Transcript Computer Networks - University of Northampton

Access to Shared Media Media Access Control protocols
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Layer 1 involves
◦ Media
 Physical , Wireless
◦ Digital data
 bit streams that travel on media,
◦ Components that put signals on media, and
 cables, connectors..
 Have I missed anything?
◦ Topologies
 Shared access
 Any examples ?
 Dedicated access
 any example ?
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Physical layer does not have any provision to:
Issue 1: Identify
◦ Which computer does data belong to?
Issue 2: Data Format
◦ How is data ORGANISED and Carried on the media
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bits/bytes, Frames
Issue 3: Access shared media
Answer: Assign these jobs to the layer above it – the Data Link
Layer
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Data Link layer components
Data Frames
MAC address
Media Access and protocols
◦ Ethernet
◦ Ring Topology protocol
 Token Ring
 FDDI
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Computer One
Application
Computer Two
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Underlying Physical Medium
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LLC (Logical Link Control)
MAC (Media Access Control)
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The OSI –Data Link Layer is divided into two
separate sub-layers:
◦ Media Access Control (MAC) – provides service to
the layer below - physical layer
◦ Logical Link Control (LLC) - provides services to
layer above - the network layer
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Every Ethernet NIC card has a
unique address – called MAC
address, which provides a way for
computers to identify themselves.
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They give hosts a permanent,
unique identification on a network.
◦ Example: 00-60-2F-3A-07-BC
How long is the MAC address
( in terms of bytes)?
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◦ Example of MAC address
 00-60-2F-3A-07-BC
◦ 48 bits (6 bytes) in length.
◦ The first 3 bytes, are administered
by the IEEE (Institution of
Electrical and Electronics
engineers) to help identify the
manufacturer.
◦ The first three bytes are called
the Organizational Unique
Identifier (OUI).
◦ The remaining 3 bytes comprise
the interface serial number and
are unique to a card
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Data bits are generated by a computer that wishes to
communicate
Stream of data bits or bit-stream is framed by layer 2
and transmitted on the cable (layer 1) by the Network
Interface Card (NIC).
Framing provides structure to the bit-stream.
Example data frame :
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Two ways to access shared media
Random Access
◦ Any device on the network that has data to send can access the
medium, if not used
Taking turns
◦ Each device on a network takes turns in a round-robin manner.
Multi-access
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1111
2222
3333
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3333
nnnn
Abbreviated
MAC
Addresses
1111
1111, 2222,… are MAC addresses
When an Data frame is sent out all devices on the
network receive it.
What do they do with it?
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1111
Nope
2222
Hey, that’s
me!
3333
Nope
nnnn
Abbreviated MAC
Addresses
Data Frame
3333
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1111
When information (frame) is transmitted, each NIC on the shared
media copies part of the transmitted frame to see if the
destination address matches the MAC address on its NIC.
If there is a match, the rest of the frame is copied, otherwise rest
of the frame is ignored.
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I have data
to send
I have data
to send
X
Collision!
Issue: What happens when multiple computers want try to transmit at the
same time?
 E.g., Computer 1 and 2 has data to send…
 Two or more computers accessing media at the same time causes
data Collision
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Collision is detected by all computers, which then stop transmitting
data
 Ethernet protocol CSMA/CD summarizes these steps..
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Listens (Carrier Sense) to the
network’s shared media to see if any
other users are “on the line” by trying
to sense a neutral electrical signal or
carrier.
If no transmission is sensed, then
multiple access allows anyone onto
the media without any further
permission required.
If two PCs detect a neutral signal
and access the shared media at the
exact same time, a collision occurs
and is detected.
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Which topology is this ?
Taking turns MAC protocols need
a permission to transfer data on
network
Permission is given in the form of
a control token (T), which is
passed from one computer to
next in a round-robin fashion
◦ Token is short, reserved frame
that cannot appear in data
There is only one token, so only
one computer can transmit at a
time
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When a computer wants to transmit,
it waits for the token
After it acquires the token, it can
send data for the preset time.
After preset time, computer transmits
token on ring
Next computer ready to transmit
receives token and then transmits
If no computer is ready to transmit
data, token circulates around ring
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IBM Token Ring
◦ Now obsolete
◦ Data transfer rate upto16Mbps
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Fibre Distributed Data Interconnect (FDDI)
◦ Uses ring topology
◦ Uses fibre optics cable between stations
◦ Transmits data at 100Mbps
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Uses pairs of fibres to form two concentric rings to
provide fault tolerance
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IEEE 802.x series standards
are defined for Layer 2 (Data
Link Layer) protocols for media
access.
Two Popular standards for local
area network are:
◦ Ethernet
 IEEE 802.3 - Wired network
 IEEE 802.11 – Wireless
network
◦ Token Ring
 IEEE 802.5
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Data Link Layer
Components
Logical Link Control
(LLC)
Ethernet
Token
Ring
Media Access Control (MAC)
Protocols
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Let’s pause here for a moment and bring the
following together:
◦ What is the data entity at layer 2 called ?
◦ What is role of MAC address in data communication?
◦ Which protocols of media access are probabilistic/
deterministic?
 CSMA/CD
 Token Ring
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