A number of standards have
been developed to assist in the transfer of Audio and video signals
over ATM. This chapter covers the following protocols:
MPEG-2
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MPEG-2 is a generic method
for compressed representation of video and audio sequences using a
common coding syntax defined in the document ISO/IEC 13818 by the
International Organization for Standardization. The MPEG-2 Video
Standard specifies the coded bit stream for high-quality digital
video. As a compatible extension, MPEG-2 Video builds on the
completed MPEG-1 Video Standard (ISO/IEC IS 11172-2), by supporting
interlaced video formats and a number of other advanced features,
including support for applications such as Direct Broadcast
Satellite, Cable Television and HDTV.
The ability of ATM to
support voice, video and data simultaneously, makes it an excellent
candidate for MPEG implementations. In December 1995, the ATM forum
issued the Video on Demand (VoD) Specification 1.0, which specifies
the implementation of MPEG-2 over ATM. This implementation supports
the transport stream MPEG coding, using AAL5 for user data and
Signalling 4.0 stack for call control.
MPEG-2
Transport Stream Header Structure
The structure of the MPEG-2
Transport Stream header is shown in the following
illustration.
4
|
8
|
Sync byte
|
Terror
|
Pay |
Trans |
|
PID (13 bits)
|
TSC |
AFC |
Continuity counter
|
Data
|
MPEG-2
Transport Stream header
Sync byte Fixed 8-bit field with the value of 0100 0111.
TError (Transport error
indicator) Indicates the
presence of at least 1 uncorrectable bit error in the associated
transport stream packet.
Pay (Payload unit start
indicator) 1-bit flag with
normative meaning for transport stream packets.
Trans (Transport
priority) 1-bit priority of
the packet compared to other packets of the same PID.
PID 13-bit
field indicating the type of data stored in the packet
payload.
TSC (Transport scrambling
control) Indicates the
scrambling mode of the Transport stream packet payload.
AFC (Adaptation field
control) Indicates whether
this transport stream packet header is followed by an adaptation
field and/or payload.
Continuity counter 4-bit field incremented with each Transport Stream packet
of the same PID.
Data byte 8-bit field containing data.
MPEG-2
Program Stream Header Structure
The structure of the MPEG-2
Program Stream header is shown in the following
illustration:
Pack start code |
32 bits
|
"01" |
2 bits
|
System clock reference base
|
3 bits
|
Marker bit |
1 bit
|
System clock reference
base |
15 bits
|
Marker bit |
1 bit
|
System clock reference base |
15 bits
|
Marker bit |
1 bit
|
System clock reference |
9 bits
|
Marker bit |
1 bit
|
Program mux rate |
22 bits
|
Marker bit |
1 bit
|
Marker bit |
1 bits
|
Reserved |
5 bits
|
Pack stuffing length |
3 bits
|
Stuffing byte |
8 bits
|
MPEG-2 Program Stream
header
Pack start code The string 0X000001BA identifying the beginning of a
pack.
System clock reference
base Indicates the intended
time of arrival of the byte. Contains the last bit of the system
clock reference base as the input of the program target
decoder.
System clock reference extension
field Indicates the number of
periods of a 27 MHz clock after a 90 kHz start.
Marker bit 1-bit field with the value 1.
Program mux rate 22 bit integer specifying the rate at which the P-STD
receives the program stream during the pack in which it is included.
This is measured in units of 50 bytes per second.
Pack stuffing rate Number of stuff bytes following this field.
Stuffing byte Fixed value that can be inserted by the encoder to meet
the requirements of the channel (for example). It is discarded by
the decoder.
DVB
ETS 800 300
Certain implementations suitable for Digital
Video Broadcasting (DVB) broadcasting systems are supported by CATV
infrastructures. Specifically, implementations of the Return Channel
for interactive services are supported by CATV. DVB involves a
standard link.
The format of the DVB packet is shown in the
following illustration:
Mpegheader (4) |
Upstream
marker (3) |
Slot
number
(2) |
MAC flag
control
(3) |
MAC flag
(26) |
Ext. flags
(26) |
MAC message
(40) |
MAC message
(40) |
MAC message
(40) |
Rsrvc.
(40) |
DVB packet
structure |
Mpeg header 4
byte Mpeg-2 transport stream header as defined in ISO 13818-1 with a
specific PID designated for MAC messages. The value of this PID is 0
x 1C. The transport scrambling control field of the MPEG header is
set to 00.
Upstream
marker 24 bit field, 3 byte marker that provides upstream
QPSK synchronization information. At least one packet with
synchronization information must be sent in every period of 3 msec.
