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Guaranteed Slice Rate: A Novel ATM Service for Best Effort Video Communications Ahmed Mehaoua University of Cambridge Centre for Communication Systems Research 10 Downing Street, Cambridge CB2 3DS United Kingdom

Abstract The transport of MPEG2 video over ATM introduces several issues that must be addressed in order to cope with this problem on an end-to-end basis. These include the design of an efficient videooriented delivery service with the associated congestion control mechanisms. In this article, we present a novel video slice-based best effort service for VBR MPEG2 communications over ATM. The proposed ’Guaranteed Slice Rate’ (GSR) service makes use of an efficient audio-visual SSCS with intelligent cell marking at the source and adaptive video packet discards during network congestion.

Keywords : AAL5, SSCS, FEC, MPEG-2, QoS.

1. INTRODUCTION Different proposals have been made for selecting the type of service under which MPEG2 encoded video streams have to be transported over ATM. For constant bit rate MPEG2 streams, the CBR class of service is the natural choice [1]. Even in this case, the scheduling algorithm employed by the switches may influence the overall quality significantly. For the open-loop (i.e. variable bit rate) case, three main approaches have been proposed. The statistical service with rate renegociation tries to maximize the multiplexing gain by capturing the VBR nature of MPEG2 [2][3][4]. According to this approach, the effective bandwidth of the source during a specific time interval is used in order to allocate resources in the network. If enough resources are not available the quality gets degraded and in that sense, the service is

statistical. The rate is renegotiated in the long time scale and the way the renegociation points are selected depends on the designed algorithm. The second approach supported by a feedbackbased available bit rate service, uses feedback information in order to change the coding rate at the output of the MPEG2 encoder to suit the available network bandwidth [5][6]. In this approach, the service is considered best effort with some minimum guarantees. In the last approach, which provides statistical service without any guarantees, like the one used in the Internet today, the overall quality relies totally on the load of the network and the switch cell drop policy during congestion [7]. Thus, no QoS can be guaranteed at all. This last ‘truly’ best effort approach makes use of the Unspecified Bit Rate (UBR) class of service as defined in [8]. This service category is expected to be a popular service for a variety of applications. Nevertheless, ATM standards do not specify any congestion control mechanisms for the basic UBR service. Switches are allowed to discard cells when their buffers overflow. The absence of network and application based congestion control can lead to poor end to end performance for UBR based applications. As a result, competitive UBR implementations are expected to enhance the classical UBR service with intelligent tagging and per-VC buffer management policies. These service improvements have been successively named UBR+ and 'Guaranteed Frame Rate’ (GFR) [9]. GFR hasbeen recently proposed in the ATM Forum to ensure minimum rate guarantees to UBR virtual circuits. The rate guarantee is provided at the frame level rather than at the cell level [10]. GFR also guarantees the ability to share

any excess capacity fairly among the GFR VCs by means of per-VC scheduling and source policing. Therefore in this paper we propose enhancements to the UBR service to better cope with MPEG2 video characteristics. This optimized best effort service, referred to ‘Guaranteed Slice Rate’ (GSR), aims to minimize video slice loss and to reduce bad throughput crossing the network. To achieve these challenges, the proposed service relies on three components: a new audio visual Service Specific convergence Sublayer with Forward Error Correction (i.e. FEC-SSCS), a Dynamic and Extended video cell marking at the source (i.e. DexPAS), and an intelligent Partial Slice Discard policy with FEC mechanism support (i.e. FEC-PSD). The article is organized as follows. Section 2 is devoted to the Dynamic and Extended Priority Assignation Scheme ‘DexPAS’. In Section 3, we present the Audio-Visual SSCS with FEC capability, FEC-SSCS. Section 4 emphasizes the multi-level cell drop mechanism called ‘FEC-PSD’, which takes benefit of both Dex-PAS and FEC-SSCS. Finally, we conclude in section 5. 2. DYNAMIC AND EXTENDED PRIORITY ASSIGNATION SCHEME (DEXPAS). To better cope with hierarchical MPEG2 video transmission requirements, we propose to extend ATM marking and prioritization capabilities. In [11], a novel virtual control field, located in the ATM cell header, has been proposed and referenced as Extended Cell Loss Priority (ExCLP). This field comprises the classical CLP bit and the adjacent PTI ATMuser-to-ATM-user bit (AUU). Manipulated individually these two single bits define only three distinctive cell types: high priority cell (i.e. ‘00’), low priority cell (i.e. (‘01’) and End Of Message (EOM) cell (‘10’). If we gather them, we better use the cell header by the definition of up to four different cell types within a single virtual circuit. In this paper, we propose to merge the Extended priority Assignation Scheme (ExPAS) derived from the ExCLP field with the Dynamic Priority Assignation Scheme introduced in [12]. This new source cell marking strategy, called ’Dynamic and

