Q&A Test

Q&A on MPEG Compression Standards
and ATM Networks


By Arran Derbyshire (arad@doc.ic.ac.uk) and
K- Chandra Rajh (ck4@doc.ic.ac.uk)


Question One.

What is the MPEG (Moving Pictures Expert Group)?
  1. It is a joint commitee of the ISO (International Standardization Organisation) and IEC (International Electrotechnical Commission).
  2. It is a joint commitee of the BSI(British Standardisation Institute) and ANSI (American National Standards Institute) .
  3. It is a joint commitee of the ISO and ANSI.
  4. None of the above.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Two.

Which of these standards are specified by the MPEG?
  1. Sampling technology needed to sample digital video.
  2. Transmission scheme for video.
  3. Compression and decompression scheme for video.
  4. All of the above.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Three.

Which system/systems has ATM been designed to provide a solution to?
  1. High speed Ethernet.
  2. Optical commmunications.
  3. The B-ISDN.
  4. Frame relay networking.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Four.

Which of these following statements are true?
  1. All macroblocks within an I-frame must be coded "Intra".
  2. Macroblocks within a P-frame may either be coded as "Intra" or "Non Intra"(temporally predicted from a previously reconstructed frame).
  3. Macroblocks within the B-frames can be independently selected as either "Intra", forward predicted, backward predicted, or both forward and backward(Interpolated) predicted.
  4. B-frames are used for predictions and hence propogate errors.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Five.

Which of the following technologies compliments the ATM concept?
  1. Optical fibre.
  2. The public switched telephone network.
  3. Circuit switching.
  4. Frequency division multiplexing.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Six.

The transmission of MPEG compressed data is supported by an appropriate packet(cell) transport/adaptation protocol. This "ATM" transport format detects packet loss, helps identify the picture area in wich the compressed information was lost. MPEG transport layer has two distinct sublayers: "data link level" and "adaptation level". Which of these following statements about "data-link" are true?
  1. The data link sublayer headers contain video-specific information to aid error recovery at the decoder.
  2. The data link sublayer format is based on a "cell relay" asynchronous time division mutiplexing concept similar to that being considered for the CCITT broadband ISDN/ATM standard, supporting features such as service multiplexing and error detection.
  3. The data link sublayer formats the HP/LP prioritized MPEG bit-stream into a series of fixed length L byte cells.
  4. The data link sublayer has been designed to permit rapid decoder resynchronization after error events that result in the loss of one or more cells.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Seven.

What allows ATM to provide high speed and real time services?
  1. A large cell size.
  2. The use of MPEG video compression.
  3. High speed circuit switching.
  4. Reduced data control complexity.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Eight.

Where is error recovery provided in an ATM network?
  1. The same place as limited error control in frame relay network.
  2. In the full error control layer.
  3. At the edges of the network.
  4. In the switching nodes.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Nine.

What advantages does the asynchronous multiplexing and switching characteristics of ATM provide?
  1. The ability to handle services of different bit rates.
  2. The ability to handle services with a varying information rate efficiently.
  3. A reduced bandwidth waste.
  4. A higher data transfer rate.
A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Question Ten.

Which of these statements about error concealment by MPEG video compression is/are true?
  1. Spatial replacement attempts to estimate lost picture regions from previously received anchor frames, using the available motion information.
  2. For P and B-frames, if motion information is available, spatial replacement is always the preferred method for concealing lost data.
  3. I-frame concealment is based on the spatial interpolation approach of lost picture elements from adjacent pixels(blocks) in the same frame.
  4. The problem of blurring is eliminated using spatial interpolation.
Answer: (A) 1 (B) 4 (C) 1,2 and 3 (D) 2 and 3 (E) 3

Click Here For The Answer.

Answer To Question One.



The correct answer is A.
MPEG stands for Moving Picture Experts group. ISO and IEC in collaboration with ITU-T and ITU-R, have created joint working groups with the purpose of specifying universal standards in several communications domains, including audio-visual digital communications. One of such groups is the ISO/IEC JTC1/SC29/WG11 known as the Moving Pictures Experts Group(MPEG), which was established in 1988 with the purpose of defining a coding algorithm suitable for storage and retrieval of of moving pictures and associated audio in digital storage media (DSM). The MPEG has been responsible for the MPEG-1 and MPEG-2 standards in the past, and is currently developing the MPEG-4 standard.

Back To The Question.
Go To The Next Question

Answer To Question Two.



