ATM PROTOCOL ADAPTATION FOR EMI/EMC ENVIRONMENT

Manoj Jain *, L.Vidyasagar *, Rajashekhar N H *, T R Ramamohan *, Capt T N Pranesh

*Central Research Laboratory, BEL, Bangalore

ABSTRACT- Traditionally telecommunication networks are primarily circuit switching based with variety of protocols developed specifically to have call control while computer networks are packet switching based with specific protocols for having traffic control and packet routing.

In contrast, the next generation networks are being evolved as a convergence of communication and computer networking. These networks will be having distributed elements interconnected through broadband connecting media. The traffic carried over the networks will be circuit switched as well as packet switched but to support this in distributed processing environment control plane messages will also be transacted as overheads.

However in this context of evolving high performance network as objective the assessment of the suitability of conventionally evolved protocol needs to be done. As user applications also will use this type of network infrastructure hence suitably adapted form of existing protocols are to be used by them. This will ensure compatibility with existing protocols at the same time will provide immunity against electromagnetic interference by interconnecting heterogeneous media.

The communication system at each end can be branched into classical 7 layers OS1 model with transactions being done between peer entities. The electromagnetic interference in the interconnecting media directly affects the physical layer but indirectly all upper layers also. It causes different types of error patterns to be introduced in the physical layer bit streams. These error patterns are to be analyzed to quantify their effect on the communication system. Various classical channel models viz., Poisson, Neymann Pearson, Pareto, Markov etc. can emulate various segments of the interconnecting media thereby giving fairly good idea of error pattern behavior at physical layer. This information is useful in selecting suitable FEC with interleaver so as to have very less residual error out of physical layer at other end. Still upper layers are affected because of this residual error. The main objective of this paper is to focus on various issues concerning adaptation of ATM layers so that overall electromagnetic compatibility of the system can be achieved.

1. Introduction

Untamed progress of modern telecommunications is accompanied by massive penetration of electronics into industrial, business, military, medical, and consumer markets, by expansion of the radio spectrum up to hundreds of GHz, and by seemingly limitless product miniaturization. These factors challenge engineers to provide electromagnetic compatibility for a myriad of interfering electronic products and systems through the protocols so as to reduce system noise, and to meet assured data transfer.

Typical applications of the protocols in the tactical and defence environment are sensing and switching operation, detection and tracking, voice circuits and microphones and services within and between the cabinets. These services are situated at in close proximity to strong electromagnetic fields generated by the high powered devices like microwave relay links, radars, telecommunication antennas and walkie talkies.

Electromagnetic Interference (EMI) is the frequency spectrum pollution, which degrades the performance of electronics equipments and network system. Electromagnetic Compatibility (EMC) is the ability of an equipment or a system to function as designed in its intended operational environment without adversely affecting the operation of other equipment/system.

There are two solutions to overcome the problem. One is we have to identify and fight EM1 problems, troubleshooting equipment affected by the interference and providing the solutions. Second is to understand and follow the measurement conditions and protocols that are described in the applicable standards.

This paper spans the major techniques and methods to meet EMC requirements of conducted and radiated emissions, immunity of digital/analog electronic and telecom protocols, products and systems. The emphasis is on the understanding of the underlying physical phenomena and hands-on skills that are necessary for successful adaptation of protocol and troubleshooting of modern protocols which comply EMVEMC standards.

In the true sense since all types of services from very slow speed data to real time video with bursty and continuous traffic are transported using the Asynchronous Transfer Protocol (ATM). ATM can support even future services, which are not yet visualized; hence it is a future proof. Apart from service integration, one of the greatest advantages

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of ATM for military communications is the potential for better utilization of link capacities, since bandwidth is allocated on demand and' full benefit of continuously variable bit rate of voice, video and data can be realized. So this paper mainly concentrates on the adaptation of protocol in ATM networks, which in turn minimizes the effects of Bit Error Rate (BER) due to EMVEMC.

2. Introduction t o ATM

ATM is a cell-switching and multiplexing technology that combines the benefits of circuit switching (constant transmission delay and guaranteed capacity) with those of packet switching (flexibility and efficiency for intermittent traffic and bandwidth). Demands that drive ATM deployment are evolution of network, high bandwidth applications, integration of traffic types and focus on business needs.

