Reduces the number of bits contained in
Sometimes programs need to send more
data in a timely fashion than the bandwidth
of the network supports.
Need to compress the data at the sender
and decompress it at the receiver.
In terms of storage, the capacity of a
storage device can be effectively increased
with methods that compresses a body of
The bandwidth of a digital communication
link can be effectively increased by
compressing data at the sending end and
decompressing data at the receiving end.
Lossless Compression – data is
compressed and can be uncompressed
without loss of information. These are
referred to as bit-preserving or reversible
Lossy Compression – aim to obtain the
best possible fidelity for a given bit-rate or
minimizing the bit-rate to achieve a given
fidelity measure. Most suited to video and
audio compression techniques
- Image quality is not reduced.
Use in: artificial images that contain sharp-
edged lines such as technical drawings,
textual graphics, comics, maps or logos.
- reduces image quality. Cannot get the
original image back & lose some
Use in: natural images such as photos of
Lossless - allows one to preserve an exact
copy of one's audio files
Usage: For archival purposes, editing,
Lossy - irreversible changes , achieves far
greater compression, use psychoacoustics
to recognize that not all data in an audio
stream can be perceived by the human
Usage: distribution of streaming audio, or
• Start by encoding the first frame using a
still image compression method.
• It should then encode each successive
frame by identifying the differences
between the frame and its predecessor,
and encoding these differences. If the
frame is very different from its
predecessor it should be coded
independently of any other frame.
In the video compression literature, a
frame that is coded using its predecessor
is called inter frame (or just inter), while a
frame that is coded independently is called
intra frame (or just intra).
• To carry sensitive information, a system
must be able to assure privacy.
• One way to safeguard data from attacks
is encrypting the data.
• Encryption – sender transform original
information (plaintext) to another form
(ciphertext) by a function that is
parameterized by a key.
• Decryption – reverses the original
process to transform the message
(ciphertext) back to its original form
Symmetric Keys – use same key to
encrypt and decrypt a message.
Asymmetric Keys -2 keys are needed
(public key and private key); 1 key to
encrypt, another key to decrypt and vice
Confidentiality Allow only authorized users to access
Authentication Verify who the sender was and trust
the sender is who they claim to be.
Integrity Trust the information has not been
No repudiation Ensure that the sender or receiver
cannot deny that a message was sent
Access Control Restrict availability to information.