1993 << Back To The X Files nº 4

__ \ / |_ / \ e n o n | o u n d a t i o n presents: \ / *------ the \ / / \ files ------* / \ Fall/1993 Issue: 4 INTRODUCTION TO BASIC DIGITAL TECHNOLOGY Written by: Erik Turbo File: 4 of 6 4. Digital Transmission 4.1 Interoffice Facilities Below is a list of interoffice facilities required for digital transmission of data. 1. VF Facilities A small number of Special Services circuits use VF, Voice Frequency, pairs for the facility between Central Offices. a. MFT Equipment The Metallic Facility Terminal (MFT) is the AT&T family of plug-in equipment developed to provide transmission and/or signaling fuctions required for metallic facilities. MFT provides the interfect between VF metallic circuits and switching systmes, station equipment, or another metallic cable. In addition to metallic facilities, micro-wave radio is utilized for both digital and analog transmission. Fiber-optic facilites are also utilized for digital transmission. 2. Carrier Facilities Carrier facilities, both analog and digital, are pair gain devices. They are more economic than using Voice Frequency (VF) metallic cable pairs. 3. Digital Transmission Systems a. Digital Hierarchy Digital terminals are connected together by an almost countless number of facility and equipment configurations. b. Digital Channel Banks The most common channel banks used by the BOCs are the AT&T D4 and D5 terminals. The channel units provide transmission and signalling features required to interface with 2 or 4 wire circuits. 4.2 T1 Carrier Line The T1 line carries DS1 signals (1.544 Mb/s) between signal digroup channel banks using four-wire bidirectional transmission over standard cable pairs. 4.3 Digital Line Coding > Bipolar Coding Bipolar coding is the basic line coding procedure used by T-carrier lines. A Bipolar code uses alternating polarties for encoding "1"'s. > Clear Channel Capability Clear Channel Capability is used to describe the capability to transmit a DSO or DS1 level signal which contains any mix of ones and zeros, including all zeros. The density requirement is no more than 15 zeros. If more than 15 zeros are used then ones must be put in or an error will result. Due to current signaling and maintenance requirements, only 56 kbps of the DSO signal is available for use by the customer. In order to provide 64 kbps Clear Channel Capability (64 CCC), the Bipolar with Eight Zero Substitution (B8ZS), is the method recommended as the North American Standard. However, interim methods such as Fractionally Controlled Mutliplexing (FCM), or Zero Byte Time Slot Interchange (ZBTSI), can be utilized as an expedient. > Bipolar Advantages - Simplifies error detection - Elimates DC components - Reduces bandwith requirements 4.4 Digital Wave Forms 0 1 1 0 0 1 0 1 +V ----------- ----- ----- | | | | | | 0V _____|___________|__________|_____|______|_____|_ -V Unipolar Non-return to zero 0 1 1 0 0 1 0 1 +V ---- ---- ----- ----- | | | | | | | | 0V _____|____|_|____|__________|_____|______|_____|_ -V Unipolar Return to Zero 0 1 1 0 0 1 0 1 +V ---- ----- | | | | 0V _____|____|_________________|_____|______________ | | | | -V |_____| |_____| Bipolar non Return to Zero 0 1 1 0 0 1 0 1 +V ____ ____ | | | | 0V _____|____|_________________|____|______________ | | | | -V |____| |____| Bipolar Return to Zero Comparison of Digital Waveforms 4.5 Error Detection + ____ ____ ____ | | | | | | 0 ____| |________ ____| |__________ ___| |________ | | | | - |____| |____| /\ < -- Noise Burst + _____/\_______________________________________/____\___________________ / \ / \ /\ /\ / \ __ / \ 0 _/________\____/____ \____/____\/____\____/____________\/__\____/_____\ \/ \/ \/ \/ \ - _______________________________________________________________________ Bi-Polar Violation | | + ____ ____ ____ ____ | | | | | | | | 0 ____| |________ ____| |___| |_ ___| |________ | | | | - |____| |____| Error Detection The error detection technique is very simple. Since each successive "1" bit is of opposite polarity, and extra pulse will show up as an error. This error detection technique is called Bipolar Violation Detection. 4.6 Analog Switching with Digital Transmission __________ 1 001010 2 _______ 3 _________ | Local | __ __ |Toll | __ |Telephone|----|-x--x--x--|------|__|----------|__|------|-x-x-x-|---|__|-. --------- | Office | |Office | |0 |__________| |_______| |0 Analog - Digital |1 Conversion |0 |1 |0 __________ 6 5 _______ 4 | _________ | Local | __ 001010 __ |Toll | __ | |Telephone|----|-x--x--x--|------|__|----------|__|------|-x-x-x-|---|__|-. --------- | Office | |Office | |__________| |_______| Key: 1,2,3,4,5 and 6 are Analog/Digital Conversion switching systems 001010 is the digital bit-stream representation of the analog signal Analog Switching with Digital Transmission diagram The diagram shows an Analog signal being converted to Digital for transmission and back to Analog for switching. This configuration causes additional noise and distortion to be added to the Analog signal. Toll Office __________ 1 ________ _________ | Local | __ 01101110 | | |Telephone|----|-x--x--x--|------|__|---------------|01101110|-------. --------- | Office | | | | |__________| |________| | | Toll Office | __________ 2 ________ | _________ | Local | __ 01101110 | | | |Telephone|----|-x--x--x--|------|__|---------------|01101110|-------. --------- | Office | | | |__________| |________| Digital Switching with Digital Transmission > Four points of analog/digital conversion have been elimated from the previous example. > The digital switch will switch the digital stream directly. Therefor, digital to analog conversion is not needed. > The introduction of a digital toll office reduces the need for some analog/digital conversion. > This reduces the cost of providing additional digital transmission facilities since no analog/digital conversion will be required at the toll office location and this will improve the overall transmission. ------------------------------------------------------------------------------ EOF ---------------- Xenon Foundation Productions 1993 -------------------EOF ------------------------------------------------------------------------------