ATM is a networking technique used in broadband networks, by which, all classes of traffic (video, speech, data) is segmented into packets called 'ATM cells' containing 53 bytes, of which 48 octets form the information field and the other 5 octets form the header.

In ATM networks, there is no concept of error detection and/or correction at the data-link level. So any user needing reliability of data transfer needs to employ his own error-control protocol. Signaling protocol is such a user which assumes the underlying link as reliable. To provide this facility to signaling and other potential users, ITU-T devised a new protocol, keeping the characteristics of ATM in mind. Signaling across ATM networks is important because a reliable protocol like Transfer Control Protocol (TCP) assumes the link to be loss-prone and slow, which won't fit into the ATM requirements. The ATM protocol was devised keeping in mind the user at large, and not only the signaling protocol. Thus the interfaces were defined to be generic nature, and any user, at any OS1 layer can use them to deliver his data reliably to the other end.

3. Protocols in the ATM networks

The ATM reference model as shown in figl, is composed of the following planes, which span all layers: Control plane, User plane and Management plane. The protocol stack for signaling resides in the control plane (C-plane). The ITU terminology also specifies that there are protocols in the user plane (U-plane) that are used by the user applications to transfer information over the VCCs established over the C-plane; and there are protocols in the management plane (M- plane) for network management functions.

Physical layer of ATM PRM is analogous to the physical layer of the OS1 reference model the ATM physical layer manages the medium- dependent transmission. ATM layer is responsible for establishing connections and passing cetls by making use of header of ATM cell. Signaling ATM Adaptation Layer provides adaptation between the signaling Protocol Data Units (PDU) and the ATM cells. Segmentation and Reassembly layer (SARI segments Common Part Convergence Sublayer (CPCS)- Service Data Units (SDU) into 48-byte ATM cell payloads on transmission, and reassembles ATM cells into CPCS-SDUs on reception. CPCS aligns it's SDUs on 48 byte boundaries (by adding padding), adds a length field and a Cyclic Redundant Code (CRC) check transmission. On reception, it verifies the length field and the CRC and strips the padding, if any. Service Specific Connection Oriented Protocol (SSCOP) implements a reliable data link protocol to provide robust transmission of signaling protocol PDUs between peer signaling entities. Service Specific Coordination Function (SSCF) provides a mapping between the SSCOP capabilities and the needs of the signaling protocol module. User Network Interface (UNI) signaling protocol specifies the procedures for the establishment, maintenance and clearing of network connections at the user part of the Broadband - Integrated Service Digital Network (B-ISDN).

ATM Adaptation Layer (AAL) layer provides the rule to interface with all of the different types of traffic such as LAN, voice, data, frame relay elc. These different interfaces are defined as a senrice of AAL bearer classes. Accompanying the various bearer classes and AAL types is a set of Quality of Service (QoS) standards intended to assure appropriate operation of the ATM network. These QoS standard include parameters for Cell Error Ratio, Cell Loss Ratio, Mis-insertion Rate, and statistical values for Cell Delay.

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4. Effect of transmission errors on ATM performance

BEP

4.1 Celt Synchronization

The cell synchronization is an important issue to be considered. Care should be taken to provide proper cell synchronization and frame synchronization on ATM cell transmission. ATM cell synchronization (cell delineation) is done as follows: Starting from an arbitrary byte (frame synchronization generally does byte alignment of cells) from the received data stream, 4 bytes are collected, Header Error Correction (HEC) is calculated and matched with the fifth byte in the stream. If there is no match, the starting point is shifted one byte at a time until a match is found, then provisional cell delineation is assumed to have occurred. If there are 6 consecutive matches, synchronization is confirmed. After synchronization is achieved, the header is continuously monitored for error and loss of synchronization is declared if there is no match for 7 consecutive headers.

CL P CMP

Average time interval between loss of synchronization events for error rates of interest is given in Table 1. It can be seen from the table that a link Bit Error Probability (BEP) of 1 E(-4) or better would be required to ensure that average time interval between synchronization loss is sufficiently long.