Extended Priority Assignation Scheme’ (DexPAS), permits the detection of multiple packet boundaries at different scale (i.e. AAL5 and MPEG2 Packetized Elementary Stream (PES) levels). The mechanism is simple to implement and sufficiently generic to be performed at any MPEG data layer (e.g. frame, slice, macroblock, or block). In this paper, the emphasis is on the slice and Control Block levels as defined in section 3. DexPAS dynamically assigns priority to the cells in respect to the MPEG picture type (e.g. (I)ntra (P)redictive or (B)i-directional predictive coded frame) and the current network congestion level (i.e. by means of feedback switch notifications). Table 1 presents the mapping of MPEG data frames onto the DexCLP field. The classical CLP mechanism restricts the priority levels to two and under utilizes ATM capabilities, whereas DexPAS extends it to up to four. Cell Type

CLP

PTI-AUU

Priority

Intra / Predictive Predictive/Bidirectional

0

0

High

0

1

Low

End of Control Block

1

0

high

End of Video Slice

1

1

high

Table 1 - DexCLP Field Mapping

Cells belonging to Intra-coded frame have the highest priority level , whereas B-frame cells have the lowest priority. Regarding to P frames, they are alternatively assigned a High or a Low priority depending on the network load. Cells having their DexCLP field initialized to '01' is referenced as 'End of control Block' (EOB). They delimit a group of subsequent video cells associated with a cell Drop Tolerance (DT). The DT is determined by the used grouping mode at the FEC-SSCS. Currently, the PTI ATM-user-to-ATM-user bit is reserved to indicate whether it is the last cell of an AAL5 Protocol Data Unit (PDU). We propose to preserve this flag to distinguish between successive video slices encapsulated in separate AAL5 PDU. Therefore, the cells having their DexCLP flag set to '11' are named ‘End Of video Slice’ (EOS). Finally, DexPAS takes advantages of both static I/PB and static IP/B priority partition in preserving critical MPEG2 data (i.e. Intra and Predictive frames), while extending ATM marking capability.

3. AUDIO VISUAL SSCS WITH FEC (FEC-SSCS). The key factor that controls the end-to-end performance is the ATM adaptation layer (AAL). AAL type 5 is currently the most commonly used by ATM network interface cards and signaling protocols. However as defined in [13], AAL5 is inappropriate for variable bit rate video and requires enhancements. AAL5 was initially designed for losssensitive applications that make use of reliable transport protocols. These Transport protocols commonly handle data losses and errors by means of retransmissions, which is useless with real-time video applications. Similarly, AAL5 only provides error detection through CRC-32 parity check and PDU length mismatch. Due to the lack of more sophisticated error detection and recovery features, AAL5 is unable to identify the positions of lost cells within a PDU. Thus, when corrupted AAL PDUs are detected, the entire packet is discarded or can be forwarded to the upper layer together with an indication of corruption (i.e. marking the Reception Status (RS) parameter of the indication primitive). Therefore, in this paper we propose enhancements to AAL5 with the introduction of Forward Error Control and Correction (FEC) capabilities at the Service Specific Convergence Sublayer (SSCS).

respectively get exactly 12, 59, 106 and 153x48-byte ATM cell payloads. Application Layer

Compressed Video Slice 6 bytes

MPEG2 System Layer

MPEG-2 PES Layer

PES packet 188 bytes

MPEG-2 TS Layer

TS Packet

+

TS Packet

TS #1

TS Packet

+

TS #2

TS #3

2 bytes FEC-SSCS

2 bytes Service Specific Convergence Sub-layer PDU 8 bytes

ATM Adaptation Layer Type 5

CPCS

Common Part Convergence Sub-layer PDU 576 bytes

SAR

1

2

3

4

5

6

7

8

5 bytes ATM Layer

Cell

9

10

11

12

53 bytes Cell

Cell

..........

Cell

EOS

Exactly 12 ATM cells (no padding)

Figure 1 - AAL5 multi-level FEC-SSCS using SGM_3 packet grouping mode

At the connection setup, the grouping mode is negotiated between the source and the destination according to the requested quality of service. SSCS Group Mode (SGM)

SSCS Group Size(SGS)

Drop Tolerance in cells

SGM_3

3

12

SGM_15

15

59

SGM_27

27

106

SGM_39

39

153

Table 2 - SSCS Grouping Modes and associated Drop Tolerance.

3.1 The SSCS Grouping Modes

3.2 The Forward Error Correction Codes

As illustrated in Figure 1, MPEG2 Transport Stream (TS) packets are passed to the SSCS by the MPEG-2 System Layer using message mode service with blocking/deblocking internal function [13].

A two-byte header and a two-byte trailer information are appended to every SSCS SDU. The header is composed of a 4-bit Sequence Number (SN), a 4-bit Sequence Number Protection (SNP), a 4-bit Payload Type (PT), and a 4-bit Control Block Length (CBL). The trailer is composed of a 2-byte Forward Error Correction field applied only to the payload.

