|
Data
Communications
|
Upper
OSI Layers
- The
session layer, the presentation layer, and the application
layer are considered user layers.
- Session
layer
- Establishes,
maintains, and synchronizes dialogs between communicating
upper layers.
- Coordinates
connection and disconnection of dialogs between applications.
- Provides
synchronization points for data exchange.
- Coordinates
who sends when
- Ensures
that the data exchange is complete before the session
closes.
- Presentation
layer functions
- Translation
-- direct or abstract syntax notation 1 (ASN.1)
- Encryption/Decryption
-- sender transforms the original information to another
form and sends the resulting unintelligible message out
over the network. Decryption reverses the encryption process
in order to transform the message back to its original
form.
- Sender
uses an encryption algorithm and a key to transform
the plaintext into a ciphertext.
- In
conventional encryption, the encryption key
(Ke) and the decryption key (Kd)
are the same and secret.
- Character-level
encryption can be substitutional and transpositional.
- Bit-level
techniques, data as text, graphics, audio, or
video are divided tino blocks of bits, then altered
by encoding/decoding, permutation, substitution,
exclusive OR, and rotation.
- An
example of bit-level encryption is the data
encryption standard (DES) which encrypts a
64-bit plaintext using a 56-bit key.
- Public
key encryption is where every user has the same
encryption algorithm and key. The decryption algorithm
and key are kept secret.
- Authentication
means verifying the identity of a sender. Digital signatures
are based on public key encryption/decryption.
- Data
compression reduces the number of bits sent.
- Lossless
compression is where the compressing and decompressing
algorithms are usually the inverse of each other.
- Run-length
encoding
- Statistical
compression -- morse code, Huffman encoding,
and Lempel-Ziv-Welch encoding.
- Relative
compression
- Lossy
compression -- Joint photographic experts group
(JPEG) and Motion picture experts group (MPEG).
- Application
layer contains whatever functions required by the user.
- Message
handling system (MHS) is the OSI protocol that underlies
electronic mail and store-and-forward handling. It is
derived from the ITU-T X.400 series. MHS is the system
used to send any message that can be delivered in a store-and-forward
manner. The MHS standard defines the format of a message.
- The
file transfer, access, and management (FTAM) protocol
is used to transfer, access, and manage (control) files.
A virtual filestore is a nonimplementation-specific
model for files and databases that can be used as an intermediary
for file transfer, access, and management.
- A
virtual terminal is an imaginary terminal (a software
model of a terminal) with a set of standard characteristics
that every host understands.
- The
OSI directory service (DS) is designed according
to the ITU-T X.500 standard. A directory is a global source
of information about many different objects. It is a distributed
database with each host holding a part of the data. The
information contained in the directory is called the directory
information base (DIB). Users gain access to the DS
through directory user agents (DUA). The DUA communicates
with one or more entities called directory system agents
(DSAs) contained within the directory system itself.
- Common
management information services (CMIS) use the common
management information protocol (CMIP). All CMIP management
occurs by monitoring and manipulating communication between
OSI entities called managed objects. A managed
object is a network resource such as a workstation, a
switch, routing hardware or software, queuing programs,
etc. The specific services provided by CMIS are called
common management information service elements (CMISEs).
These service elements fall into three categories: management
association services, management notification services,
and management operation services.
|
|