Coder input level (OB)

Link BEP lntewal between Loss

1 E(-3) 25 Secs

1 E(-5) 70 Years

SQR in DB

Table I: Effect of BEP on Synchronization

4.2 Cell loss and misrouting probability

ATM is suitable for military communication but having several issues, the main one being the effect of inter node link transmission errors on the overall QoS in the tactical environment. Inter node link may be wired media or wireless media. Basically, defence tactical links are poorly engineered because of various constraints and the average BEP of a typical tactical radio link is about 7 orders higher than that of Fiber Optic links. At this high link error rates, the QoS parameters like Cell Loss Priority (CLP) and Cell Misrouting Probability (CMP) are highly degraded and give very poor performance.

In ATM Cell single error in the header is corrected and cell is rQUted to the correct destination. In the case of multiple errors in the header, the errors are either uncorrectable in which case the cells are discarded and lost, or the cell header as received

after error correction turns out to be a valid header but not the intended one and results in cell misrouting. Hence, header requires some additional protection to have low CMP in the tactical environment. In the case of civilian networks with fiber optic links with extremely low error rate of the order of l.OE(-lO), both CLP, and CMP are extremely low. Values of CLP, and CMP for values of BEP of interest in the present context of tactical communications are given in the table 2.

Table 2: CLP and CMP Comparison for BEP

4.3 Effect of cell payload error on voice, video, and data

0 17.6 I

-24 4.5 27

I -36 I 0 Table 3 SQR as a function of speech signal level

The effect of cell payload errors is more severe in the case of video compared to speech. In the case of compressed video, errors in cell payload will cause degradation in picture quality and would be perceptually objectionable at high error rates and a minimum BEP of 1E(-6) would be required to ensure acceptable picture quality.

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The effect of payload BEP on data is however, more severe. If the cell contains one or more errors and hence is errored, the cell has to be retransmitted. Since messages at higher ATM adaptation layer may be made up of several cells, the entire message needs to be retransmitted causing drastic reduction in thruput at high error rates. The table 4 shows computed values of probability of cell payload error, and probability that a 10 cells higher layer message is transmitted without error.

Link BEP

1E(-3)

1 E(-4)

1 E(-5)

1 E(-6)

Prob (cell in error) delivered correctly)

Prob ( I O cell message

0319 0 02

0.0376 0 68

0 00383 0 962

3 8E(-4) 0 996

It can be seen from the above tables that a link BEP of 0.001 is not acceptable for both speech and data payload in cells.

4.4 Inference

It is clear from the above discussions that both the cell header and payload cannot tolerate a tactical link BEP of 0.001, and a BEP improvement by a factor of 100 to 1000 would be required to meet performance objectives in respect of CLP, CMP, speech and video quality, and data thruput. Hence, both header and payload are to be protected by suitable Error Correction methods to achieve minimum QoS that improves the BER performance to an acceptable level.

5. Points to be considered for protocol adaption

5.1 Handling error condition

Protocols like TCP/IP, Frame Relay, ATM etc. are designed to operate in a low error rate environment. They have only error detection capabilities and generally on detection of error they discard the full frame/packet. This reduces processing power requirement at lower layers so that it can operate at much higher bit rates.

Normal congestion notification and avoidance methods in these protocols generally use packet discard strategy hence congestion control ' algorithm senses the packet loss and attribute it to congestion notifications. Then to avoid congestion they throttle back the packet transmission rate at source itself. But these poses serious problems in case the packet losses are because of high bit error rates as in this case also congestion control

will behave similarly but this time to adverse the situation even more. Congestion control algorithms have to be modified for differentiating between the two situations to allow for optimum packet transmission under all conditions.

5.2 Selection of layer to be protected

The rule of thumb for layer selection is that if attention is given on lower layer it takes less time to take corrective action but at the expense of more overheads and processing power. Different types of applications requires importance to be given to protect particular layer against errors. For example, in the adaptation of ATM technology for defence environment, it will be worthwhile to adapt layer 2 to make it more robust. It will not only improve data transfer but also the control plane transactions. Data transfer itself can use assured mode service of SSCOP for high reliability with selective retransmission along with modified error recovery and resync procedures so as to be able to resume transfer after an interruption of service.