The definition of the field is as follows:
- Bit 0: Upstream Marker
Enable:
Possible valuues 1Slot marker pointer is
valid. 0Slot marker pointer is not valid.
- Bits 1 - 3: MAC Message
Framing - Bit 1 relates to the first MAC message slot
within the MPEG frame, bit 2 to the second MAC message within the
MPEG frame, and bit 3 to the last MAC message within the MPEG
frame. Possible values:
0 - A MAC message
terminates in this slot. 1 - A MAC
message continues from this slot into the next, or the slot is
unused. If the slot is unused, the first two bytes of the slot are
0 x 0000.
- Bits 4 - 7:
Reserved
- Bits 8 - 23: Upstream
Slot Marker Pointer - A 16 bit unsigned integer which
indicates the number of downstream ?symbol? clocks between the
next Sync byte and the next 3 msec time marker. Bit 23 is
considered the most significant bit of this field.
Slot Number4 A 16
bit field which is defined as follows:
- Bit 0: Slot Position
Register Enable (msb)
Possible valuues 1Slot marker
pointer is valid. 0Slot marker pointer is not valid.
- Bits 1-3:
Reserved
- Bit 4: Set to
the value ?1.? This bit is equivalent to M12 in the case of OOB
downstream.
- Bit 5: Odd
Parity - This bit provides odd parity for upstream slot position
register. It is equivalent to M11 in the case of OOB
downstream.
- Bits 6 - 15:
Upstream Slot Position Register - 10 bit counter which counts from
0 to n with bit 6 the msb. These bits are equivalent to M1 - M10
in the case of OOB downstream.
MAC flag
control 24 bit field (b0 (msb), b1, b2 . . . b23) that
provides control information used in conjunction with the ?MAC
Flags? and ?Extension Flags? fields. The definition of the MAC Flag
Control field is as follows:
- b0 - b2 -
Channel 0 control field.
- b3 - b5 -
Channel 1 control field.
- b6 - b8 -
Channel 2 control field.
- b9 - b11 -
Channel 3 control field.
- b12 - b14 -
Channel 4 control field.
- b15 - b17 -
Channel 5 control field.
- b18 - b20 -
Channel 6 control field.
- b21 - b23 -
Channel 7 control field.
MAC flags 26 byte
field containing 8 slot configuration fields (24 bits each) which
contain slot configuration information for the related upstream
channels followed by two reserved bytes. The first 3 bytes
correspond to MAC Flag Set 1, the second 3 bytes to MAC Flag Set 2,
etc.
Ext. flags A 26
byte field used when one or more 3.088 Mbit/s or 6.176 Mbit/s
upstream QPSK links are used. The definition of the Extension Flags
field is identical to the definition of the MAC Flags field (above).
The Extension Flags field contains the MAC Flags from 9 to
16.
MAC message The
MAC Message field contains a 40 byte message in hexadecimal
code.
Reserved field C
(Rsrvc.) Reserved Field C is a 4 byte field reserved for
future use.
DSMCC
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The Digital Storage Media
Command and Control (DSM-CC) specification is a set of protocols
which provides the control functions and operations specific to
managing ISO/IEC 11172 (MPEG-1) and ISO/IEC 13818 (MPEG-2) bit
streams. The concepts and protocols are, however, considered to
apply to more general applications. (Compliant with ISO standard
13818-6 6/12/95.)
The format of the header is
shown in the following illustration:
8
|
16
|
Protocol discriminator
|
DSMCC type
|
Message ID
|
Transaction ID (32
bits)
|
Download ID (32
bits)
|
Reserved
|
Adaptation length
|
Message length
|
DSMCC header
structure
Protocol discriminator This field indicates that the message is an MPEG-2
message.
Dsmcc type MPEG-2 DSMCC type. Possible types are: UN
configuration. UN primitive. UU configuration. UU
primitive.
Message ID The message type.
Transaction ID A field used for session integrity and error
processing.
Download ID An optional field replacing the transaction ID fields if
the message type is a download message.
Reserved A
reserved field, the value of which is always set to zero.
Adaptation length This field indicates the length of the adaptation
part.
Message length The length of the message including the adaptation
part.
ATM Circuit
Emulation
Circuit Emulation was
developed to facilitate the transmission of constant bit rate (CBR)
traffic over ATM networks. Since ATM is a packet- rather than
circuit-oriented transmission technology, it must emulate circuit
characteristics in order to provide support for CBR traffic. The
goal of Circuit Emulation is to connect between CBR equipment across
an ATM network, without the CBR equipment realizing it.
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