The correct answer is D.
The MPEG specifies a standard compression, transmission and decompression scheme for video. The basic job of MPEG compression is to take analogue or digital video signals and convert them to packets of digital information that are more efficient to transport on ATM networks. There are three main parts of MPEG specifications, namely, systems, video and audio. The video part defines the syntax and semantics of the compressed video bit stream. The audio part defines the same for the audio bitstream, while the system part address the problem of multiplexing the audio and video streams into a single system stream with all the necessary timing information. The MPEG-2 specification consists of a fourth part called DSMCC, which defines a set of protocols for retrieval and storage of MPEG data to and from a digital storage medium.

Back To The Question.
Go To The Next Question

Answer To Question Three.



The correct answer is E.
ATM has been designed to be the transfer mode of the B-ISDN (broadband integrated systems digital network) because of it's ability to transfer all data types, including broadband data, efficiently. It is not a solution to 'high speed Ethernet', it has the ability to replace Ethernet local area networks. It is also not a solution to optical communications, since optical communications are actually responcible for the ATM system concept itself. Frame relay is an older system concept to ATM, in fact ATM has partially been developed from it. Therefore, ATM is certainly not a solution to it.

Back To The Question.
Go To The Next Question

Answer To Question Four.



The correct answer is C.
There are different reduandancies present in the video signal data - spatial, temporal, psychovisual and coding. Spatial reduandancy occurs because neighboring pixels in each individual frame of a video signal are related. The pixels in consecutive frames of a signal are also related, leading to temporal redundancy. The human visual system does not treat all the visual information with equal sensitivity, leading to psychovisual redundancy. Finally, not all paramerters occur with the same probability in an image. As a result, they would not require an equal number of bits to code them, leading to a coding redundancy. In the MPEG compression standard, at the highest level of hierarchy, the video bitstream consists of a series of video sequences. Each video sequence consists of a variable number of groups of pictures. A group of pictures (GOP) contains a variable number of pictures/frames. With MPEG, the video frames are broken down into 8x8 regions called blocks. Four of these regions can be put together to create a 16x16 macroblock. The macroblocks are then grouped together into runs of macroblocks called slices.

Frames that can be predicted from previous frames are called P-frames. But what happens if transmission errors occur in a sequence of P-frames? To avoid the propagation of transmission errors, a complete frame which does not rely on information from other frames is transmitted approximately once every 12 frames. These stand-alone frames are "intra coded" and are called I-frames. There is also a third kind of frame which predicts pixel values from frames that occur both before and after it; these bidirectional frames are called B-frames. For the P-frame coding, the encoder searches the previous frame in half pixel increments for other macroblock locations that are a close match to the information that is contained in the current macroblock. If no matching macroblocks are found in the neighbouring region, the macroblock is intra coded and the DCT coefficients are encoded. If a matching block is found, the coefficients are not transmitted, but a motion vector is used instead. For the B-frame coding, the encoder searches both the previous and future frame for encoding. The B-frames themselves never used for predictions and hence do not propagate errors.

Back To The Question.
Go To The Next Question

Answer To Question Five.



The correct answer is A.
Optical fibre compliments the ATM concept because ATM requires the high error performance of optical fibre for it's reduced error control systems. The public switched telephone network is mainly unsuitable for ATM because of it's poor quality, and so it is generally not a complimentry technology. Circuit switching does not complement ATM because it is an entirely different transfer mode which is any many ways is inferior to ATM. Frequency division multiplexing is not used with ATM, time division multiplexing is. Therefore it does not compliment ATM.

Back To The Question.
Go To The Next Question

Answer To Question Six.



The correct answer is D
The data link sublayer format is based on a "cell relay" asynchronous time division multiplexing concept similar to that being considerd for the CCITT broadband ISDN/ATM standard, supporting features such as service multiplexing and error detection. The data link sublayer formats the HP/LP prioritised MPEG bit-streams into a series of fixed length L byte cells. The data link header contains generic transport information such as a priority indicator, a service ID and a sequence number. The adaptation sublayer has been designed to to permit rapid decoder resynchronisation after error events that result in the loss of one or more cells. The adaptation header contains video-specific information to aid error recovery at the decode, e.g. , frame type indicators, slice/macroblock IDs etc.

Back To The Question.
Go To The Next Question

Answer To Question Seven.



The correct answer is B.
Reduced data control complexity reduces delays in ATM networks yielding a higher transfer rate and the ability to handle real time applications. ATM actually has a relatively small cell size, set at an optimal size to limit the delay for real time services. MPEG video compression reduces redunancies in video images, thus allowing for more efficient usage of communication channels. It does not allow ATM to provide high speed communications for arbitrary data. High speed circuit switching is a completely different high speed transfer mode for which ATM is an alternative to.

Back To The Question.
Go To The Next Question

Answer To Question Eight.



The correct answer is D.
ATM provides error recovery at the edges of the network which is provided by full error control implicitly. Frame relay networks implement limited error control in the switching nodes of the network and provide error recovery at the edges of the network. ATM does not provide error recovery in the switching nodes, in order to reduce delays in the network.

