity checking
In this code, a k-bit dataword is changed to an n-bit codeword where n = k + 1. The extra bit, called the parity bit, is selected to make the total number of Is in the codeword even.
The encoder uses a generator that takes a copy of a 4-bit dataword (a0, a1 a2 and a3) and generates a parity bit r0.
If the number of 1s is even, the result is 0; if the number of 1s is odd, the result is 1.In both cases, the total number of 1s in the codeword is even.
The checker at the receiver calculates the syndrome s0. s0 equals 0 when the number of Is in the received codeword is even; otherwise, it is 1.
Behrouz A. Forouzan” Data communications and Networking

20. 2. Cyclic Redundancy Check (CRC)
In a cyclic code, if a code-word is cyclically shifted (rotated), the result is another code- word.
Example:
if is a code-word and we cyclically left-shift, then is also a code-word.
Behrouz A. Forouzan” Data communications and Networking

21. 10.# Figure 10.14: CRC encoder and decoder A CRC code with C(7, 4)
Behrouz A. Forouzan” Data communications and Networking
10.21

22. The Process of CRC At the Sender:
The encoder, the dataword has k bits (4 here).The codeword has n bits ( 7).
The size of the dataword is augmented by adding n-k (3 here) 0s to the right-hand size of the word.
The generator uses a divisor of size n-k+1 (4 here), predefined and agreed upon.
The generator divides the augmented dataword by the divisor ( modulo-2 division).
The remainder is appended to the dataword to create the codeword.
Behrouz A. Forouzan” Data communications and Networking

23. The Process of CRC At the Receiver:
The decoder does the same division process as the encoder.
The remainder of the division is the syndrome.
If the syndrome is all 0s, there is no error, the dataword separated from the received codeword and accepted.
Otherwise, everything is discarded.
Behrouz A. Forouzan” Data communications and Networking

24. 10.# Figure 10.15: Division in CRC encoder
Behrouz A. Forouzan” Data communications and Networking

25. 10.# Figure 10.16: Division in the CRC decoder for two cases
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26. 3. CHECKSUM
Checksum is an error-detecting technique that can be applied to a message of any length.
Like CRC, checksum based on the concept of redundancy.
Behrouz A. Forouzan” Data communications and Networking

27. Example
Suppose the message is a list of five 4-bit numbers that we want to send to a destination.
In addition to sending these numbers, we send the sum of the numbers.
For example, A sender wants to send the set of numbers
(7, 11, 12, 0, 6) the process will be as follow:
Sum up the numbers = = 36
Then convert the decimal number 36 in binary = (100100)2.
To change it to a 4-bit number ( as the sending numbers) we add the extra leftmost bit to the right four bits as shown below. This is called sum in one’s complement .

28. Example Cont. Then complements the result to get the checksum.
6= ( 0110) 2 ,
the complement of 6 = = 9
Or (1001)2 = 9 = (1001)2
The sender sends the five data numbers and the checksum
(7, 11, 12, 0, 6, 9).
If there is no corruption in transmission, the receiver receives (7, 11, 12, 0, 6, 9) and adds them in one’s complement to get 15.
Behrouz A. Forouzan” Data communications and Networking

29. 10.# Figure 10.16: Example 10.24 100100 36 101101 45 10 10 _________ 10
_________ 10
_________

30. 4. Flow Control
Another important design issue that occurs in the data link layer (and higher layers as well) is what to do with a sender that systematically wants to transmit frames faster than the receiver can accept them.
This situation can occur when the sender is running on a fast, powerful computer and the receiver is running on a slow, low-end machine.
Behrouz A. Forouzan” Data communications and Networking

31. Two approaches are commonly used.
1. In the first one, feedback-based flow control, the receiver sends back information to the sender giving it permission to send more data, or at least telling the sender how the receiver is doing.
2. In the second one, rate-based flow control, the protocol has a built-in mechanism that limits the rate at which senders may transmit data, without using feedback from the receiver.

32. Flow control schemes
Behrouz A. Forouzan” Data communications and Networking

33. NOISELESS CHANNELS
Let us first assume we have an ideal channel in which no frames are lost, duplicated, or corrupted.
Two protocols are working in the ideal channel:
Simplest Protocol
Stop-and-Wait Protocol
Behrouz A. Forouzan” Data communications and Networking

34. Figure 11.6 The design of the simplest protocol with no flow or error control
Behrouz A. Forouzan” Data communications and Networking

35. Algorithm 11.1 Sender-site algorithm for the simplest protocol
Algorithm Receiver-site algorithm for the simplest protocol
Behrouz A. Forouzan” Data communications and Networking

36. Stop-and-Wait Protocol
A sender sends one frame, waits for an acknowledgment that confirms its receipt by the receiver, then it sends the next frame. The waiting time is called time-out.
Figure Design of Stop-and-Wait Protocol

37. Figure Flow diagram
Behrouz A. Forouzan” Data communications and Networking

38. NOISY CHANNELS
Although the Stop-and-Wait Protocol gives us an idea of how to add flow control, noiseless channels are nonexistent.
We discuss three protocols in this section that use error control.
Stop-and-Wait Automatic Repeat Request(ARQ)
Go-Back-N ARQ
Selective Repeat ARQ
Automatic Repeat Request (ARQ) - any time an error is detected in an exchange, specified frames are retransmitted.
Behrouz A. Forouzan” Data communications and Networking

