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Data Link Layer - Overview of LLC and MAC
Introduction to 802.11
Professor Richard Harris
School of Engineering and Advanced
Technology (SEAT)
Objectives
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You will be able to describe the objectives of the 802
project.
You will be able to discuss the 802.11 LAN standards
You will be able to describe the framing methodology for
802 standards
You will be able to discuss the various fields used for
802 and 802.11 standards in particular.
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References
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Chapters 13 and 14, Forouzan
Chapter 4, Tanenbaum
Cisco CCNA course notes
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Presentation Outline
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Introduction to Project 802
Revision of key concepts from the CCNA learning
material on Ethernet
The wireless LAN revolution 802.11x
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Project 802 of the IEEE
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The Computer Society of the IEEE began a special
project in 1985 that was known as Project 802
Its purpose was to set standards that would enable
intercommunication between equipment from a variety
of manufacturers.
Project 802 is a way of specifying functions of the
physical layer and the data link layer of major LAN
protocols.
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The IEEE Standard for LANs - 1
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The following diagram shows the relationship between the traditional ISO model and
the 802 standard.
It will be seen that the 802 standard has sub-divided the Data Link Layer into two
parts, viz:
LLC Link Layer Control
MAC (Media Access Control) Layer
• The IEEE has actually created several physical layer standards for
different LAN protocols
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The IEEE Standard for LANs - 2
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Data Link Layer
Logical Link Control (LLC)
For IEEE 802, flow control, error control, and part of the framing
duties are all brought together in this LLC sub-layer. Framing is also
performed in the MAC sub-layer.
The LLC provides a single data link control protocol for all IEEE
LANs. (Contrast this with the MAC layer where we have different
protocol versions for different LANs. See previous slide.)
The LLC makes the MAC sub-layer transparent.
Framing
LLC provides a PDU similar to HDLC
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Framing
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The LLC header contains a control field like HDLC and is
used for flow and error control.
The two access point fields (DSAP and SSAP) define the
upper layer protocol at the source and destination that
uses LLC.
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Media Access Control - MAC
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In general, multiple access methods include random access,
controlled access and channelisation.
For IEEE 802, there is a sub-layer called Media Access Control
(MAC) that defines the specific access method for each LAN.
Eg. It defines CSMA/CD as the media access method for Ethernet LANs.
It defines the token passing method for Token Ring and Token Bus
LANs.
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Evolution of Ethernet
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There have been a number of variants for Ethernet that
have evolved over time. The above figure shows this
evolution.
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The MAC Sub-layer
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Data Link Layer
In standard Ethernet, the MAC sub-layer looks after the
operation of the access method. It also frames data
received from the upper layer and passes them to the
physical layer.
LLC
MAC
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MAC Sub-layer Frame Format
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The Ethernet frame contains the following 7 fields:
Preamble
SFD (Start Frame Delimiter)
DA (Destination Address)
SA (Source Address)
Length or type of PDU (Protocol Data Unit)
Upper layer data
CRC (Cyclic Redundancy Check)
Ethernet doesn’t provide any mechanism for frame
acknowledgements so it is “unreliable”.
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The MAC Frame Layout
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Notes:
The preamble is actually added by the Physical Layer and isn’t a
formal part of the frame.
The length or type field can contain either piece of information!
Originally, meant to specify the type of the upper layer protocol
using the MAC frame, it can now also specify the number of
bytes in the data field.
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Minimum and Maximum Lengths
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Notes:
The minimum length is essential since it is needed for the
correct operation of CSMA/CD.
The maximum length is 1518 bytes
This comprises 18 bytes of header and trailer so the maximum
payload can be 1500 bytes.
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Implementations of Ethernet
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Slot Time
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“For an Ethernet network, the round trip time required
for a frame to travel from one end of a maximum length
network to the other, plus the time needed to send the
jam sequence is called the slot time.”
It is defined in bits.
It is the time needed for a station to send 512 bits
The slot time therefore depends on the data rate.
For standard Ethernet, 10-Mbps this value is 51.2s
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Slot Time & Maximum Network Length
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The relationship between these two entities depends on the
propagation speed of the signal in the particular medium.