Back To The Question.
Go To The Next Question

Answer To Question Nine.



The correct answer is C.
Services requiring different bit rates are loaded into cells which are lauched with other cell streams asynchronously at the correct rate. Services with a varying information rate can be handled efficiently by ATM because cells need only be sent when non-redundant data (i.e. information) is available. This ability to handle information efficiently leads to a more efficient use of network channels, and therefore a reduction in bandwidth wastage. Asynchronicity does not imply high speed transfer, rather efficient transfer. Synchronous transfer modes could provide similar transfer rates to ATM, but with less flexibility and therefore less efficiency.

Back To The Question.
Go To The Next Question

Answer To Question Ten.



The correct answer is E
Two distinct methods are available for error concealment at the decoder: temporal replacement and spatial replacement. Temporal replacement attempts to estimate lost picture regions from previously received anchor frames, using the closest available motion information. For P and B-frames, if motion information is available, temporal replacement is always a preferred method for concealing lost data. If motion vectors for the affected area are not available, they are estimated from the top and/or bottom picture regions. However, for I-frame there is no motion information, so that significant differences might exist between the current intra-coded frame and the previously decoded frame. Accordingly, I-frame concealment is based on a spatial interpolation approach where lost picture elements are interpolated with adjacent pixels in the same frame. In spatial interpolation, intra-frame redundancy between blocks is exploited, while a potential problem of blurring remains due to insufficient high order DCT coefficients for active areas.

Back To The Question.
Back To Question One

Abbreviations.

DCT --- Direct Cosine Transform.
ISO --- International Standardization Organization.
IEC --- International Electrotechnical Commission.
GOP --- Group Of Pictures.
ISDN --- Integrated Services Digital Networks.
ATM --- Asynchronous Transfer Mode.
MPEG --- Moving Pictures Expert Group.
ITU-T--- International Telecommunications Union - Telecom standards sector.
CCITT--- Consutive Commitee for International Telecommunication and Telegraphy.
DSM --- Digital Storage Media.

References.


  1. ISO IS 13818-1 -ITU-T Recomandation H.222.0, Information technology - Generic coding of moving picture and associated audio information, part 1: systems,November,1994.
  2. ISO IS 13818-2 -ITU-T Recomandation H.222.0, Information technology - Generic coding of moving picture and associated audio information, part 2: systems,November,1994.
  3. D. Le Gall. MPEG: A video compression standard for multimedia applications. Communications of the ACM, April 1991.
  4. ISO WD 13818-9 -Draft ITU-T Recomandation H.222.x, Information technology- Generic Coding of Moving pictures and associated audio information, part9: Real time Interface specifications, November 1994.
  5. Advance Television Research Consotium, "Advanced Digital Television: Prototype Description", FCC WPI certification Document,feb 1992.
  6. H. Sun and W. Kwok, "Adaptive Concealment for Block-based compression video".
  7. W. Kwok and H .Sun, "Multi-directional Interpolation for Spatial Error Concealment," to be submitted.
  8. H .Sun and j.Zdepski ,"Adaptive Error Concealment Algorithm or MPEG compressed video", SPIE Proc. Visual Comm. and Image Processing 92, Vol. 1818, Nov. 18-20, 1992, pp. 814-824.
  9. F. Kishino, K. Manabe, Y. Hayashi, and H. Yasuda, "Variable Bit-rate Coding of Video Signals for ATM Networks," IEEE J.Selected Areas in Comm., Vol 7., No. 5 ,june 1989.
  10. ASYNCHRONOUS TRANSFER MODE Solution for Broadband ISDN 2nd Edition, Martin de Prycker, Ellis Horwood Publishing.
  11. "The Lines Unleashed" in Byte - May 1996, plus articles: "The New WAN" by Salvatore Salamone, "The Price of WAN Connectivity" by Liza Henderson and "Playing the ATM Card" by Lane F. Cooper.
  12. "Asynchronous transfer mode: the ultimate broadband solution?" by M. Jeffrey, Electronics & Communications Engineering Journal June 1994.
  13. "Management of patient records with NHS local area supported by ATM" by Constantinos TSIBANIS, Department of Computer Science at Bristol.
  14. "ISDN" by Thomas A. Fine from "Recent advances in networking"
  15. "ATM Asynchronous Transfer Mode" by Brendan McKeon from "The OSI Reference Model"
  16. Is ATM the future of all global communications?
  17. Why has communications evolved towards the ATM concept?
  18. ATM transport and cell-loss concealment techniques for MPEG video.
  19. What is MPEG video compression standard?

Last modified by arad@doc.ic.ac.uk and ck4@doc.ic.ac.uk on 7th of June 1996.