39. 1. Stop-and-Wait ARQ
Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expires.
To detect and correct corrupted frames, we need to add CRC to the frame. When the frame arrives at the receiver site, it is checked and if it is corrupted, it is silently discarded. The sender then resends the frame.
We use sequence numbers to number the frames. This helps the receiver to know the lost or duplicated frames.
Behrouz A. Forouzan” Data communications and Networking

40. 1. Stop-and-Wait ARQ
The sequence numbers are based on modulo-2 arithmetic.
The acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected.
Behrouz A. Forouzan” Data communications and Networking

41. Figure 11.10 Design of the Stop-and-Wait ARQ Protocol
Behrouz A. Forouzan” Data communications and Networking

42. Example 11.3
Figure shows an example of Stop-and-Wait ARQ. Frame 0 is sent and acknowledged. Frame 1 is lost and resent after the time-out. The resent frame 1 is acknowledged and the timer stops. Frame 0 is sent and acknowledged, but the acknowledgment is lost. The sender has no idea if the frame or the acknowledgment is lost, so after the time-out, it resends frame 0, which is acknowledged.
Behrouz A. Forouzan” Data communications and Networking

43. Figure 11.11 Flow diagram for Example 11.3
Behrouz A. Forouzan” Data communications and Networking

44. 2. Go-Back-N ARQ
In the Go-Back-N Protocol, the sequence numbers are modulo 2m, where m is the size of the sequence number field in bits.
Sliding window defines the range of sequence numbers that is the concern of the sender and receiver - sender and receiver deals with only a part of the range of sequence numbers
The send window is an abstract concept defining an imaginary box of size 2m − 1 with three variables: Sf, Sn, and Ssize.
The send window can slide one or more slots when a valid acknowledgment arrives.
Behrouz A. Forouzan” Data communications and Networking

45. The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn.
The window slides when a correct frame has arrived; sliding occurs one slot at a time.
ACKs are cumulative - more than one frame can be acknowledged by a single ACK
Behrouz A. Forouzan” Data communications and Networking

46. Figure 11.12 Send window for Go-Back-N ARQ
Behrouz A. Forouzan” Data communications and Networking

47. Figure 11.13 Receive window for Go-Back-N ARQ
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48. Figure 11.15 Window size for Go-Back-N ARQ
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49. In Go-Back-N ARQ, the size of the send window must be less than 2m;
Note
In Go-Back-N ARQ, the size of the send window must be less than 2m;
the size of the receiver window is always 1.
Behrouz A. Forouzan” Data communications and Networking

50. Figure 11.14 Design of Go-Back-N ARQ
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51. Example 11.7
Figure shows what happens when a frame is lost. Frames 0, 1, 2, and 3 are sent. However, frame 1 is lost. The receiver receives frames 2 and 3, but they are discarded because they are received out of order.
The sender receives no acknowledgment about frames 1, 2, or 3.
Its timer finally expires. The sender sends all outstanding frames (1, 2, and 3) because it does not know what is wrong.
This means that when ACK 2 arrives, the sender is still busy with sending frame 3.
Behrouz A. Forouzan” Data communications and Networking

52. Figure 11.17 Flow diagram for Example 11.7
Behrouz A. Forouzan” Data communications and Networking

53. 3. Selective Repeat ARQ
Go-Back-N ARQ simplifies the process at the receiver site.
The receiver keeps track of only one variable, and there is no need to buffer out-of-order frames; they are simply discarded.
However, this protocol is very inefficient for a noisy link. In a noisy link a frame has a higher probability of damage, which means the resending of multiple frames.
This resending uses up the bandwidth and slows down the transmission.
For noisy links, there is another mechanism that does not resend N frames when just one frame is damaged; only the damaged frame is resent.
This mechanism is called Selective Repeat ARQ.
Computer networks / Andrew S. Tanenbaum-- 5th ed

54. Figure 11.18 Send window for Selective Repeat ARQ
Figure Receive window for Selective Repeat ARQ

55. Figure 11.20 Design of Selective Repeat ARQ
Behrouz A. Forouzan” Data communications and Networking

56. Example 11.8
This example is similar to Example 11.3 in which frame 1 is lost. We show how Selective Repeat behaves in this case. Figure shows the situation. One main difference is the number of timers. Here, each frame sent or resent needs a timer, which means that the timers need to be numbered (0, 1, 2, and 3). The timer for frame 0 starts at the first request, but stops when the ACK for this frame arrives. The timer for frame 1 starts at the second request, restarts when a NAK arrives, and finally stops when the last ACK arrives. The other two timers start when the corresponding frames are sent and stop at the last arrival event.
A NAK is sent once for each window position and defines the first slot in the window.
In Selective Repeat, ACKs are sent when data are delivered to the network layer. If the data belonging to n frames are delivered in one shot, only one ACK is sent for all of them.
Behrouz A. Forouzan” Data communications and Networking

57. Figure 11.23 Flow diagram for Example 11.8
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58. Behrouz A. Forouzan” Data communications and Networking