Data Link Layer

B. Konkoth

PDU

Protocol Data Unit A unit of data which is specified in a protocol

of a given layer

Layer 5, 6, 7 – Data Layer 4 – Segment Layer 3 – Packet Layer 2 – Frame Layer 1 - Bit

Data Link Layer

Provides a well-defined service interface to the network layer.

Determines how the bits of the physical layer are grouped into frames (framing).

Deals with transmission errors . Regulates the flow of frames. Performs general link layer management.

Link Layer: Introduction

Some terminology: hosts and routers are nodes

(bridges and switches too) communication channels

that connect adjacent nodes along communication path are links wired links wireless links LANs

PDU is a frame

“link”

2 sub-layers in Data Link Layer

logical link control (LLC)

media access control (MAC)

logical link control (LLC)

starts and maintains connections between devices.

when you send data from your workstation to a server on the same network segment, LLC sub-layer establishes a connection with that server

media access control (MAC)

The MAC Layer enables multiple devices to share the media

MAC sub-layer maintains physical device addresses for communicating with other devices (commonly referred to as MAC addresses)

Most LANs have more than one computer, and the MAC sub-layer determines who may speak and when Ethernet – CSMA/CD Token ring – Token passing FDDI – Dual ring

IEEE 802 Project

IEEE 802

Institute of Electrical and Electronic Engineers (IEEE) began a project in February of 1980, known as Project 802

for the year and month it began

It is a set of standards given to the various LAN architectures such as Ethernet and Token Ring

802.11

Specifications developed for wireless LAN technology

Examples are 802.11a, 802.11g, 802.11n

802.2 Standard defining Logical Link Control

(LLC)

responsible for the data transmission between computers or devices on a network

provides a general interface between the different protocols (IPX, TCP/IP, etc.) and the different network types (Ethernet, Token Ring, etc.)

802.3

802.3 is the standard for popular Ethernet networks today

802.3 specifies the physical media and the working characteristics of Ethernet

Data are transmitted using CSMA/CD

CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:

If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission

Human analogy: listen, don’t interrupt others!

CSMA/CD (Collision Detection)

collision detection: measure signal strengths, compare

transmitted, received signals: easy in wired LANs

difficult in wireless LANs: receiver shut off while transmitting

Most used protocol in Ethernet

CSMA/CD

CSMA/CD doesn’t stop collisions from happening, but it helps manage the situations when collisions occur.

In fact, collisions are a very normal part of Ethernet operation. It’s only when collisions begin to occur frequently that you need to become concerned.

802.5

The 802.5 standard was modeled after the IBM Token Ring network

Introduced a unique access method: token passing

Token Ring A token is generated by the first computer

that comes online on the network.

When a workstation wants to transmit data, it grabs the token and then begins transmitting

The destination computer receives the data frame, modifies it, and sends it back to the sender, indicating that the transmission of data was successful.

When the workstation completes transmitting, the token is released back to the network