Difference between pages "TDMA" and "Gzip"

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(TDMA Vs. CDMA)
 
 
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{{Wikify}}
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{{expand}}
  
'''TDMA - Time Division Multiple Access'''
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== File format ==
 +
The gzip file (.gz) format consists of:
 +
* a file header
 +
* optional headers
 +
** extra fields
 +
** original file name
 +
** comment
 +
** header checksum
 +
* a body, containing a DEFLATE-compressed payload
 +
* a file footer
  
 +
=== File header ===
 +
The file header is 10 bytes in size and contains:
 +
{| class="wikitable"
 +
! align="left"| Offset
 +
! Size
 +
! Value
 +
! Description
 +
|-
 +
| 0
 +
| 2
 +
| 0x1f 0x8b
 +
| Signature (or identification byte 1 and 2)
 +
|-
 +
| 2
 +
| 1
 +
|
 +
| Compression Method
 +
|-
 +
| 3
 +
| 1
 +
|
 +
| Flags
 +
|-
 +
| 4
 +
| 4
 +
|
 +
| Last modification time <br> Contains a POSIX timestamp.
 +
|-
 +
| 8
 +
| 1
 +
|
 +
| Extra flags
 +
|-
 +
| 9
 +
| 1
 +
|
 +
| Operating system <br> Value that indicates on which operating system the gzip file was created.
 +
|}
  
== Why use TDMA?==
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==== Compression method ====
TDMA (Time Division Multiple Access), is used in the largest available networks in the world.  It is a digital communication method allowing many users to access a single communication channel.  TDMA is aimed at dealing with multiple access to the same communication medium.  Each individual user is given a unique time slot within the defined communication channel.  This methodology increases the efficiency of transmission by allowing multiple users simultaneous access to a time slot.  A
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significant benefit is TDMA can be easily adapted to transmission of data as well as voice communication.
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TDMA offers the ability to carry date rates of 64 kbps to 120 MBPS, which enables options of communication such as fax, voiceband data, sms, as well as bandwidth intensive apps.  TDMA allows the mobile device to have extended battery life, since the cellular device is only transmitting a portion of the time during conversations.  In addition, TDMA is the most cost effective technology for upgrading an analog system to digital (http://www.iec.org/online/tutorials/tdma/topic04.html).
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{| class="wikitable"
 +
! align="left"| Value
 +
! Identifier
 +
! Description
 +
|-
 +
| 0 - 7
 +
|
 +
| Reserved
 +
|-
 +
| 8
 +
| "deflate"
 +
| zlib compressed data
 +
|}
  
==How it works==
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==== Flags ====
  
It’s necessary for TDMA to rely upon that fact that the audio signal has been digitized. These signals are divided into a number of milliseconds and distributed into time slots. TDMA is also the access technique used in the European digital cellular standard, GSM, and the Japanese digital standard, personal digital cellular (PDC). The reason for choosing TDMA for all these standards was that it enables some vital features for system operation in an advanced cellular or PCS environment. Today, TDMA is an available, well-proven technique in commercial operation in many systems.
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{| class="wikitable"
A single channel can carry all four conversations if each conversation is divided into relatively short fragments, is assigned a time slot, and is transmitted in synchronized timed.
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! align="left"| Value
 +
! Identifier
 +
! Description
 +
|-
 +
| 0x01
 +
| FTEXT
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| If set the uncompressed data needs to be treated as text instead of binary data. <br> This flag hints end-of-line conversion for cross-platform text files but does not enforce it.
 +
|-
 +
| 0x02
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| FHCRC
 +
| The file contains a header checksum (CRC-16)
 +
|-
 +
| 0x04
 +
| FEXTRA
 +
| The file contains extra fields
 +
|-
 +
| 0x08
 +
| FNAME
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| The file contains an original file name string
 +
|-
 +
| 0x10
 +
| FCOMMENT
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| The file contains comment
 +
|-
 +
| 0x20
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|
 +
| Reserved
 +
|-
 +
| 0x40
 +
|
 +
| Reserved
 +
|-
 +
| 0x80
 +
|
 +
| Reserved
 +
|}
  
==Pros & Downfaults==
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<b>Note:</b> The FHCRC bit was never set by versions of gzip up to 1.2.4, even though it was documented with a different meaning in gzip 1.2.4.
  
