Unit 2 Data Link Layer - Functions and its Prototocols

 UNIT: Data Link Layer

 

Fig – 7 layers of OSI Model

 

 

1. Introduction to Data Link Layer

Definition:
The Data Link Layer (DLL) is the second layer in the OSI model. It provides node-to-node delivery of frames and ensures reliable data transfer over the physical link.

Functions:

Framing- Divides the stream of bits received into data units called frames.

Physical addressing (MAC) - If frames are to be distributed to different systems on the same network, data link layer adds a header to the frame to define the sender and receiver.

Error detection & correction - Used for detecting and retransmitting damaged or lost frames and to prevent duplication of frames. This is achieved through a trailer added at the end of the frame.

Flow control - If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender ,the Data link layer imposes a flow control mechanism.

Access control (MAC) - Used to determine which device has control over the link at any given time.

2. Link Layer Addressing

Link Layer Address / MAC Address:

• Unique identifier burned into NIC (Network Interface Card).
• 48-bit address (usually written in hexadecimal, e.g., 00:A0:C9:14:C8:29)
• Assigned by IEEE

 

Types of Addressing:

Type	Description
Unicast	One-to-one (specific destination)
Multicast	One-to-many (group of devices)
Broadcast	One-to-all (all devices on LAN)

 

Unicast Address : Each host or each interface of a router is assigned a unicast address. Unicasting means one-to-one communication. A frame with a unicast address destination is destined only for one entity in the link.

Multicast Address : Link-layer protocols define multicast addresses. Multicasting means one-to many Communication but not all.

Broadcast Address : Link-layer protocols define a broadcast address. Broadcasting means one to-all communication. A frame with a destination broadcast address is sent to all entities in the link.

 

 

MAC address vs IP address

 

 3. Error Detection and Error Correction

Types of Errors:

• Single-bit error: Only one bit is altered.
• Burst error: Two or more bits in a sequence are altered.

Error Detection Techniques:

Method	Description
Parity Bit	Adds one extra bit to make sum even/odd
Checksum	Sum of data segments
CRC (Cyclic Redundancy Check)	Polynomial division method; highly effective

Error Correction:

• Uses redundancy to detect and correct errors.
• Hamming Code is a popular technique for single-bit error correction.

 

 

4. Data Link Control

Manages the flow and reliability of data transfer between devices.

Flow Control:

Ensures sender does not overwhelm receiver.

• Stop-and-Wait ARQ
• Sliding Window Protocol
  (Go-Back-N, Selective Repeat)

Error Control:

• Uses Acknowledgements (ACK) and Negative Acknowledgements (NAK)
• Retransmission of lost or corrupted frames

 

5. Medium Access Control (MAC)

Determines how multiple devices access the shared transmission medium.

Techniques:

Protocol Type	Example Protocols	Description
Random Access	ALOHA, CSMA/CD	Devices transmit at will; collisions possible
Controlled Access	Polling, Token Passing	Devices take turns to access medium
Channelization	FDMA, TDMA, CDMA	Divide channel into frequency/time/code

 

6. Wired LANs: Ethernet

Ethernet Overview:

• Most widely used LAN technology
• Based on IEEE 802.3
• Uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) in shared Network Control Protocol mediums

 

 

 

Ethernet Frame Format:

Field	Description
Preamble	Synchronization
Destination MAC	Receiver’s MAC address
Source MAC	Sender’s MAC address
Type/Length	Type of protocol (e.g., IPv4)
Data	Actual payload
CRC	Error checking

 

7. Wireless LANs (WLAN)

Overview:

• Based on IEEE 802.11
• Uses radio frequency instead of cables
• Devices communicate via Access Point (AP) or in ad hoc mode

Characteristics:

• Uses CSMA/CA (Collision Avoidance)
• Supports roaming, authentication, encryption

 

8. Other Wireless Networks

Technology	Description
Bluetooth (802.15)	Short-range, PAN for connecting devices
WiMAX (802.16)	Long-range wireless broadband
Zigbee	Low power, IoT sensor communication
Cellular Networks	4G, 5G networks using base stations

 

 

 

 

 

 

 

 

 

 

 

 

9. Connecting Devices and Virtual LANs (VLANs)

Connecting Devices:

Device	Function
Repeater	Regenerates signal to extend distance
Hub	Basic device that broadcasts data to all
Bridge	Filters traffic, connects two LAN segments
Switch	Multi-port bridge, connects devices in LAN
Router	Connects different networks, routes packets
Gateway	Protocol converter (e.g., between networks)

Virtual LANs (VLANs):

• Logical subgroup within a physical LAN
• Devices in the same VLAN can communicate as if they were on the same LAN
• Reduces broadcast traffic, improves security

 

 

 

 

 

Data Link Layer Protocols

 

 

Fig:- Data Link Layer Protocols

 

A. Synchronous Data Link Protocol (SDLC) –

·         SDLC is a computer communication protocol and was created and developed by IBM in 1975.

·          it supports multipoint links with error recovery or correction.

·         It is the predecessor to HDLC and is used to carry SNA (Systems Network Architecture) traffic.

·         It is also used to establish point-to-point (one-to-one) or point-to-multipoint (one-to-many) connections between all of the remote devices and the mainframe computers at the central locations.

·         Additionally, it is used to ensure that data units move correctly and in the right order from one network point to the next.

Basic Configurations followed by SDLC nodes:

1.      Point-to-Point

2.      Multipoint

3.      Loop

4.      Hub go-ahead

B. High-Level Data Link Protocol (HDLC) –

·         HDLC is a protocol that many Wide Area protocols..

