Data Link Layer
B. Konkoth
PDU
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Protocol Data Unit
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A unit of data which is specified in a protocol of a
given layer
Layer 5, 6, 7 –
Layer 4 –
Layer 3 –
Layer 2 –
Layer 1 -
Data
Segment
Packet
Frame
Bit
Data Link Layer
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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:
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hosts and routers are nodes
(bridges and switches too)
communication channels that
connect adjacent nodes along
communication path are links
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wired links
wireless links
LANs
PDU is a frame
“link”
2 sub-layers in Data Link Layer
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logical link control (LLC)
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media access control (MAC)
logical link control (LLC)
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starts and maintains connections between devices.
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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)
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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
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Ethernet – CSMA/CD
Token ring – Token passing
FDDI – Dual ring
IEEE 802 Project
IEEE 802
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Institute of Electrical and Electronic Engineers
(IEEE) began a project in February of 1980, known
as Project 802
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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
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Specifications developed for wireless LAN
technology
Examples are 802.11a, 802.11g, 802.11n
802.2
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Standard defining Logical Link Control (LLC)
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responsible for the data transmission between
computers or devices on a network
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provides a general interface between the different
protocols (IPX, TCP/IP, etc.) and the different
network types (Ethernet, Token Ring, etc.)
802.3
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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:
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If channel sensed idle: transmit entire frame
If channel sensed busy, defer transmission
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Human analogy: listen, don’t interrupt others!
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CSMA/CD (Collision Detection)
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collision detection:
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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
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CSMA/CD doesn’t stop collisions from happening, but
it helps manage the situations when collisions occur.
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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
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The 802.5 standard was modeled after the IBM Token
Ring network
Introduced a unique access method: token passing
Token Ring
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A token is generated by the first computer that
comes online on the network.
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When a workstation wants to transmit data, it
grabs the token and then begins transmitting
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The destination computer receives the data frame,
modifies it, and sends it back to the sender,
indicating that the transmission of data was
successful.
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When the workstation completes transmitting, the
token is released back to the network