Computer Networks 7

Framing problem

ensures receiver is able to extract the same message from the signal as the transmitter sent

Efficient encoding practices

minimize number of transmitted bits, redundant information; improves bandwidth utilization, storage requirement

Presentation Format

agreed format of message exchanged between applications

Data Compression

improves encoding efficiency by reducing redundancy in transmitted data

Lossless Compression

original data can be perfectly reconstructed

Lossy Compression

some information is discarded to achieve higher compression ratios

Presentation formatting

transformation of network data from the representation used by the application program into a form that is suitable for transmission over a network

Marshalling

process of transferring and formatting a collection of data structures into an external data representation type appropriate for transmission in a message

Unmarshalling

process of reformatting the transferred data upon arrival to recreate the original data structures at the destination

Elements of arguemnt marshalling system

Defines data types which can be transferred, conversion strategy, add tags and stubds

Tags

any additional information included in a message that helps the receiver decode the message

Stubs

piece of code that impements argument marhalling

Canonical Intermediate Form conversion strategy

sending host translates from its internal representation to the external representation before sending data and the receiver translates from this external representation into its local representation when receiving data

Receiver Makes Right conversion strategy

sender transmit data in its own internal format, receiver is responsible for translating the data from the sender’s format into its own local format

Run Length Encoding (RLE) compression technique

replace consecutive occurrences of a given symbol with only one copy of the symbol and add counter of how many times that symbol occurs, e. g: AAABBCDDDD → 3A2B1C4D

Differential Plus Code Modulation compression technique

first output a reference symbol and then, for each symbol in the data, to output the difference between that symbol and the reference symbol(s), e. g: AAABBCDDDD → A0001123333

Dicitonary Based Module compression technique

build a dictionary (table) of strings that you expect to find in the data and then to replace each of these strings when it appears in the data with the corresponding index to the dictionary

Video Compression (MPEG)

MPEG takes a sequence of video frames as input and compresses them into three types of frames: I, P and B frames

I frame (intrapicture)

JPEG compressed version of the corresponding frame in the video source

P frame (predicted picture)

specifies the differences from the previous I frame

B frames (bidirectional predicted picture)

gives an interpolation between the previous and subsequent I or P frames