Newsletter: Asynchronous Transfer Mode (ATM)
BER Testing for T1 E1
Welcome to a January 2011 issue of GL Communications’ Newsletter providing information and insight into our
ATM BERT Software for T1 E1. Using our various T1 E1 platforms, this software permits
BER testing over ATM switching and transmission systems.
Asynchronous Transfer Mode, or ATM is a form of transmission and switching that is commonly used on DSL (Digital
Subscriber Lines), fiber optic lines, and 3G mobile infrastructure. Transmission systems could be any digital facility including:
T1, E1, DSL, T3, E3, OC3, STM1, OC12, STM4, and other higher rates.
ATM is based on segmenting all information regardless of type (voice, data, picture, or video) into “cells”
of length 48 bytes plus 5 overhead bytes (53 total). At the receiving end the reassembly of these cells puts the information
back to its original state. To handle all information types many higher layer protocols have been developed called ATM
Adaptation Layers, or AALs.
ATMs basic advantages include lower latency (compared to IP), efficient and fast switching speeds, little or no
congestion, and versatile traffic type handling. Its major disadvantages are the large overhead consumed for each cell and
the finite time required to pack and unpack a 53 byte cell (versus a single byte in TDM).
Of course, the competing and proliferating technology is IP (Internet Protocol). IP will likely dominate in the future as
it invades all forms of transmission and switching. But for now, a significant amount of network transmission and switching
remains ATM. One major ATM application is the reuse of copper subscriber lines for internet data (the world is replete with
copper subscriber lines!). Another is in the mobile broadband system infrastructure called 3G – think “
smartphones”. All this means that most traffic gets converted to ATM at some time during its traversal.
See sidebar for how voice and internet traffic is handled.
For a quick graphical tutorial on ATM – see “
Voice and internet traffic transport over networks:
- Switched or Dial-Up – Voice traditionally uses the public switched telephone network (PSTN) using TDM technology,
Dial-up internet uses PPP which is an adaptation of IP
- DSL use DSLAMs and the ATM protocol AAL2, usually for both voice and internet data
- Cable modems use a transmission and encapsulation protocol called DOCSIS. In this scheme, voice and data are
converted to IP
- Wireless is usually 3G technology. In 3G, transmission from the tower into the network is initially ATM
(see 3G newsletter for more information)
In the above figure, there is an implicit end to end connection between the endpoints. ATM is inherently connection
oriented, therefore any traffic that relies on connections is easily accommodated (adapted), i.e. voice, modem, and fax.
Traffic that is inherently connectionless must be adapted, thus the necessity of higher ATM Adaptation Layers, or AAL.
To test an end to end ATM derived virtual circuit, one can use GL’s ATM T1 E1 BERT software with any of
GL’s T1 E1 platforms, as shown below.
The application transmits a BERT pattern using the simplest ATM Adaptation Layer, i.e. AAL0. The BERT pattern is
inserted in its entirety into the 48 byte payload of the cell.
The application allows an ATM virtual circuit to be tested by sourcing test traffic and verifying at the receive end. Other
features are: Bit Error Insertion, Looping back incoming traffic (using T1 E1 Loopback), and configuring ATM headers for
UNI & NNI interfaces. The application is capable of generating various Pseudo Random Bit Sequence (PRBS) patterns, all
ones, all zeroes, alternate ones & zeroes, 1:1, 1:7, and user-defined bit patterns. In addition, single bit error insertion,
auto error insert rate from 10-2 to 10-9, invert & non-invert selections, and scrambling options
(according to ITU-T G.804) are provided.
- User-defined header configuration supported
- User-defined traffic rate to the accuracy of 1% of total bandwidth
- Supports different PRBS patterns, All one’s, All zero’s, alternate 1’s and 0’s, 1:1, 1:7,
and User -defined pattern. User defined pattern length can be 2 to 32 bits in length
- Supports inverting payload data, and scrambling. Scrambling is according to ITU-T G.804
- Supports single bit error insertion, and error rate insertion .
- Provides detailed statistics such as Rx/Tx cell count, total cell count, rejected cell count, pass cell count, idle cell count,
cell rate, and HEC error count
- Provides throughput details, error, and alarm LEDs for easy analysis
- Tx and Rx settings for multiple ports can be independently controlled or coupled
- Capable to save and load the configuration settings
Brief Description of T1 E1 ATM BERT Features
ATM Header Configuration
The GUI provides configuration of ATM header fields such as GFC (Generic Flow Control), VPI (Virtual Path Identifier),
VCI (Virtual Channel Identifier), PT (Payload Type), and CLP (Cell Loss Priority). ATM header fields for UNI will have GFC
(Generic Flow Control) enabled, while for NNI interface GFC field is disabled.
Selecting BERT Patterns (Payload)
Payload at the Tx configuration allows user to select specific Bit Error Rate test pattern for transmission.
T1E1 ATM BERT support various BERT patterns; QRSS, 29-1,
211-1, 215-1, 220-1, 223-1, all ones, all zeros, 1:1, 1:7, alternate 1s
and 0s, and user-defined pattern from 3 to 32 bits length. While at Rx configuration these patterns are used to verify the
incoming BERT pattern. Pattern Sync is achieved only if BERT pattern matches configuration options, configurable header
lengths and header information.
The Traffic Rate option supports Bandwidth Rate defined as:
- Percent with range starting from 1 to 100% of current bandwidth
- Cell Ratio, where users can set the amount of ATM traffic cells and idle cells
ATM BERT allows users to insert single bit error or a random error rate from 10-2 to 10-9 into the
outgoing (TX) BERT cell stream.
BERT Results are displayed in 2 ways – BERT Results with LEDs and Statistics. LEDs give users a quick way of viewing
the test status. The three Status LEDs reflect current as well as history status. These LEDs indicate the Traffic status (whether
traffic is being received or not), Pat Sync status (In Sync or Sync Loss) and Bit Error Status (Whether Bit Errors are present).
Bert Results provides a more detailed view of the test results. Various results like the Bit Error Rate (the average bit
error rate since the start of the test calculated as a ratio of the total bit errors occurred to the total bits received), the
number of times/number of seconds Bit Error, Sync Loss, No Rx Data events occurred, Number/Seconds of Error Free events
etc. are displayed.
For comprehensive information on GL's ATM BERT Software for T1 E1, please refer to
T1 E1 ATM BERT web page.
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