Normally this is about 2 x 108 m/sec (about 2/3rds of the rate in air)
so:
Slot Time
Max_Length = Propagation Speed 
2
Max_Length  (2 108 )  (51.2 106 / 2)  5120m
By including delay times in repeaters and interfaces, together with
the time required for the jam sequence we find that this reduces the
theoretical length to about 48% of the theoretical calculated value.
Max_Length  2500m
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Implementation 10Base5
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The following slides show implementations of Ethernet.
The slides are reproduced directly from Forouzan’s text.
(Chapter 13).
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10Base2 Implementation
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10Base-T implementation
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10Base-F implementation
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Cable Topologies for Ethernet
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(a) Linear, (b) Spine, (c) Tree, (d) Segmented.
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Wireless LANs
Introducing 802.11x
What is IEEE 802.11?
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IEEE has defined the specifications for a wireless LAN,
called IEEE 802.11, which covers the physical and data
link layers.
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The 802.11 Architecture - 1
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The IEEE 802.11 standard defines two types of service:
BSS – Base Service Set
This is made up of stationary or mobile wireless stations and an
optional central base station.
The central base station is called an Access Point
ESS – Extended Service Set
This is made up of two or more BSS’s connected through a
distribution system that is often a wired LAN – it can be of any type.
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The 802.11 Architecture - 2
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There are two types of BSS illustrated below:
Ad hoc network
Infrastructure network
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Example of an Extended Service Set
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Station Types
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The standard defines three types of stations based on
their mobility:
No transition
Deemed to be stationary or moving only within a BSS
BSS-transition
Said to be a station that can move from one BSS to another BSS
but constrained to being inside one ESS.
ESS-transition
A station can move from one ESS to another.
Note:
The standard does not guarantee that communication will be
continuous during the move.
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The MAC Sub-layer
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The standard defines two MAC sub-layers:
The Distributed Coordination Function (DCF)
The Point Coordination Function (PCF)
The figure below shows the various relationships
involved:
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Distributed Coordination Function
(DCF)
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DCF uses CSMA/CD as the access method.
However:
Wireless LANs cannot implement CSMA/CD for at least three
reasons:
1. For collision detection we need the station to be able to
simultaneously send and receive signals. This leads to significant
bandwidth requirements.
2. Collision may not be detected because of the hidden station
problem (See later slide)
3. The distance between stations can be great and signal fading may
prevent a station at one end from hearing the collision at the other
end. (See later slide)
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Point Coordination Function (PCF)
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It is an optional access method. This can be
implemented in an infrastructure network (NOT an ad
hoc network).
We implement it on top of the DCF and is appropriate for time
sensitive applications.
PCF is centralised.
It is a contention free polling method.
The Access Point performs polling for stations that can be
polled.
The stations are polled sequentially.
Data is sent to the AP.
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Fragmentation
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Since wireless network environments tend to be quite
noisy, it is recommended that frames be fragmented
(smaller pieces) since if a frame is to be retransmitted, it
is better to send a smaller one than a larger one!
Small frames
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The Frame Format
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There are 9 fields in the MAC layer frame:
Frame Control: the FC field is 2 bytes long and defines the frame type and
some control info
D: The duration of the transmission to set the Network Address Vector (NAV)
Addresses x 4 (each 6 bytes). These depend on the To DS and From DS
subfields
Sequence control: Sequence number used in flow control.
Frame body: It can be from 0 to 2312 bytes and can contain info as defined by
the FC field.
Frame Check Sequence (FCS) is 4 bytes and contains a CRC 32 error detection
sequence.
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The Frame Format – subfields of FC
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Frame Types
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In the 802.11 standard, there are basically three types of
frame:
Management frames
Used for the initial communication between stations and access
points
Control frames
Used for accessing the channel and acknowledging frames
Data frames
Used for carrying data and control information
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Addressing Scheme
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There are 4 cases corresponding to the flag values of 0 or 1 in the
To DS and From DS flags.
Notes:
Address 1 is always the address of the next device.
Address 2 is always the address of the previous device
Address 3 is the address of the final destination if it is not defined by
Address 1
Address 4 is the address of the original source station if it is not the
same as Address 2.
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Summary of the Addressing Schemes
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Hidden Station Problem
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Exposed Station Problem
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The Physical Layers
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