OSI Model

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OSI Model

• Source
• J. Henshall, OSI Explained, 2nd edition
• A. Tanenbaum, Computer Networks, 3rd edition
• Prof Ivo Jirasek CPSC 441Networking course
• Groove Systems
• Othman Laraki, 15.020 Competition in Telecoms
  Recitation 2
• Prof M T Harandi, Lou, Vaidya, Gupta, UIUC,
  Networking

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Background

• ARPANET Grandparent of all computer networks
• A research network sponsored by U.S. Department
  of Defense
• It eventually connected hundreds of universities
  and government installations using leased
  telephone lines

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Contd.

• When satellite and radio networks were added
  later, the existing protocols had trouble
  interworking with them, so a new reference
  architecture was needed
• Thus the ability to connect multiple networks
  together in a seamless way was one of the major
  design goals from very beginning

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World of Networking

• Macro Picture
• Types of Networks
• LAN, WAN, MAN, PAN, etc.
• Components of Networks
• Computers, Servers, Satellites, Cables, Routers,
  Modems, etc.
• Micro Picture
• Network Architectures
• Communication Protocols

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Two Paramount Concepts

• Protocols
• Computer-communications architecture

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Protocols

• Protocols are used for communication between
  entities in different systems
• Entity Anything capable of sending and receiving
  information
• User application programs, file transfer
  packages, email facilities etc.
• System A physically distinct object that
  contains one or more entities
• Computers, terminals, remote sensors etc.

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Key Elements

• Syntax includes such things as data format and
  signal levels
• Semantics includes control information for
  coordination and error handling
• Timing includes speed matching and sequencing

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• Protocols
• Each layer is designed as if it communicates
  directly with the same layer on a different
  computer (called peer processes)
• The rules of how two peer layers talk to each
  other is called a protocol.
• Different peer layers will use different
  protocols
• Data is passed from one layer to the layer below
  until it reaches the bottom layer where the
  physical communication takes place.
• After receiving data from the network, the data
  passes up the corresponding layers of the
  computer
• It appears that two peer layers are communicating
  directly

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• Analogy - 2 Philosophers example
• One English speaking philosopher wishes to send a
  message to a colleague in France (Peers in layer
  3)
• Since they have no common language, they both
  hire a translator. The translators (Peers in
  layer 2) decide which language they will
  translate to.
• The translators send their message to their
  secretary (Peers in Layer 1) who may decide to
  send the message by fax, or email or telephone
  (Layer 1 protocol)
• Note that logically each layer communicates with
  its peer. In practice, each layer communicates
  with the layers below them

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• Analogy - Things to remember
• Message is passed from layer 3 down through
  layers 2 and 1 before transmission
• At the opposite side, the message travels up
  through layer 1 and 2 before arriving at layer 3
• Layer 3 does not care how the message is
  transformed or how it is transmitted by the lower
  layers
• Layer 2 is only concerned with transforming the
  message (i.e. translation) and does not concern
  itself with how the message is transmitted
• Layer 1 is only concerned with transmitting the
  message and does not concern itself with the
  contents of the message

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• Advantages of Protocols
• A protocol between two layers should be detailed
  enough to allow anyone to develop hardware or
  software for that layer so that it correctly
  obeys each protocol
• Thus protocols allow interoperability between
  different types of computer equipment
• PC can communicate with a Macintosh
• Different manufacturers make different types of
  network cards (Layer 1) that can inter-operate
• Therefore users are not restricted to one
  particular manufacturer, allowing greater choice
  and increasing competition between the various
  manufacturers

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• Different protocols are designed to address
• Physical transmission
• Cable, fiber optic, radio frequency
• Encoding techniques
• How do you differentiate between the end of one
  message and the beginning of another?
• Network to Network Communication
• How do you route a message from one computer to
  another computer a number of hops away?
• Reliability
• Messages can get corrupted and lost during
  transmission

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ISO

• International Standards Organization Responsible
  for developing standards, over a wide range of
  technical areas
• ISO adopted the title OSI for the set of
  standards concerned with computer communications

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The goal of OSI

• Will remove two constraints from the end user
• By reducing the dependency on a single supplier
  (commercial)
• The users of any two computer systems may
• Exchange files
• Exchange electronic messages
• Log on to the others system
• Submit jobs to the others system

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Advantages

• Isolated systems and closed groups of
  internetworking systems will be opened to each
  other as OSI products become widely available
• A wide range of communication based services will
  be sustained amongst a global community of
  computer systems

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ISORM

• As a basis for the development of these
  standards, ISO developed a reference model
  (ISORM) to partition the problem into discrete
  layers
• ISORM provides a conceptual framework for
  understanding the complex processes involved in
  computer communication
• OSI model has seven layers

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Fundamental Principles of OSI

• A layer is created where a different layer of
  abstraction is needed
• Each layer performs a well defined function
• The function of each layer is chosen with an eye
  toward defining internationally standardized
  protocols

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Contd.

