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.
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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.
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” Link Layer: Introduction
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 • 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