·          It was first developed and created by ISO in 1979.

·          Generally speaking, this protocol is based on SDLC.

 it offers both reliable and unreliable service with best effort.

·          Bit-oriented HDLC is a protocol that can be used for both point-to-point and multipoint communications.

 

 

 

C. Serial Line Interface Protocol (SLIP) –

·         The only purpose of SLIP, an older protocol, is to add a framing byte to the end of an IP packet.

·         It functions essentially as a data link control facility that is necessary for the dial-up IP packet transfer between an Internet Service Provider (ISP) and a home user.

·         It is an encapsulation of TCP/IP that was created specifically to work over multiple router connections and serial ports for communication.

·         It has some drawbacks, such as the absence of mechanisms for error detection or correction.

1.      Two new special characters are introduced: END (decimal 192) and ESC (decimal 129). The two-byte sequence of ESC and octal 334 or ESC and octal 335, depending on whether the

2.      Data byte code represents the END or ESC character, is sent in a data packet.

3.      Since SLIP lacks a standard specification, there is no maximum packet size. However, for both sending and receiving, the standard value is 1006 bytes of datagram.

4.      When using SLIP, both the sender and the recipient should be aware of each other’s IP addresses.

5.      For IP addressing, it only supports static assignment.

6.      Data is transferred synchronously.

7.      A payload (data) and a flag serving as an end delimiter make up a SLIP frame.

Advantages:

1.      It can support a variety of network configurations, including host-host, host-router, router-router, and others.

2.      Its low overhead makes it simple to use in microcontrollers.

3.      Due to the widespread use of TCP/IP and the fact that it is a simple packet protocol, it is simple to implement.

Disadvantages:

1.      It doesn’t perform any data authentication, and using SLIP, IP addresses cannot be assigned dynamically.

2.      No type identification method is offered by SLIP. No indication of the protocol type sent can be made. Therefore, SLIP connections can only support one protocol at a time.

3.      It lacks a system for detecting or fixing errors during data transmission.

4.      There is no way for hosts to exchange addressing information over a SLIP connection.

5.      No compression features are offered by SLIP to increase packet throughput. Although CSLIP was a variant used for the same purpose, it did not find widespread adoption.

D. Point to Point Protocol (PPP) –

The same functionality as SLIP is essentially provided by PPP, which is a protocol. The most reliable protocol is used to transport not only IP packets but also other kinds of packets. Additionally, dial-up and leased router-router lines may need it. In essence, it offers a framing technique to describe frames. This character-oriented protocol also has an error-detection function. Additionally, it is employed to deliver the NCP and LCP protocols. While NCP is used to negotiate network-layer protocols, LCP is used to bring lines up, negotiate options, and bring them down. It is necessary for similar serial interfaces to HDLC.

Frame format of PPP

Features of PPP:

1.      Packet Framing - Network layer data packet formulation within data link block.

2.      Multi-Protocol - Yield information from any NCP network layer upwards at same time as demultiplex.

3.      Bit Transparency - Should carry certain bit pattern in field of data.

4.      Error Detection - No modification.

E. Link Control Protocol (LCP) –

It was first created and developed by IEEE 802.2. Additionally, it is employed to offer HDLC-style services on LAN. For the purposes of establishing, configuring, testing, maintaining, and ending or terminating links for the transmission of data frames, LCP is essentially a PPP protocol.

Fig :- Frame format of LCP

 

 

F. Link Access Procedure (LAP) –

LAP protocols, or data link layer protocols, are necessary for framing and moving data over point-to-point links. It also has a few features for reliability services. LAPB (Link Access Procedure Balanced), LAPD (Link Access Procedure D-Channel), and LAPF are the three main types of LAP (Link Access Procedure Frame-Mode Bearer Services). Actually, IBM SDLC, which is being submitted to the ISP purely for standardisation, is where it came from.

G. Network Control Protocol (NCP) –

NCP was an additional older protocol that ARPANET used. In essence, it enables users to access computers and a few other devices at distant locations and transfer files between two or more computers. PPP is typically made up of a collection of protocols. For every higher-layer protocol that PPP supports, NCP is always available. TCP/IP took over from NCP in the 1980s.

Some more Data Link Layer protocols

1.      Address Resolution Protocol (ARP)

2.      Cisco Discovery Protocol (CDP)

3.      Fiber Distributed Data Interface (FDDI)

4.      Link Layer Discovery Protocol (LLDP)

5.      Multiprotocol Label Switching (MPLS)

6.      Split multi-link trunking (SMLT)

7.      Shortest Path Bridging (IEEE 802.1aq)

8.      Unidirectional Link Detection (UDLP)

 

 

 

 

Bottom of Form

 

 

 

 

Summary Chart

Topic	Key Idea
Data Link Layer	Node-to-node communication
Link Layer Addressing	MAC address – unique hardware address
Error Detection/Correction	Parity, CRC, Hamming code
Data Link Control	Flow & error control (ACK, ARQ, sliding window)
MAC Protocols	CSMA/CD, CSMA/CA, Token Ring
Ethernet	IEEE 802.3 standard, wired LAN
Wireless LANs	IEEE 802.11, uses Access Points
Other Wireless Networks	Bluetooth, WiMAX, Zigbee
Connecting Devices	Hub, Switch, Router, etc.
VLAN	Logical segmentation of network
DataLink Layer Protocols	Protocols and its features