• The layer boundaries need to be chosen to
  minimize the information flow across the
  interfaces
• The number of layers need to be large enough that
  distinct functions need not be thrown together in
  the same layer out of necessity, and small enough
  that architecture does not become unwieldy

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Layer Cooperation

• When two computer systems, end systems are
  involved in OSI communication, they do by obeying
  the rules or Standards, associated with each
  layer of the ISORM
• The standards dictate how the functionality of
  layers can be achieved, by the cooperation of two
  implementations of a layer standard, one on each
  end-system

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Why such a Layering?

• Three main reasons
• Conceptual Simplicity
• Modularity of code (facilitates writing software
  for a layer independent of other layers)
• Packet processing well organized

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OSI 7-Layer Protocol Model

• Protocol
• Specifies the sequence of messages to be
  exchanged
• Specifies the format of data in messages

Message Sent
Message Received
Application
Presentation
Session
OSI Layered Protocol Model
Transport
Network
Data link
Physical
Communication Channel
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Features

• A fundamental rule of the ISORM is that a
  potential user of an OSI application can access
  OSI only by requests to an application entity
• No other layer can be accessed directly by a user
• The stack of entities on both of the cooperating
  endsystems can be regarded as a black-box with
  access only at the top
• The general rule of operation is that two peer
  entities of a particular layer (one on each
  end-system) cooperate to provide services to the
  next higher layer, and do so by the use of
  services offered by the next lower layer

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Protocol Layers

• Physical ? how to transmit bits
• Data link ? how to transmit frames
• Network ? how to route packets to the node
• Transport ? how to send packets to the
  application
• Session ? manage connections
• Presentation ? encode/decode msgs, security
• Application ? everything else!

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Physical Layer

• Physical is responsible for sending bits from one
  computer to another
• Is not concerned with the meaning of the bits
• Defines electrical details (what represents a 0
  or 1)
• Mechanical connections shape and number of
  connector

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Contd.

• What signals are sent on which pins
• Devices at the physical layer
• Simple Hubs (passive and active)
• Couplers , T connectors, terminators, cables,
  and cabling, repeaters
• Repeaters, multiplexers

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Contd.

• The following are addressed at the physical layer
• Network connections
• multipoint, point to point
• Physical topologies
• bus, star, or ring
• Analog / digital signaling
• Bit synchronization
• Baseband / Broadband
• Multiplexing

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Data Link Layer

• Provides for error free transfer of FRAMES over a
  single link from one device to another
• Link
• the circuit established between two adjacent
  nodes, with no intervening nodes
• Path
• a group of links that allows a message to move
  from origin to destination

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Links and Paths
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Contd.

• Adds Cyclic Redundancy Check (CRC) to detect
  damaged frames
• Adds control information
• frame type
• segmentation details
• Detects when a frame is lost and asks for
  retransmission

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Contd.

• Broadcast networks
• all devices on the LAN receive the data
  transmission
• Point to Point
• only the destination computer receives the
  message
• Uses physical address

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Contd.

• Split into two sub-layers
• Media Access Control (MAC)
• Controls how devices share the same media
• Logical Link Control (LLC)
• establishing and maintaining links between
  communicating devices
• synchronization
• flow control
• error checking

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Network Layer

• Makes routing decisions for devices that are
  farther than one link away
• Translates logical address into physical address
• Routers work at the network layer
• Example IP addressing

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Transport Layer

• Responsible for process to process (end to end)
  delivery of messages
• Breaks messages into segments
• Can be Connection-oriented or Connection-less
• Example TCP or UDP

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Session Layer

• Allows applications on different computers to
  share a connection
• Provides for checkpoints (if a connection is lost
  only the required info is resent)
• Dialog control who can transmit

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Presentation Layer

• Handles the format of the data
• Protocol conversion
• Data translation (ASCII)
• Compression
• Encryption

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Application Layer

• Provides services to support user applications
  such as
• FTP (file transfer)
• TELNET (remote login)
• SMTP (simple mail transfer protocol ) e-mail

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OSI 7-Layer Protocol Summary
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Internet 5-Layer ModelTCP/IP
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Encapsulation of Upper-layer Data
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Message Encapsulation TCP over an Ethernet
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The Programmer's Conceptual View
TCP Transmission control protocol UDP User
datagram protocol
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Reference Model TCP/IP
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TCP/IP Model Transport Layer
TCP (Transmission Control Protocol) ? reliable,
connection-oriented, error-free byte stream
delivering handles flow control UDP (User
Datagram Protocol) ? unreliable, connectionless
No TCPs flow control applications where prompt
delivery more important than accurate delivery
(speech, video, )
Allows peers on the source and destination hosts
to carry on a conversation. Protocols TCP and
UDP
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Physical Layer

• Function Provides a virtual link, called virtual
  bit pipe, for transmitting a sequence of bits
  between any pair of connected nodes.
• Physical interface module (modem) maps the
  incoming bits from the data link layer into
  signals appropriate for the channel, and at the
  receiving end, maps the signals back into bits.
• Interface module between the data link module and
  the modem (e.g., RS-232-C interface or X.21
  physical layer) is responsible for initiating
  data sending/receipt by providing separate wires
  between the two modules for control signals.

