1. ملاحظة: الكلام المسطر + الرسومات ذات السهم الاحمر*مهمة للمذاكرة
Physical Media
Data Link layer
Chapter 5
ملاحظة: الكلام المسطر + الرسومات ذات السهم الاحمر*مهمة للمذاكرة

2. Physical Media
Data Link layer
The main task of the data link layer is to transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer.
It accomplishes this task by having the sender break up the input data into data frames.
Switch and bridge work on Data link layer.

3. Physical Media
Data Link layer
The data link layer uses the services of the physical layer to send and receive bits over communication channels. It has a number of functions, including:
Providing a well-defined service interface to the network layer.
Dealing with transmission errors.
Regulating the flow of data so that slow receivers are not swamped by fast senders.

4. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
The data link layer has a number of specific functions it can carry out:
The function of the data link layer is to provide services to the network layer.
Framing
Error Control
Flow Control

5. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Services Provided to the Network Layer

6. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Services Provided to the Network Layer
The principal service is transferring data from the network layer on the source machine to the network layer on the destination machine.
Three reasonable possibilities that are commonly provided are:
Unacknowledged connectionless service.
Acknowledged connectionless service.
Acknowledged connection-oriented service.

7. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Framing
Since it is too risky to count on timing to mark the start and end of each frame, other methods have been devised.
In this section we will look at four methods:
Byte count.
Flag bytes with byte stuffing.
Flag bits with bit stuffing.
Physical layer coding violations.

8. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Byte count : لابد من فهم الطريقة

9. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Flag bytes with byte stuffing

10. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Flag bits with bit stuffing

11. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Error Control
Error control: The usual way to ensure reliable delivery is to provide the sender with some feedback (acknowledgement.)

12. Data Link Layer Design Issues
Physical Media
Data Link Layer Design Issues
Flow Control
The usual solution is to introduce flow control to throw the sender into sending no faster than the receiver can handle the traffic.

13. Elementary Data Link Layer Protocols
Physical Media
Elementary Data Link Layer Protocols
As far as the data link layer is concerned, the packet passed across the interface to it from the network layer is pure data, whose every bit is to be delivered to the destination's network layer.
An Unrestricted Simplex Protocol
A Simplex Wait and Watch Protocol
A Simplex Protocol for a Noisy Channel

14. Elementary Data Link Layer Protocols
Physical Media
Elementary Data Link Layer Protocols
An Unrestricted Simplex Protocol شرح
Data are transmitted in one direction only.
The protocol consists of two distinct procedures, a sender and a receiver.
The sender runs in the data link layer of the source machine, and the receiver runs in the data link layer of the destination machine.
No sequence numbers or acknowledgements.

15. Elementary Data Link Layer Protocols
Physical Media
Elementary Data Link Layer Protocols
A Simplex Wait and Watch Protocol شرح
The communication channel is still assumed to be error free however, and the data traffic is still simplex.
Protocols in which the sender sends one frame and then waits for an acknowledgement before proceeding are called stop- and-wait.

16. Elementary Data Link Layer Protocols
Physical Media
Elementary Data Link Layer Protocols
A Simplex Protocol for Noisy Chanel شرح
Now consider the normal situation of a communication channel that makes errors.
Frames may be either damaged or lost completely.

17. Sliding Window Protocol
Physical Media
Sliding Window Protocol
The three protocols are bidirectional protocols that belong to a class called sliding window protocols.
The three differ among themselves in terms of efficiency, complexity, and buffer requirements, as in all sliding window protocols, each outbound frame contains a sequence number, ranging from 0 up to some maximum.
A One Bit Sliding Window Protocol
A Protocol Using Go Back N
A Protocol Using Selective Repeat

18. Sliding Window Protocol
Physical Media
Sliding Window Protocol
A One Bit Sliding Window Protocol: A sliding window protocol with a maximum window size of 1. Such a protocol uses stop-and-wait since the sender transmits a frame and waits for its acknowledgement before sending the next one.
A Protocol Using Go Back N: Requiring a sender to wait for an acknowledgement before sending another frame. This technique is known as pipelining.
A Protocol Using Selective Repeat: It works well if errors are rare, but if the line is poor, it wastes a lot of bandwidth on retransmitted frames. In this protocol, both sender and receiver maintain a window of acceptable sequence numbers.

19. EXAMPLE DATA LINK PROTOCOLS
Physical Media
EXAMPLE DATA LINK PROTOCOLS
Within a single building, LANs are widely used for interconnection, but most wide-area network infrastructure is built up from point-to-point lines. Here we will examine the data link protocols found on point-to-point lines in the Internet in two common situations.
The first situation is when packets are sent over SONET optical fiber links in wide-area networks. These links are widely used, for example, to connect routers in the different locations of an ISP’s network.
The second situation is for ADSL links running on the local loop of the tele-phone network at the edge of the Internet. These links connect millions of individuals and businesses to the Internet.

20. ADSL (Asymmetric Digital Subscriber Loop)
Physical Media
ADSL (Asymmetric Digital Subscriber Loop)
ADSL connects millions of home subscribers to the Internet at megabit/sec rates over the same telephone local loop that is used for plain old telephone service.
A device called a DSL modem is added on the home side. It sends bits over the local loop to a device called a DSLAM (DSLAccess Multiplexer), pronounced ‘ ‘dee-slam,’’ in the telephone company’s local office.

21. The Data Link Layer in the Internet
Physical Media
The Data Link Layer in the Internet
The Internet needs point-to-point links for these uses, as well as dial-up modems, leased lines, and cable modems, and so on.
A standard protocol called PPP (Point-to-Point Protocol) is used to send packets over these links.
PPP is defined in RFC 1661 and further elaborated in RFC 1662 and other RFCs (Simpson, 1994a, 1994b).
Two such protocol are widely used in the internet:
SLIP- Serial line IP
PPP-Point to Point protocol

22. HDLC (High Level Data Link Layer Control)
Physical Media
HDLC (High Level Data Link Layer Control)
HDLC is an actual protocol designed to support both half duplex and full duplex communication over point to point and multipoint links.
A bit old but are still heavily used. They are all derived from the data link protocol first used in the IBM mainframe world: SDLC (Synchronous Data Link Control) protocol.
These protocols are based on the same principles. All are bit oriented.

23. Physical Media
HDLC Frame Format
Flag Field: The flag field of an HDLC frame is an 8 bit sequence with a bit pattern
Address Field: Contains the address of the secondary station that is either originator or the destination of the frame.
Control Field: Used for flow and error control.
Information Field: Contain the user’s data from the network layer.
FCS Field: Frame Check Sequence(FCS) is HDLC error detection field.

24. Bit Stuffing HDLC uses a process called bit stuffing.
Physical Media
Bit Stuffing
HDLC uses a process called bit stuffing.
Bit stuffing is a process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag.
Example: The sequence after
Bit Stuffing becomes