TDMA can be wasteful of bandwidth because time slots are allocated to specific conversations whether or not anyone was speaking at the given moment.  There is an enhanced version however, EDTMA, which attempts to correct this problem.  Unlike TDMA which waits to determine whether a subscriber is transmitting, ETDMA assigns subscribers using a dynamic method.  The data is sent through pauses which normal speech contains.  If the subscriber has something they would like to transmit, it is placed as one bit in the buffer queue.  The system then scans the buffer and notices the user has something to transmit, allocating the bandwidth accordingly.  However, if there is nothing to transmit, it goes to the next subscriber.  This technique can be 10 times more efficient as analog transmission of TDMA. 
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==== Extra flags ====
 +
If compression method is 8 the following extra flags can be defined:
 +
{| class="wikitable"
 +
! align="left"| Value
 +
! Identifier
 +
! Description
 +
|-
 +
| 0x02
 +
|
 +
| compressor used maximum compression, slowest algorithm
 +
|-
 +
| 0x04
 +
|
 +
| compressor used fastest algorithm
 +
|}
  
==Cell Phone Providers==
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==== Operating System ====
 +
{| class="wikitable"
 +
! align="left"| Value
 +
! Identifier
 +
! Description
 +
|-
 +
| 0
 +
|
 +
| FAT filesystem (MS-DOS, OS/2, NT/Win32)
 +
|-
 +
| 1
 +
|
 +
| Amiga
 +
|-
 +
| 2
 +
|
 +
| VMS (or OpenVMS)
 +
|-
 +
| 3
 +
|
 +
| Unix
 +
|-
 +
| 4
 +
|
 +
| VM/CMS
 +
|-
 +
| 5
 +
|
 +
| Atari TOS
 +
|-
 +
| 6
 +
|
 +
| HPFS filesystem (OS/2, NT)
 +
|-
 +
| 7
 +
|
 +
| Macintosh
 +
|-
 +
| 8
 +
|
 +
| Z-System
 +
|-
 +
| 9
 +
|
 +
| CP/M
 +
|-
 +
| 10
 +
|
 +
| TOPS-20
 +
|-
 +
| 11
 +
|
 +
| NTFS filesystem (NT)
 +
|-
 +
| 12
 +
|
 +
| QDOS
 +
|-
 +
| 13
 +
|
 +
| Acorn RISCOS
 +
|-
 +
| 255
 +
|
 +
| unknown
 +
|}
  
There are several cell phone companies competing to sell their phones and advertise their network coverage area.  They will primarily be competing within two categories: TDMA and [[CDMA]].  The pros and cons of TDMA have been mentioned, and the companies that offer TDMA are the following:  AT&T, Cingular, Nextel, T-Mobile.  The companies that support [[CDMA]] are:  ALLTEL, Amp'd Mobile, Cricket Wireless, ESPN, Quest, Sprint, Verizon, Virgin Mobile.  As we can see, more companies are supporting [[CDMA]].  The question that arises is, what makes the two so different? 
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=== Optional headers ===
 +
==== Extra fields ====
 +
<b>TODO: add description</b>
  
 +
The extra field are variable of size and contains:
 +
{| class="wikitable"
 +
! align="left"| Offset
 +
! Size
 +
! Value
 +
! Description
 +
|-
 +
| 0
 +
| 2
 +
|
 +
| Extra field data size <br> Value in bytes.
 +
|-
 +
| 2
 +
| ...
 +
|
 +
| Extra field data
 +
|}
  
==TDMA Vs. [[CDMA]]==
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==== Original file name ====
 +
This is the original name of the file being compressed, with any directory components removed, and, if the file being compressed is on a file system with case insensitive names, forced to lower case.
  
TDMA is better for international plans and debately has better battery life.  [[CDMA]] claims it has better battery life and coverage, however, Cingular, in the US, is the largest TDMA carrier.  All of this started however ever since [[CDMA]] was introduced in 1989, and the wireless world has been in debate over merits of TDMA and [[CDMA]].  Those who are for [[CDMA]] have claimed that its technology has bandwidth efficiency of up to 13 times that of TDMA and between 20 to 40 times that of analog transmission.    
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Contains an ISO 8859-1 (LATIN-1) string with end-of-string character.
  