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Data Link Layer

• Function Converts the unreliable bit pipe
  provided by the physical layer into a virtual
  communication link for sending packets error
  free.
• The sending DL module places some control bits
  called header at the beginning of each packet and
  some more overhead bits called trailer at the end
  of each packet, resulting in a longer string of
  bits called a frame.
• Some of the overhead bits perform error
  detection/correction, and some request
  retransmissions when error occurs.
• For multiaccess links (e.g., Ethernet), there is
  a need for a new sublayer, called the MAC
  sublayer, to arbitrate the access to the link so
  that each node can successfully transmit its
  frames without interference from the other nodes.

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Network Layer

• The transport layer provides packets and control
  information (on how to handle the packets, e.g.,
  where the packets should be delivered) to the
  network layer. The network layer then uses the
  control information to generate the packet
  header.
• Function 1 Addressing
• Function 2 Routes packets from their sources
  through the network to their destinations
• Function 3 Deals with different types of
  networks, e.g., fragmentation and defragmentation

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Layer Entity

• An implementation of a layer standard must be
  capable of understanding and acting on messages
  exchanged with an implementation of the same
  layer standard on another cooperating end-system
• When such an implementation is invoked, it is
  known as Layer Entity

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Design Issues for the Layers

• Every layer needs a mechanism for identifying
  senders and receivers
• Any computer process needs to determine with whom
  it wants to talk out of dense network of
  computers

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Rules for Data Transfer

• Simplex Data travel in only one direction
• Half-duplex Data travel in either direction but
  not simultaneously
• Full-duplex Data travel in both directions
  simultaneously
• Networks need to determine how many logical
  channels the connection correspond to, and what
  are their priorities

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Error Control

• Physical communication circuits are not perfect
• Both ends of the connection must agree which of
  the error-detecting and error-correcting codes
  are to be used out of many codes
• The receiver must have some way of telling the
  sender which messages have been correctly
  received and which have not

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Preserving the Order of Messages

• To deal with a possible loss of sequencing
• There needs to be explicit provision for the
  receiver to put back pieces together properly
• An obvious solution is to number the pieces

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Synchronization of Speed

• How to keep a fast sender from swamping a slow
  receiver with data
• Solution
• Some kind of feedback from receiver to sender,
  either directly or indirectly, about the
  receivers current situation

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Interfaces and Services

• The function of each layer is to provide services
  to the layer above it
• The active elements in each layer are often
  called as entities
• An entity can be a software entity (such as a
  process) or a hardware entity such as an
  intelligent I/O chip
• Entities in the same layer on different machines
  are called peer entities

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Services

• The entities in layer n implement a service used
  by layer n1
• In this case, layer n is called service provider
  and layer n1 is called service user
• Services are available at SAP (service access
  points)
• The layer n SAPs are the places where layer n1
  can access the services offered

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SAP

• Each SAP has an address that uniquely identifies
  it
• To call someone you must know the callees SAP
  address
• To send a letter you must know the addressees
  SAP address

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Data Units

• At a typical interface, the layer n1 entity
  passes an IDU (Interface data unit) to the layer
  n entity through the SAP
• IDU Service Data Unit Control information
• SDU information passed across the network to
  the peer entity and then up to layer n1

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Contd.

• In order to transfer SDU, the layer n entity may
  have to fragment it into several pieces, each of
  which is given a header and sent as a separate
  PDU (Protocol Data Unit) also known as Packet
• PDU headers are used by the peer entities to
  carry out their peer protocols
• They identify which PDUs contain data and which
  contain control information

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Service Primitives (Operations)

• Request An entity wants service to do work
• Indication An entity is to be informed about an
  event
• Response An entity wants to respond to an event
• Confirm The response to an earlier request has
  come back

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Physical Layer

• Source
• George Varghese CS 423 Lectures 2-4 Physical
  Layers
• A. Tanenbaum, Computer Networks, 3rd edition
• Prof Ivo Jirasek CPSC 441 Networking course

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Introduction
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Contd.

