AMACC TARLAC
Data link layer
JLD 1
AMACC TARLAC
Data link layer
OSI model layer 2
TCP/IP model part of Network Access layer
Application HTTP, FTP,
Data TFTP, SMTP
Presentation stream etc Application
Session
Transport Segment TCP, UDP Transport
Network Packet IP Internet
Data link Frame Ethernet,
WAN
Network Access
Physical Bits technologies
JLD 2
AMACC TARLAC
Functions of data link layer
Encapsulates packets by adding a frame header and
trailer including appropriate addressing.
Controls access to the transmission medium.
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Hops
There may be a different layer 2 protocol in use on each
hop of a journey.
Different media, different types of link, different
bandwidths, LAN/WAN affect the choice of protocol.
Different protocols have different frames.
The router removes the old frame and adds a new
header and trailer for the next hop.
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Sublayers
Sets up the frame
header and trailer
Network
to encapsulate the
packet.
Logical link Identifies network
control layer protocol.
Data link
Media access Adds layer 2
control address.
Marks frame start
and end.
Physical
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AMACC TARLAC
Standards
Institute of Electrical and Electronics Engineers
IEEE 802.2 Logical link control
IEEE 802.3 Ethernet
IEEE 802.5 Token ring
IEEE 802.11 Wi-fi
International Telecommunication Union (ITU)
Various WAN standards: HDLC, ISDN, Frame relay
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Point to point link
Only two devices on the network
Full duplex: both can send at the same time, no problem
with media access
Half duplex: data can only travel one way at a time so
one device can send at a time. Simple media access
control.
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Shared medium
Star with hub
Physical bus
Needs media access control.
If there is no control there will be many collisions and the
frames will be destroyed.
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Contention based media access
Non-deterministic, first come first served.
Each device “listens” and sends when the medium
seems to be clear.
Low overhead.
Collisions occur.
Need a way of re-sending lost frames.
Becomes inefficient on large networks.
E.g. traditional Ethernet.
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AMACC TARLAC
Variation on contention based
Traditional Ethernet uses CSMA/CD (collision detection):
collisions are allowed and detected, frames sent again.
Wi-fi uses CSMA/CA (collision avoidance): when the
medium is clear, host sends signal to say it is about to
use the medium. It then sends.
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Different environments
Fragile environment e.g. satellite link – frames are likely
to be lost – need large overhead of control mechanisms
to make sure data arrives.
Protected environment e.g. modern LAN – frames not
often lost – do not need such elaborate control
mechanisms
Therefore need different layer 2 protocols
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Addressing needs
Point to point link – only one possible destination.
Minimal addressing.
Multi-access network – need full addressing system.
Therefore need different layer 2 protocols.
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Layer 2 frame format
All protocols have the same general form but there are
variations.
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PPP frame
Point to point links. Minimal addressing. Control
mechanisms.
Start Minimal Packet Check
address and
stop
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Ethernet frame
Multi-access links. Full addressing.
No control field.
Same for all Ethernet types/bandwidths.
Timing Addresses Layer 3 Check
48 bits Packet and
and protocol
start each stop
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AMACC TARLAC
802.11 Wi-Fi
LAN wireless protocol
Fragile environment – lots of interference, risk of lost
frames, contention.
Every transmission needs to be acknowledged.
No acknowledgement – re-send frame.
Lots of control mechanisms in frame.
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End to end
PC sends packet to server
Packet header has PC IP address and source and
server IP address as destination.
Frame header has PC MAC address as source and
router MAC address as destination.
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JLD 18
AMACC TARLAC
The End
JLD 19
