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

 Specifications developed for wireless LAN technology  Examples are a, g, n

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  is the standard for popular Ethernet networks today  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 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