• The Physical Layer performs bit by bit
  transmission of the frames given to it by the
  Data Link Layer
• The specifications of the Physical Layer include
• Mechanical and electrical interfaces
• Sockets and wires used to connect the host to the
  network
• Voltage levels uses (e.g. -5V and 5V)
• Encoding techniques (e.g. Manchester encoding)
• Modulation techniques used (e.g. square wave)
• The bit rate and the baud rate

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Anatomy
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Significance
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Definitions
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Sine waves
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Contd.
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Bandwidth Inter Symbol Interference
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NYQUIST Limit

• Suppose we know the bandwidth (H) of a channel
  and the number of signal levels (V) being used.
  What is the maximum number of bit we could
  transmit?
• Nyquists Limit says
• Max_bps 2Hlog(base 2)(V)
• For example, if the bandwidth is 3100Hz and we
  are using 16 level modulation then the maximum
  number of bits per second is max_bps 2 ?
  3100 ? log2(16) 24800 bps

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Baud Rate and Bit Rate
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Contd.

• The maximum number of times a signal can change
  in a second is called the baud rate.
• The number of bits (1s and 0s) transmitted in a
  second is called the bit rate.
• In the examples we have seen so far, the bit rate
  and the baud rate are the same. This is not
  always the case.
• The bit rate can be higher than the baud rate if
  we use more than two signal levels

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Wire Propagation Effects

• Propagation Effects
• Signal changes as it travels
• Receiver may not be able to recognize it

Original Signal
Final Signal
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Propagation Effects Attenuation

• Attenuation signal gets weaker as it propagates
• Attenuation becomes greater with distance
• May become too weak to recognize

Signal Strength
Distance
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Propagation Effects Distortion

• Distortion signal changes shape as it propagates
• Adjacent bits may overlap
• May make recognition impossible for receiver

Distance
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Propagation Effects Noise

• Noise thermal energy in wire adds to signal
• Noise floor is average noise energy
• Random signal, so spikes sometimes occur

Signal Strength
Noise
Noise Floor
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Time
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SNR

• Want a high Signal-to-Noise Ratio (SNR)
• Signal strength divided by average noise strength
• As SNR falls, errors increase

Signal Strength
Signal
SNR
Noise Floor
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Interference

• Interference energy from outside the wire
• Adds to signal, like noise
• Often intermittent, so hard to diagnose

Signal
Signal Strength
Interference
Time
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Termination

• Interference can occur at cable terminator
  (connector, plug)
• Often, multiple wires in a bundle
• Each radiates some of its signal
• Causes interference in nearby wires
• Especially bad at termination, where wires are
  unwound and parallel

Termination
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time
When noise effects an analogue signal, it is hard
to deduce the original signal.
The original digital signal can be deduced
despite the noise.
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Modulation

• Because attenuation is frequency dependent,
  modems use a sine wave carrier of a particular
  frequency, and then modulate that frequency

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Contd.

• Amplitude modulation Two different amplitudes of
  sine wave are used to represent 1's and 0's.
• Frequency modulation Two (or more) different
  frequencies, close to the carrier frequency, are
  used.
• Phase modulation The phase of the sine wave is
  changed by some fixed amount.

Binary Signal
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Contd.

• Transfer speeds
• Bit rate BPS
• Baud
• Bandwidth
• Compression
• appears to increases speed by decreasing the
  number of bits sent (usually some data does not
  compress well)
• sending and receiving modem must use same
  compression standard

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Transmission Modes
Parallel Mode
Serial Mode
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Contd.
Parallel Mode
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Modems

• Modulation demodulation
• Used to connect a digital computer to an Analog
  phone system
• Can be installed internally a card inserted into
  the motherboard
• Can be connected to the serial port (external
  modem)

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Modems Modern modems use combinations of
amplitude, frequency, and phase modulation to
achieve high data rates Modems can support auto
answer, auto dial, auto disconnect, and auto
redial

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Conversion Using Modem
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Interface Standards Many different groups
contribute to interface standards International
Telecommunications Union (ITU) (formerly
CCITT) Electronics Industries Association
(EIA) Institute for Electrical and Electronics
Engineers (IEEE) International Organization for
Standards (ISO) American National Standards
Institute (ANSI)

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Interface Standards All interface standards
consist of four components 1. The electrical
component 2. The mechanical component 3. The
functional component 4. The procedural component
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Interface Standards The electrical component
deals with voltages, line capacitance, and other
electrical characteristics The mechanical
component deals with items such as the connector
or plug description. A standard connector is the
ISO 2110 connector, also known as DB-25

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Interface Standards The functional component
describes the function of each pin or circuit
that is used in a particular interface The
procedural component describes how the particular
circuits are used to perform an operation

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Channel Types

• A channel is any conduit for sending information
  between devices
• There are three basic types of channel simplex,
  half-duplex and full-duplex
• A simplex channel is unidirectional, which means
  data can only be sent in one direction. For
  example, a TV channel only carries data from the
  transmitter to your TV set. Your TV set cannot
  send information back

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Contd.