Let us define four grouping modes at the SSCS (i.e. 'SSCS_Group_Mode’ (SGM)), which ensure integer numbers of ATM cell payloads at the SAR layer with no byte stuffing. For every mode, a number 'N' of MPEG-2 TS packets are grouped to built a SSCS SDU. As illustrated in Table 2, 'N' parameter may have the following values: 3, 15, 27 and 39. After appending the SSCS header/trailer and CPCS trailer information, we

The FEC scheme uses a Reed-Solomon (RS) code [14], which enables the correction of up to 4 erroneous bytes in each block of 564 bytes (e.g. 3x188). The addition of a sequence number modulo-16 of 4 bits enables the receiver entity to detect and locate up to 15 consecutive SSCS PDU losses. When losses are detected, dummy bytes are inserted in order to preserve the bit count integrity at the receiver

side. The SNP contains a 3-bit CRC generated using the generator polynomial g(X)=X3+X+1, and the resulting 7-bit codeword is protected by an even parity check bit. The SNP field is then capable of correcting single bit errors and detecting multiple bit errors. The PT field specifies the type of embedded information for discrimination purpose (I-frame, P-frame, Bframe, Audio, Data, Headers, FEC information).

necessary at the sender and the receiver. The destination checking process is also pipelined and the correct SSCS PDUs are immediately transmitted to the upper layer with no latency.

3.3 The Control Blocks (CB)

After calculating the correction codes, the SSCS-PDU are forwarded to the Common Part Convergence Sublayer (CPCS). The 8-byte CPCS trailer information is appended to the CPCS SDU and no byte padding is required. The resulting CS PDU is passed to the Segmentation And Reassembly (SAR) layer. The under-laying SAR protocol will subsequently segment the CS-PDU into exactly twelve (12), (59), (106) or (153) 48-byte ATM SDUs depending on the SGM. These numbers represent the Drop Tolerance (DT) for a Control Block (CB). The ATM layer will then marked the CLP field of every cell using the ‘AUU’ and the ‘SCLP’ parameters of the AAL_DATA_Request [13].

Let us define a Control Block (CB) at the SSCS level as a two dimensional matrix of ’P’ cells column by ’M’ rows onto which consecutive fixed length SSCS PDUs are written row by row (see Figure 2). P

(cell Payloads)

Virtual Column

SSCS Header

AAL-SDU

RS-FEC

CPCS Trailer

1

SSCS Header

AAL-SDU

RS-FEC

CPCS Trailer

2

AAL-SDU

RS-FEC

CPCS Trailer

SSCS Header

M

3

SSCS Header

4 5 . .

SSCS Header

AAL-SDU

RS-FEC

CPCS Trailer

M-1

SSCS Header

AAL-SDU

RS-FEC

CPCS Trailer

M

XORing Results ca lculated per Column and at the Cell basis

Since we are dealing with variable length encoded video slices, it is unlikely to have an exact number of SSCS SDUs to fill up the last virtual matrix. Therefore, we propose to specify the current length of the Control Block (CBL) in the SSCS trailer.

M+1

Writing and Reading Order Localisation (Parity and Sequence Number Checks)

4. PARTIAL VIDEO SLICE DISCARD WITH FEC SUPPORT (FEC-PSD).

Correction (RS and XOR Codes) Erreonous or Lost Cell

FEC Information

Figure 2 - The Virtual Control Block structure.

The corresponding Common Part convergence Sublayer (CPCS) trailer is then appended to each SSCS PDU. A single redundancy row is then appended at the tail of the virtual matrix, which is obtained by simply XORing the columns at the cell basis [15]. A single cell loss per column can then be recovered at the destination using redundant codes. The parameter ’M’ which represents the height of the matrix, is referenced as ’Control Block Length’ (CBL) and is negotiated at the call set up with reference to the protection level requested by the application. Lower is its value and higher is the recovery power of the FECSSCS protocol. The drawback is a proportional increase of the control information overhead. Since the FEC information is obtained using XORing method, the data matrix is only a virtual data structure and no buffering is

In [16], we have proposed enhancement to the Partial Packet Discard (PPD) mechanism to support Forward Error Correction mechanisms. The new scheme, called ’Partial video Slice Discard with FEC support' (FEC-PSD) performs at both Control Block (CB) and video slice levels. Unlikely to the classical PPD [17], FECPSD will stop cell drops as early as the switch congestion stops and the Drop Tolerance is not exceeded. Therefore, the proposed switch mechanism reacts intelligently to the congestion by allowing partially discarded video slices to be transmitted and recovered at the destination using FEC codes. FEC-PSD scheme runs per-Virtual Circuit and employs four state variables and one counter to ensure fairness among video connections. Two of them are associated with the slice level and the remaining ones with the control block level.

International Conference on Telecommunication Systems Modeling and Analysis, Nashville, USA, March 1997, pp. 425-430.

5. CONCLUSION In this paper, we have proposed several enhancements to the classical UBR service to support best effort MPEG2-encoded video communications over ATM. The newly defined service named Guaranteed Video Slice Rate (GSR) better takes into account MPEG2 properties and requirements. The service relies on three optimized components, which are: a novel Audio-Visual SSCS providing cell loss detection, localization and correction at the AAL5; an extended and video slice-based priority data partition mechanism and an intelligent partial video packet discard scheme. The objective of this best effort video service is twofold, firstly offering a simple to use and cost-effective delivery service for delay flexible and loss tolerant video applications (i.e. video-security, videophony, multimedia email, video clips retrieval, … ), and secondly optimizing network bandwidth utilization by eliminating bad throughput. 6. REFERENCES

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