5.3 Selection of suitable FEC for upper layer

Upper layers should also have suitable Forward Error Correction (FEC) coding so that they have better immunity to errors without resorting to Automatic Request Queue (ARQ) technique. ARQ techniques are suitable again for low error rate channels only. They also cause long delays in communication path and require suitably sized buffers at both ends. As error rate increases, ARQ alone will reduce the throughput of channel to unacceptable limits. A suitable combination of forward error correcting codes for reducing overall error rate and a powerful error detection code on top of it for ARQ or selective retransmission methods can be a suitable alternative for this situation. Iterative error correction codes like turbo codes can be used for FEC because of their adaptability to various types of error patterns and also varying error rates.

6. Possible adaption of ATM protocol for EMllEMC

There are the several issues in adapting ATM for the tactical networks. Because of the hostile environment overall Quality of Service of the connection is seriously affected. Attempts to reduce the effect of EM1 are to be done at each layer of ATM protocol stack.

6.1 Physical Layer:

As user data and control plane data both, which are raw in nature are passed through physical layer, hence suitable FEC method has to be employed without considering the boundary of the cell format. An interleaver of suitable depth and

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robust sync bits are added to it for taking care of bursts. This scheme is essential as normal Physical layer Frame Synchronization does not work well at extremely low BER. Also cell synchronization does not work properly at such BER hence overall bit error rate of Physical bit stream only has to be improved so as to have good cell sync. characteristics as mentioned at para 4.1

6.2 A TM Layer:

ATM header of 40 bits has to be compressed to one or two bytes as number of virtual channels on each links is going to be very few. This saving in bandwidth is essential to cater for the overheads due to physical layer FEC. A cell based FECICRC detection and cell sequencing can be tried out for cell level. Also error correction and selective retransmission can be done at cell level.

6.3 AA L Layer:

AAL need not be adapted so as to use general- purpose integrated circuits for AAL processing. Also it may not be feasible to adapt this layer if the user is directly pumping the cells.

6.4 SAAL Layer 2:

The SSCOP provides assured mode data service for upper layers. Suitable adaption of SSCOP is to be done, so that in the case of protocol mismatches it can recover without bringing down the assured mode service. For user Variable Bit Rate data SSCOP with modification can be used instead of plain AAL5. Various parameters for layer-2 management like Poll Timer, Max CC, Max PD, clear buffers etc. are to be tailored for this situation.

6.5 UN1 layer31

Adapted form of Q.2931 has to be used for User Network Interface. Various parameters like Message Retransmission, Information Elements processing etc. are to modified to have less effect of errors. Redundancy in information elements can be done. Small FEC overheads can be added to have message integrity and to reduce cell losses because of residual errors.

6.6 Upper Layer for Data Transmission:

For transferring the data, end-to-end upper layers protocols like FTP etc. can be adapted to have more error protection so that they should be able to resume the transfer of data even after the interruption in service. As these layers are outside the ATM domain hence any other application layer adaptation can be tried.

7. Implementation

A novel approach for improving BER performance over tactical A I M links is now discussed. This approach works at Physical layer but also considers payload as ATM to take advantage of it. The design uses the technique of interleaving and inserting Forward Error Correction (FEC) code in the incoming data (2 Mbps E l data) stream, and provides high BER performance with less overhead penalty. This method utilizes full link capability by inserting FEC codes in sync time slots and idle cells bandwidth, without exceeding the bandwidth limit. By this technique, a link enhancement of the order of lo3 to lo4 times can be achieved over tactical ATM links. Various design issues like degrading effects of transmission errors on ATM QOS Performance and possible FEC design issues are examined.

7.1 Selection of Suitable FEC

As discussed before it is clear that both the cell header and payload cannot tolerate a tactical link BEP of 0.001 and a BEP improvement by a factor of 100 to 1000 would be required, hence a suitable FEC mechanism is required to protect complete cell to achieve minimum QoS.

Fig 2: Probability of Cell loss Vs signal to noise ratio.

Fig 3: Probability of Cell Loss Vs bit error rate.