• A half-duplex channel allows information to flow
  in either direction (but not simultaneously)
• Devices at either end of the channel must take it
  in turns to transmit information whilst the other
  listens. For example, a walky-talky either
  transmits or receives but not both at the same
  time

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Contd.

• A full-duplex channel allows data to be sent in
  both directions simultaneously
• A full-duplex channel can be formed from two
  simplex channels carrying data in opposite
  directions. This may make it more expensive than
  a half-duplex channel
• There is no waiting for turns or for the devices
  swap roles, as is the case with a half-duplex
  channel. This means full-duplex can be faster
  and more efficient

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Terminal-to-Mainframe Computer Connections A
point-to-point connection is a direct, unshared
connection between a terminal and a mainframe
computer A multipoint connection is a shared
connection between multiple terminals and a
mainframe computer The mainframe is called the
primary, and each terminal is called a secondary

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Terminal Mainframe Connections


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Terminal-to-Mainframe Computer Connections To
allow a terminal to transmit data to a mainframe,
the mainframe must poll the terminal Two basic
forms of polling include roll-call polling and
hub polling In roll-call polling, the mainframe
polls each terminal in a round-robin fashion In
hub polling, the mainframe polls the first
terminal, and this terminal passes the poll onto
the next terminal

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OSI Model Data Link Layer

• Source
• A. Tanenbaum, Computer Networks, 3rd edition
• Prof Ivo Jirasek CPSC 441 Networking course
• Shivkumar Kalyanaraman, Biplab Sikdar, ECSE-4730
  Computer Communication Networks (CCN), RPI
• CS 851 Seminar, University of Virginia

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OSI
Application
Presentation
LOGICAL LINK sublayer
Session
Transport
Framing Error control Flow control
Network
Data Link
Physical
MEDIA ACCESS sublayer
Transmission/reception of frames
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Link Layer Setting the Context
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Contd.

• two physically connected devices
• host-router, router-router, host-host
• unit of data frame

M
adapter card
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Link Layer Services - 1

• Framing, link access
• encapsulate datagram into frame, adding header,
  trailer
• implement channel access if shared medium,
• physical addresses used in frame headers to
  identify source, destination

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Link Layer Services - 2

• Flow Control
• pacing between sender and receivers
• Error Detection
• errors caused by signal attenuation, noise.
• receiver detects presence of errors
• signals sender for retransmission or drops frame
• Error Correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission

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Error Correction

• Includes enough redundant information along with
  each block of data sent to enable receiver to
  deduce what the transmitted character must have
  been
• Codeword (n) message (m) redundant bits (r)

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Hamming Distance

• Consider 10001001 Sender sent it
• 10110001 Receiver received
  it
• ____________________
• 3 bits differ
• The number of bit positions in which two
  code-words differ is called Hamming Distance

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Contd.

• If two Code-words are a Hamming distance d
  apart, it will require d single bit errors to
  convert one into another

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Error Detecting Codes

• Polynomial Code (Cyclic Redundancy Check) method
• Treats bit stream as polynomials
• Example 110001 1 x5
• 1 x4
• 1 x0

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Logical Gates

• AND 0 0 0, 0 1 0, 1 1 1
• OR 0 0 0, 0 1 1, 1 1 1
• XOR 0 xor 0 0, 0 xor 1 1, 1 xor 1 0

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Generator Polynomial

• When the polynomial code method is employed,
  sender and receiver AGREE upon certain Generator
  Polynomial, in ADVANCE

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Checksumming Cyclic Redundancy Check

• View data bits, D, as a binary number
• Choose r1 bit pattern (generator), G
• Goal choose r CRC bits, R, such that
• ltD,Rgt exactly divisible by G (modulo 2)
• receiver knows G, divides ltD,Rgt by G. If
  non-zero remainder error detected!
• can detect all burst errors less than r1 bits
• Widely used in practice (ATM, HDCL)

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Multiple Access protocols - 1

• single shared communication channel
• two or more simultaneous transmissions by nodes
  interference
• only one node can send successfully at a time
• Multiple access protocol
• distributed algorithm that determines how
  stations share channel, i.e., determine when
  station can transmit

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Multiple Access protocols - 2

• Multiple access protocol (cont.)
• communication about channel sharing must use
  channel itself!
• What to look for in multiple access protocols
• synchronous or asynchronous
• information needed about other stations
• robustness (e.g., to channel errors)
• performance

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MAC Protocols A Taxonomy

• Three broad classes
• Channel Partitioning
• divide channel into smaller pieces (time slots,
  frequency)
• allocate piece to node for exclusive use
• Random Access
• allow collisions
• recover from collisions
• Taking turns
• tightly coordinate shared access to avoid
  collisions

