1. Computer Networks Chapter 3 & 4

2. Data Link layer Switch and bridge work on 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. 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. 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.

4. 2. 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:
Character count.
Flag bytes with byte stuffing.
Starting and ending flags, with bit stuffing.
Physical layer coding violations.
3. Error control: The usual way to ensure reliable delivery is to provide the sender with some feedback (acknowledgement.)
4. 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.

5. 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: 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.

6. 2. A Simplex Wait and Watch Protocol: 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.
3. A Simplex Protocol for a Noisy Channel: Now consider the normal situation of a communication channel that makes errors. Frames may be either damaged or lost completely.

7. 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 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.

8. 2. A Protocol Using Go Back N: Requiring a sender to wait for an acknowledgement before sending another frame. This technique is known as pipelining.
3. 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.

9. 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.

10. 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.

11. 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

12. The Data Link Layer in the Internet: Two such protocol are widely used in the internet
SLIP- Serial line IP
PPP-Point to Point protocol
The Medium Access Sub—Layer: Networks can be divided into two categories
Point-to-point connections
Those using broadcast channels
Broadcast channels are sometimes referred to as multi access channels or random access channels. Multi-access channel belong to a sub layer of the data link layer called the MAC (Medium Access Control) sub layer.

13. The Channel Allocation Problem
How to allocate a single broadcast channel among competing users. It is of two types:
Static channel allocation in LANs and MANs
Dynamic channel allocation in LANs and MANs

14. Ethernet The types of Ethernet cables are:
Ethernet is connection media access method that allows all host on a network to share the same bandwidth of a link. Ethernet uses both Data link layer and Physical layer specification.
The types of Ethernet cables are:
Straight Through Cable: The straight through cable use to connect :
Host to switch or hub
Router to switch or hub

15. Straight Through Cable

16. 2. Crossover cable: The crossover cable can be used to connect :
Switch to Switch
Hub to Hub
Host to Host
Router direct to host

17. Crossover cable

18. 3. Rolled Cable: Although rolled cable isn’t use to connect any Ethernet connection together, you use rolled Ethernet cable to a router console serial port.
Host connect to Router or Switch

19. Wireless LANs
Although Ethernet is widely used, it is about to get some competition. Wireless LANs are increasingly popular, and more and more office buildings, airports, and other public places are being outfitted with them. Wireless LANs can operate in one of two configurations, with a base station and without a base station.
The standard states that each conformant wireless LAN must provide nine services. These services are divided into two categories: five distribution services and four station services.

20. The five distribution services are as follows
Association
Disassociation
Reassociation
Distribution
Integration
The remaining four services are as follows.
Authentication
Deauthentication
Privacy
Data delivery

21. Broadband Wireless
Erecting a big antenna on a hill just outside of town and installing antennas directed at it on customers' roofs is much easier and cheaper than digging trenches and stringing cables. Thus, competing telecommunication companies have a great interest in providing a multimegabit wireless communication service for voice, Internet, movies on demand, etc.
However, some people prefer to call it a wireless MAN (Metropolitan Area Network) or a wireless local loop

22. Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances (using short wavelength radio transmissions in the ISM band from  MHz) from fixed and mobile devices, creating personal area networks(PANs) with high levels of security.