Those who favor TDMA point out that there has been no successful major trial of [[CDMA]] technology that supports the capacities it claims. Not to mention, theoretical improvements in bandwidth efficiency claimed [[CDMA]] is now being approached by enhancements to TDMA technology. TDMA's evolution allows capacity increases of 20 to 40 fold over analog in the near future.  [[CDMA]] is a very expensive technology that needs $300,000 per base station, compared to $80,000 for TDMA.  Lastly, TDMA is the proven leader as the most economical digital migration path for existing AMPS networks. No one has the final word in this debate, however, it is evident that TDMA will remain the dominant technology in the wireless market.
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==== Comment ====
 +
Contains an ISO 8859-1 (LATIN-1) string with end-of-string character. Line breaks should be denoted by a single line feed character.
 +
 
 +
==== Header checksum ====
 +
The header checksum contain a CRC-16 that consists of the two least significant bytes of the CRC-32 for all bytes of the gzip header up to and not including the CRC-16.
 +
 
 +
=== File footer ===
 +
The file footer is 8 bytes in size and contains:
 +
{| class="wikitable"
 +
! align="left"| Offset
 +
! Size
 +
! Value
 +
! Description
 +
|-
 +
| 0
 +
| 4
 +
|
 +
| Checksum (CRC-32)
 +
|-
 +
| 4
 +
| 4
 +
|
 +
| Uncompressed data size <br> Value in bytes.
 +
|}
  
 
== External Links ==
 
== External Links ==
  
http://en.wikipedia.org/wiki/Time_division_multiple_access <br>
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* [http://www.gzip.org/format.txt The gzip file format], by the [http://www.gzip.org/ gzip project]
http://www.iec.org/online/tutorials/tdma/topic04.html <br>
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* [http://www.gzip.org/algorithm.txt The gzip compression algorithm], by the [http://www.gzip.org/ gzip project]
http://www.cellphoneinfo.com/index.html
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* [http://tools.ietf.org/html/rfc1952 RFC1952: GZIP file format specification version 4.3], by [[IETF]]
 +
* [http://en.wikipedia.org/wiki/Gzip Wikipedia: gzip]
 +
 
 +
[[Category:File Formats]]

Revision as of 02:05, 28 November 2013

Information icon.png

Please help to improve this article by expanding it.
Further information might be found on the discussion page.

Contents

File format

The gzip file (.gz) format consists of:

  • a file header
  • optional headers
    • extra fields
    • original file name
    • comment
    • header checksum
  • a body, containing a DEFLATE-compressed payload
  • a file footer

File header

The file header is 10 bytes in size and contains:

Offset Size Value Description
0 2 0x1f 0x8b Signature (or identification byte 1 and 2)
2 1 Compression Method
3 1 Flags
4 4 Last modification time
Contains a POSIX timestamp.
8 1 Extra flags
9 1 Operating system
Value that indicates on which operating system the gzip file was created.

Compression method

Value Identifier Description
0 - 7 Reserved
8 "deflate" zlib compressed data

Flags

Value Identifier Description
0x01 FTEXT If set the uncompressed data needs to be treated as text instead of binary data.
This flag hints end-of-line conversion for cross-platform text files but does not enforce it.
0x02 FHCRC The file contains a header checksum (CRC-16)
0x04 FEXTRA The file contains extra fields
0x08 FNAME The file contains an original file name string
0x10 FCOMMENT The file contains comment
0x20 Reserved
0x40 Reserved
0x80 Reserved

Note: The FHCRC bit was never set by versions of gzip up to 1.2.4, even though it was documented with a different meaning in gzip 1.2.4.

Extra flags

If compression method is 8 the following extra flags can be defined:

Value Identifier Description
0x02 compressor used maximum compression, slowest algorithm
0x04 compressor used fastest algorithm

Operating System

Value Identifier Description
0 FAT filesystem (MS-DOS, OS/2, NT/Win32)
1 Amiga
2 VMS (or OpenVMS)
3 Unix
4 VM/CMS
5 Atari TOS
6 HPFS filesystem (OS/2, NT)
7 Macintosh
8 Z-System
9 CP/M
10 TOPS-20
11 NTFS filesystem (NT)
12 QDOS
13 Acorn RISCOS
255 unknown

Optional headers

Extra fields

TODO: add description

The extra field are variable of size and contains:

Offset Size Value Description
0 2 Extra field data size
Value in bytes.
2 ... Extra field data

Original file name

This is the original name of the file being compressed, with any directory components removed, and, if the file being compressed is on a file system with case insensitive names, forced to lower case.

Contains an ISO 8859-1 (LATIN-1) string with end-of-string character.

Comment

Contains an ISO 8859-1 (LATIN-1) string with end-of-string character. Line breaks should be denoted by a single line feed character.

Header checksum

The header checksum contain a CRC-16 that consists of the two least significant bytes of the CRC-32 for all bytes of the gzip header up to and not including the CRC-16.

File footer

The file footer is 8 bytes in size and contains:

Offset Size Value Description
0 4 Checksum (CRC-32)
4 4 Uncompressed data size
Value in bytes.

External Links