Goal efficient, fair, simple, decentralized
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Channel Allocation Problem

• How to allocate a single broadcast channel among
  competing users?
• Static
• FDM /TDM (Frequency/Time Division Multiplexing)
• FDM Radio/TV broadcasts
• TDM POTS (Plain Old Telephone System)
• Dynamic
• Pure/ Slotted ALOHA
• Carrier Sense Multiple Access (CSMA) Protocols
• Collision free protocols

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Performance of Fixed Assignment Protocols - 1

• Fixed assignment protocols are ideal for
  continuous streams such as video or audio
• For packet switched data
• A perfect multiple access scheme would always
  use the channel when there are packets waiting
  (static multiplexing)
• If ? is the arrival rate and ? is the service
  rate then delay/station

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Performance of Fixed Assignment Protocols - 2

• Fixed assignment protocols divide the channel
  into N separate independent, ?/N identical
  subchannels
• If each user has arrival rate ?/N, each
  user/subchannel pair helps to calculate mean
  delay

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Contd.

• And the mean delay for a packet is
• So, if we use fixed assignment protocols for
  packet switched data, mean delay goes up by a
  factor of N!!

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Performance of Fixed Assignment Protocols - 3

• This analysis is only appropriate for TDMA due to
  the discrete-time (slotted) nature of TDMA but
  the rough factor of N still holds
• Fixed assignment protocols are not appropriate
  for multiple access in a packet switched network
  with a large number of users
• Packet arrivals are fairly random, so there will
  be many times when packets are waiting at one
  user while other users are idle
• The idle resources (time slots or bandwidth or
  both are wasted in this case)

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Dynamic Channel Allocation Protocols

• Pure ALOHA
• Slotted ALOHA
• CSMA
• CSMA/CD (old ETHERNET)
• Switching (Fast ETHERNET)
• Token passing (Token Ring )

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Random Access Protocols - 1

• When node has packet to send
• transmit at full channel data rate R.
• no a priori coordination among nodes
• Two or more transmitting nodes -gt collision,
• Random access MAC protocol specifies
• how to detect collisions
• how to recover from collisions (e.g., via delayed
  retransmissions)

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Random Access Protocols - 2

• Examples of random access MAC protocols
• ALOHA
• slotted ALOHA
• CSMA and CSMA/CD

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ALOHA

• Back in 1970, the University of Hawaii built a
  network out of radios that broadcast signals.
• Basic Idea
• Anyone may transmit whenever they want.
  (Continuous time model.)
• Each radio detects collisions by listening to its
  own signal. A collision is detected when a sender
  doesn't receive the signal that it just sent.
• After a collision, wait a random amount of time
  and transmit the same frame again. This technique
  is known as backoff.

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Pure (unslotted) ALOHA - 1

• Unslotted Aloha simpler, no synchronization
• pkt needs transmission
• send without awaiting for beginning of slot
• Collision probability increases
• pkt sent at t0 collide with other pkts sent in
  t0-1, t01

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In Pure Aloha, frames are transmitted in
completely arbitrary fashion
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Slotted Aloha

• Time is divided into equal size slots ( pkt
  trans. time)
• Node with new arriving pkt transmit at beginning
  of next slot
• If collision retransmit pkt in future slots with
  probability p, until successful.

Success (S), Collision (C), Empty (E) slots
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Contd.

• Time is divided into slots can only transmit at
  start of slot
• Requires sync of clocks
• Still poor at hi-loads

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Performance Comparison

• S throughput goodput
• (success rate)

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Data Link Layer Protocols

• Source
• A. Tanenbaum, Computer Networks, 3rd edition
• Prof Ivo Jirasek CPSC 441 Networking course
• Shivkumar Kalyanaraman, Biplab Sikdar, ECSE-4730
  Computer Communication Networks (CCN), RPI
• CS 851 Seminar, University of Virginia

129
CSMA

• We can improve the performance of our simple
  network greatly if we introduce carrier sensing
  (CS). With carrier sensing, each host listens to
  the data being transmitted over the cable
• A host will only transmit its own frames when it
  cannot hear any data being transmitted by other
  hosts
• When a frame finishes, an interframe gap of about
  9.6?sec is allowed to pass before another host
  starts transmitting its frame

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Carrier Sense Multiple Access (CSMA) - 1

• In some shorter distance networks, it is possible
  to listen to the channel before transmitting
• In radio networks, this is called sensing the
  carrier
• The CSMA protocol works just like Aloha except
  If the channel is sensed busy, then the user
  waits to transmit its packet, and a collision is
  avoided
• This really improves the performance in short
  distance networks!