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Various FEC codes suitable for this application are RS (Reed-Solemn) Codes, BCH (Bose, Chaudhuri and Hocquenghem) codes, Viterbi code and Turbo codes. After detailed analysis it is observed that BCH codes can be selected as suitable option considering it’s effectiveness and implementation simplicity meeting above requirements. Figure 2 and Figure 3, shows the simulation results of BCH (63,k,t) FEC code performance

As extrapolated from figure 2 and figure 3 BCH(63,51,2) code can provide an improvement factor of 550 at 1 E(-3) BER. The improved BEP is 1.8 X 1E(-6) and CLP is 0.5 X 1E(-5).The throughput for data cells is 0.998 with FEC overheads taking 25% of bandwidth.

7.2. Design Implementation

By considering various design issues, ATM Link Enhancer is implemented in Altera Flex Series FPGA. The basic design consists of Basic Error Control mechanism (FEC addition), Scrambler/Descrambler, Row-Column interleaver, Frame Alignment Signal (FAS)/Multiple Frame Alignment Signal (MFAS) adder, Robust SYNC (Army Radio Engineered Network SYNC) adder, in-built Pseudo Random Bit Sequence (PSRB) generator and various counters for status checking. Fig 4 is the block diagram of ATM link Enhancer.

Enhancer 2MB ATM link (Trans) 2MBBMB ATM link

(as per ITU-T G.804)

Fig 4: ATM Link Enhancer

In the transmit direction, ATM Link Enhancer takes the 2MB E l stream of data as per ITU-T. It checks the frame synchronization and removes the sync time slots TSO and TS16 (used for signaling and synchronization on E l stream as per ITU-T). After removing the E l stream overheads cell delineation is done. Now useful data is put into the ATM cell buffer after HEC correction and idlehnassigned cells rejection. Then a 64-bit proprietary frame structure is generated by taking 51 bits of actual data from the cell buffer/idle cells, 12 FEC bits from BCH (63,51,2) encoder and 1 sync bit from FAS/MFAS adder (AREN sync). In between, scrambling is done on the data bits and interleaving is done on data and FEC bits. Then this proprietary E l stream data is transported out.

The reverse process takes place at the receiving end. The proprietary E l stream data is checked for

, frame synchronization by using the AREN sync algorithm. Then the useful data is extracted and

put into the cell buffer. Subsequently cell buffer is read after cell delineation and header error correction. E l data stream is generated from cell buffer as per ITU-T and sent out. The Design uses around 2000 Logic cells and 8KB Memory bits in Altera EPF1 OK50 FPGA.

8. Conclusion

The Physical layer adaption as suggested in implemented solution is the most suitable solution as the effect of EMVEMC is more on this layer and if proper care is taken at this layer itself then residual error will be very minimum. But to have fully robust link all other possible adaption at other layers as discussed should be tried out. After suitable adaption these protocols can provide solution that allows for optimum performance and efficient utilization of band width over noisy network channels because of growing electro magnetic interference.

For larger networks with growing complexities these solutions can be further extended to either Artificial Intelligence based expert systems or Mobile Agents based distributed systems. These systems will get the QoS constraints like Delay, Delay Variation and Throughput etc. from the user application, the information about physical channel error status and other characteristics of physical layer and then adaptively change the protocol to best fit into the situation.

Acknowledgements

Authors are thankful to Mr. H.Ramakrishna (GGM), Mr.V.G.Rao (Consultant) and the Management of Central Research Laboratory, Bangalore for creating good environment for research work. Authors also wish to acknowledge the suggestions and support from the colleagues Mrs.V.Sharamila, Mr.Vinayaka Jha and Mr.Rohit Jain.

References

1. “ATM: Signalling in Broadband Networks” by Uyless Black

2. “ATM Networks” by Rainer Handel and Stefen Schrodar

3. “Error Control Coding” by Shu Lin and Daniel J.

4. 1.363, 6-ISDN AAL Specification, ITU-T 5. (2.21 10, SSCOP Specification, ITU-T

6. (2.2130, SSCF at UNI, ITU-T

7. Q.2931, Basic Call Control, ITU-T

8. EMI/EMC Training Report by BEL

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