Local

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Local Metropolitan Area Networks

• ACOE322
• Lecture 7
• Upper OSI layers

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Upper OSI layers
7 Application 6 Presentation 5 Session 4 Transport
3 Network 2 Data Link 1 Physical
Upper OSI (Host layers) Provide accurate data
delivery between computers
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Transport Layer

• It is responsible for source-to-destination
  (end-to-end) delivery of the entire message.
• Whereas the network layer oversees end-to-end
  delivery of individual packets, it does not
  recognize any relationship between those packets.
  
• Ensures that the whole message arrives intact and
  in order, overseeing both error control and flow
  control at the source-to-destination level.

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Transport Layer (Cont.)
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The responsibilities of Transport Layer

• Service-point addressing
• Source-to-destination delivery means delivery not
  only from one computer to the next but also from
  a specific process(running program) on one
  computer to a specific process(running program)
  on the other.
• The transport layer header must include a type of
  address called a service-point address (or port
  address).
• The network layer gets each packet to the correct
  computer the transport layer gets the entire
  message to the correct process on that computer.

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The responsibilities of Transport Layer (cont.)

• Segmentation and reassembly
• A message is divided into transmittable segments,
  each segment containing a sequence number.
• These numbers enable the transport layer to
  reassemble the message correctly upon arriving at
  the destination and to identify and replace
  packets that were lost in the transmission.
• Flow control
• The transport layer is responsible for flow
  control.
• It is performed end-to-end rather than across a
  single link.

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The responsibilities of Transport Layer (cont.)

• Connection control
• Can be either connectionless or
  connection-oriented.
• A connectionless transport layer treats each
  segment as an independent packet and delivers it
  to the transport layer at the destination
  machine.
• A connection-oriented transport layer makes a
  connection with the transport layer at the
  destination machine first before delivering the
  packets.
• After all the data is transferred, the connection
  is terminated.
• Error control
• It is performed end-to-end rather than across a
  single link.

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User Layers

• The session, presentation and application layers
  are known as user layers
• Mainly implemented by software
• In most protocols like TCP/IP and Novell, these
  layers are implemented by a single layer called
  application layer

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Session Layer

• It is responsible for establishing, maintaining,
  and synchronising dialogs between communication
  upper layers
• Also helps to handle upper level problems such as
  inadequate disk space or out of paper for the
  printer
• Although it is considered as user layer, but it
  is often implemented within the operating system

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Session layer
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Services of the Session Layer

• Coordinate connection and disconnection of
  dialogs between application
• Provide synchronisation points for data exchange
• Coordinate who sends first and when
• Ensure that the data exchange is complete before
  the session closes

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Session and Transport Interaction

• The transport layer can make an abrupt
  disconnection while session layer has an
  obligation to the user and cannot disconnect
  until the session can be brought to a conclusion
• The communication with the transport layer can be
  of 3 types
• One-to-one there is one session layer connection
  for each transport layer connection
• Many-to-one multiple session layer connections
  share the services of one transport layer
  connection
• One-to-many one session layer connection needs
  many transport layer connections to handle the
  task

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Synchronization points

• Mechanism for recovering data that have been
  delivered but mishandled
• Reference points are introduced into the data to
  control flow of information and allow recovery
  from software or operator errors
• These reference points may call for user
  acknowledgment or just may provide a go-back
  facility for data recovery

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Two types of synchronization points

• Major synchronization points
• Divide an exchange into a series of dialogs
• Each point must be acknowledged before the
  session can continue
• If an error occurs, data can be recovered only up
  to the last major point
• A session layer activity can be a single dialog
  or several dialogs separated by major
  synchronization points

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Two types of synchronization points

• Minor synchronization points
• Are inserted into the middle of dialogs and may
  or may not require confirmation
• If an error occurs, the control can go back one
  or more minor synchronization points within a
  dialog to recover the data

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Presentation Layer

• Functions performed includes
• Translation
• Encryption/decryption
• Authentication
• Compression

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Presentation Layer
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Translation

• The internal representation of a piece of
  information might vary enormously from one
  machine to the other (e.g. one may be using ASCII
  and the other using EBCDIC)
• Therefore translation is require so that the two
  machines can communicate
• Two methods direct or indirect

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Direct and Indirect Translation

• Direct translation performs the translation at
  the receiver
• Indirect translation performs the translation at
  the sender and at the receiver
• The direct method is not acceptable in most
  cases, because if a computer is communicating
  with several other computers, it may need several
  conversion tables
• The indirect is recommended by OSI and the
  recommended model is called abstract syntax
  notation 1 (ASN.1)
• ASN.1 not only takes care of translation but also
  handles other formatting problems such as the
  diverse nature of data (text, program) and the
  diversity in data storage (store data in
  different format)

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Encryption/Decryption

• To assure privacy for transmitting sensitive
  information
• Microwave, satellite and other wireless media
  cannot be protected from unauthorised reception
  of the transmission, cable transmission cannot
  totally avoid that either.
• To alter the information before transmitting so
  that only an authorised receiver can understand
  it
• Encryption means that the sender transforms the
  original information to another form and sends
  the resulting message out over the network
• Decryption reverses the encryption process in
  order to transform the message back to its
  original form

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Encryption/Decryption Methods

• Conventional Methods the encryption key (Ke) and
  the decryption key (Kd) are the same and secret
• Public Key Methods every user has the same
  encryption algorithm and key, however, the
  decryption algorithm and key are kept secret