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Contd.

• Improves performance when higher medium
  utilization
• When a node has data to transmit, the node first
  listens to the cable (using a transceiver) to see
  if a carrier (signal) is being transmitted by
  another node.

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Carrier Sense Multiple Access (CSMA) - 2

• How long does a blocked user wait before trying
  again to transmit its packet?
• Three basic variants
• 1-persistent Blocked user continuously senses
  channel until its idle, then transmits
• 0-persistent Blocked user waits a randomly
  chosen amount of time before sensing channel again

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Carrier Sense Multiple Access (CSMA) - 3

• P-persistent Let T end-to-end propagation
  delay
• If channel is idle then transmit packet
• If channel busy then toss coin with P(heads)
  P
• Heads Transmit at first idle
• Tails wait until first idle plus T, sense,
  repeat
• Human analogy Dont interrupt others

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Persistent and Non Persistent CSMA
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CSMA collisions
Collisions can occur propagation delay means two
nodes may not hear each others transmission
Collision entire packet transmission time wasted
Note role of distance and propagation delay in
determining collision probability
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CSMA/CD (Collision Detection)

• CSMA improves performance, but still it wastes
  the channel during collisions
• In some very short distance networks (e.g. coax
  LANs), it is possible to listen while
  transmitting (in addition to listening before
  transmitting)
• If we detect a collision while transmitting, we
  can abort the transmission and free up the
  channel sooner
• This idea was proposed by R. Metcalfe and Boggs
  at Xerox PARC in the mid 1970s under the name
  Ethernet
• Human analogy the polite conversationalist

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Taking Turns MAC protocols - 1

• 1. Channel partitioning MAC protocols
• share channel efficiently at high load
• inefficient at low load delay in channel access,
  1/N bandwidth allocated
• 2. Random access MAC protocols
• efficient at low load single node can fully
  utilize channel
• high load collision overhead
• 3. Taking turns protocols
• look for best of both worlds!

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Taking Turns MAC protocols - 2

• Token Passing
• Control token passed from one node to next
  sequentially.
• Token message
• Concerns
• token overhead
• latency
• single point of failure (token)

• Polling
• Master node invites slave nodes to transmit in
  turn
• Request to Send, Clear to Send messages
• Concerns
• polling overhead
• latency
• single point of failure (master)

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Reservation-based protocols - 1

• Distributed Polling
• Time divided into slots
• Begins with N short reservation slots
• reservation slot time equal to channel end-end
  propagation delay
• station with message to send posts reservation
• reservation seen by all stations

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Reservation-based protocols - 2

• After reservation slots, message transmissions
  ordered by known priority

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141

• MAC Protocols
• in
• WIRELESS MEDIUM

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Why Need MAC in Wireless also?

• Wireless medium is an open, shared, and broadcast
  medium
• Multiple nodes may access the medium at the same
  time
• Medium Access Control Protocol
• Define rules to force distributed nodes to access
  wireless medium in an orderly and efficiently
  manner

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Classification of Wireless MAC Protocols
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Distributed MAC Protocols

• Collision avoidance mechanisms
• Collision avoidance with out-of-band signaling
• Collision avoidance with in-band control messages
• Two distributed random access protocols
• DFWMAC Distributed Foundation Wireless MAC (used
  in IEEE 802.11)
• EY-NPMA Elimination Yield-Nonpreemptive Priority
  Multiple Access (used in HyperLan)

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Centralized MAC Protocols

• Work for centralized wireless networks
• Base station has explicit control for who and
  when to access the medium
• All nodes can hear from and talk to base station
• All communications go through the base station
• The arbitration and complexity are in base
  station

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Features of Ideal MAC Protocol

• Limited Delay
• High Throughput
• Fairness
• Stability
• Scalability
• Robustness against channel fading
• Low power consumption
• Support for multimedia

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Wireless MAC Issues

• Location Dependent Carrier Sensing
• Hidden Terminal
• Exposed Terminal
• Capture Effect

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Hidden Terminal Problem

• Node B can communicate with A and C both
• A and C cannot hear each other
• When A transmits to B, C cannot detect the
  transmission using the carrier sense mechanism
• If C transmits to B, collision will occur at B

B
C
A
D
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Exposed Terminal Problem

• Node C can communicate with B and D both
• Node B can communicate with A and C
• Node A cannot hear D
• Node D can not hear B
• When C transmits to D, B detect the transmission
  using the carrier sense mechanism and postpone to
  transmit to A, even though such transmission will
  not cause collision

B
C
A
D
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Capture Effect
Power Difference Of A and D signals
D
B
A
C

• A and D transmit simultaneously to B, the signal
  strength received by B from D is much higher than
  that from A, and Ds transmission can be decoded
  without errors. This will result unfair sharing
  of bandwidth.