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Conventional Method

• Can be divided into two categories
• Character-level encryption
• Bit-level encryption
• There are two methods in character-level
  encryption
• Substitutional
• Transpositional

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Substitutional Character-level Encryption

• The simplest form of ciphering
• In mono-alphabetic substitution, also known as
  Caesar Cipher, each character is replaced by
  another character in the set
• The mono-alphabetic encryption algorithm simply
  adds a number to the ASCII code of the character
  and the decryption algorithm simply subtracts the
  same number
• Mono-alphabetic substitution is very simple and
  can be broken easily

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Substitutional Character-level Encryption

• In poly-alphabetic substitution, each occurrence
  of a character can have different substitute
• One poly-alphabetic encryption technique is to
  find the position of the character in the text
  and use that value as the key
• It is not very secure as well, even though the
  words replaced by different characters but their
  position is still the same the code can easily
  be broken by someone with more experience

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Transpositional Character-level Encryption

• Transpositional encryption is a more secure
  method in which the characters retain their
  plaintext form but change their positions to
  create the ciphertext
• The text is organised into a 2-dimensional table
  and the columns are interchanged according to a
  key
• The key defines which columns should be swapped
• Again this encryption is not very secure either,
  one can still break it through trial and error

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Bit-level Encryption

• Data as text, graphics, audio, or video are first
  divided into blocks of bits, then altered by
• encoding/decoding
• permutation
• exclusive OR
• rotation
• others

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Public Key Encryption

• In conventional method, the decryption algorithm
  is always the inverse of the encryption algorithm
  and uses the same key
• Anyone who knows the encryption algorithm and key
  can deduce the decryption algorithm
• Security can only be assured only if the entire
  process is kept secret
• In public key, anyone can encrypt information but
  only an authorised receiver can decrypt it
• The decryption algorithm is designed in such a
  way that it is not the inverse of the encryption
  algorithm
• Figure 23.21 shows the idea every customer can
  use them but the decryption algorithm and key are
  kept secret and used only by the bank

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Authentication

• Means verifying the identity of a sender
• There are many methods, only digital signature
  will be discuss here
• Digital signature is based on public key
  encryption/decryption
• See Figure 23.25 to get an idea of how digital
  signature works.
• If the customer claims never have made such a
  transaction, the bank can take C1 out of its file
  and apply Kp-2 (public key) to it to show that it
  creates P. This decryption is not possible unless
  the customer had originally applied Ks-1 (secret
  key) to P to create C1

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Compression

• Reduces the number of bits sent
• Becomes important when data that are not pure
  text such as audio and video are send
• Can be divided into two broad categories
• lossless
• lossy

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Lossless Compression

• The compressing and decompressing algorithms are
  usually the inverse of each other
• After decompressing, we will get the exact data
  as they were before compressing
• Some of the techniques used in lossless
  compression
• Run-length encoding when data contain strings of
  repeated symbols, the strings can be replaced by
  a special marker, followed by the repeated
  symbol, followed by the number of occurrences
  (see Figure 23.27)

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Lossless Compression

• Statistical compression uses short codes for
  frequent symbols and long codes for infrequent
  symbols
• Relative compression when there is little
  difference between consecutive frames, only the
  difference between the consecutive frames are
  send instead of the entire frame

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Lossy Compression

• The decompressed information does not produce
  exactly the same information as the original but
  something very close
• In video transmission, if an image does not have
  sharp discontinuities, after transformation to a
  mathematical expression, most of the information
  is contained in the first few terms
• Using the terms sent, it is possible to reproduce
  the original frame with enough accuracy
• Some methods are
• Joint photographic experts group (JPEG) for
  pictures and graphics
• Motion picture experts group (MPEG) for video

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Application Layer

• Contains whatever functions are required by the
  user
• No standardisation in general is possible
• However, ITU-T has recognised some common
  applications for which standardisation is
  possible. Two of them are
• Message Handling System (MHS)
• File Transfer, Access, and Management (FTAM)

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Application Layer
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Message Handling System (MHS)

• Is the OSI protocol that underlies electronic
  mail and store-and-forward handling
• Is the system used to send any message that can
  be delivered in a store-and-forward manner
• Store-and-forward instead of opening an active
  channel between the sender and receiver, the
  protocol provides a delivery service that
  forwards the message when a link becomes
  available

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MHS structure

• Each user communicate with a program or process
  called a user agent (UA) which is unique to each
  user
• Each user has message storage (MS), which
  consists of disk space in a mail storage system
  and is usually referred to as a mailbox
• The message communicates with a series of
  processes called message transfer agents (MTAs)
• The combined MTAs make up the message transfer
  system (MTS)

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File Transfer, Access, and Management (FTAM)

• It is used to
• transfer (copy)
• access (read, write, or modify)
• manage (control) files
• To allow the interaction of different file
  systems, it uses the concept of virtual files and
  virtual filestores
• A virtual filestore is a non implementation-specif
  ic model for files and databases that can be used
  as an intermediary for file transfer, access, and
  management
• Each transaction requires an initiator and
  responder

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W. Stalling, Local and Metropolitan Area
Networks, 6th edition, Prentice Hall, 2000
References

• B.A. Forouzan, Data Communications and
  Networking, 3rd edition, McGraw-Hill, 2004

• W. Stallings, Data and Computer Communications,
  7th edition, Prentice Hall, 2004

• F. Halsall, Data Communications, Computer
  Networks and Open Systems, 4th edition, Addison
  Wesley, 1995