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151
MACA A New Channel Access Method for Packet
RadioPhil Karn 1990
152
Goals , New Ideas, and Main Contributions

• Goals
• Try to overcome hidden exposed terminal
  problems
• New idea
• Reserve the channel before sending data packet
• Minimize the cost of collision (control packet is
  much smaller than data packet)
• Main Contribution
• A three-way handshake MAC protocol MACA

CSMA/CA
MA/CA
MACA
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Fundamental Assumptions

• Symmetry
• A can hear from B ? B can hear from A
• No capture
• No channel fading
• Packet error only due to collision
• Data packets and control packets are transmitted
  in the same channel

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Three-Way Handshake

• A sends Ready-to-Send (RTS)
• B responds with Clear-to-Send (CTS)
• RTS and CTS announce the duration of the data
  transfer
• A sends DATA PACKET
• Nodes overhearing RTS keep quiet for some time to
  allow A to receive CTS
• Nodes overhearing CTS keep quiet for some time to
  allow B to receive data packet

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Contd.
D
B
A
C
E
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More Details for MACA

• A sends out RTS and set a timer and waits for CTS
• If A receives CTS before timer go to zero, OK!
  sends data packet
• Otherwise, A assumes there is a collision at B
• Double the backoff counter interval
• Randomly pick up a timer from 1,backoff counter
• Send next RTS after timer go to zero
• B sends out CTS, then set a timer and waits for
  data packet
• If data packet arrives before timer go to zero,
  OK!
• Otherwise, B can do other things
• C overhears As RTS, set a timer which is long
  enough to allow A to receive CTS. After the timer
  goes to zero, C can do other things
• D overhears Bs CTS, set a timer which is long
  enough to allow B to receive data packet.
• E overhears As RTS and Bs CTS, set a timer
  which is long enough to allow B to receive data
  packet.
• RTS and CTS can also contain info to allow sender
  A to adjust power to reduce interference
• Note no carrier sense

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Hidden Terminal Problem Still Exists (1)

• Data packet still might suffer collision

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Hidden Terminal Problem Still Exists (2)

• Data packet still might suffer collision

E
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Exposed Terminal Problem Still Exists

• Node C can not receive CTS

D
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Summary

• MACA did not solve hidden exposed terminal
  problems
• MACA did not provide specifications about
  parameters
• What are RTS, CTS packet sizes ?
• How to decide timers?
• What is initial backoff window size?
• A lot things need to do if using MACA

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MACAW A Media Access Protocol for Wireless
LANsV. Bharghavan, A. Demers, S. Shenker, and
L. Zhang (Sigcomm 1994)
162
Goals, New Ideas, and Main Contributions

• Goals
• This paper refined and extended MACA
• New Idea Information sharing to achieve fairness
• Main Results
• Modified control messages
• Four-way handshake (reliable, recover at MAC
  layer)
• Five-way handshake (relieve exposed terminal
  problem)
• RRTS (unfairness)
• Modified back-off algorithms
• Multiplicative Increase and Linear Decrease
  (MILD)
• Synchronize back-off counter using piggyback
  message
• Multiple stream model (V-MAC)

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Four-Way Handshake

• Sender sends Ready-to-Send (RTS)
• Receiver responds with Clear-to-Send (CTS)
• Sender sends DATA PACKET
• Receiver acknowledges with ACK
• RTS and CTS announce the duration of the transfer
• Nodes overhearing RTS/CTS keep quiet for that
  duration
• Sender will retransmit RTS if no ACK is received
• If ACK is sent out, but not received by sender,
  after receiving new RTS, receiver returns ACK
  instead of CTS for new RTS

destination
source
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Comparison of with ACK without ACK
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Revisit Exposed Terminal Problem

• RTS/CTS/DATA/ACK cannot solve exposed terminal
  problem
• When overhearing RTS, the node needs to wait
  longer enough to allow the data packet being
  completely transmitted even it does not overhear
  CTS
• To relieve exposed terminal problem,
• Let exposed terminal know the DATA packet needs
  to be transmitted
• Extra message DS (data send)
• Five Handshaking to let exposed terminal know how
  long it should wait

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Five-Way Handshake

• Sender sends Ready-to-Send (RTS)
• Receiver responds with Clear-to-Send (CTS)
• Sender sends DATA SENDING (DS)
• Sender sends DATA PACKET
• Receiver acknowledge with ACK
• RTS and CTS announce the duration of the transfer
• Nodes overhearing RTS/CTS keep quiet for that
  duration

B
A
C
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Unfairness

• Using RTS/CTS/DATA/ACK or RTS/CTS/DS/DATA/ACK
  might cause unfairness
• A sends data to B D sends data to C
• A and D have enough data to send
• C can hear from B and D, but