Digital Video Broadcasting DVB

1
Digital Video Broadcasting (DVB)

• Garry Walton
• DERA
• Kevin Hodson
• Delta Communication Ltd
• July 2000

2
Digital video Broadcasting (DVB)

• Covers
• Overview of DVB
• Architecture
• Detailed Description
• Make up of DVB Transmission stream
• Protocols
• IP over DVB
• Reliable multicast file transfer
• Security
• Virtual Private networks
• Conditional Access
• IP Security
• Return Channel Options
• Fielded DVB systems
• Tutorial on MPEG2 and TCPIP

3
Overview of DVB

• DVB is the European standard for digital video
  broadcasting.
• It is used extensively across the world.
• DVB is a transport mechanism for an MPEG2 encoded
  stream (more on this later) providing features
  which are not part of the MPEG2 standard.
• DVB can also carry private data and control
  messages.
• DVB is now seen as being very flexible and can
  carry Internet traffic.
• The DVB standards also specify mechanisms for
  interactive service with return channels provided
  by PSTN, ISDM, GSM, DECT, LMDS, cable and
  satellite.

4
Worldwide adoption of DVB
5
The DVB Transmission Stream

• The DVB stream consists of a series of fixed
  length packets which make up a Transport Stream
  (TS).
• The packets support streams or data sections.
• Streams carry higher layer packets derived from
  an MPEG stream.
• Data sections are blocks of data carrying
  signalling and control data.
• DVB is actually a support mechanism for MPEG.
• It carries multiple streams, some synchronised to
  each other (for lip sync), others running
  independently.
• One MPEG stream needing higher instantaneous data
  can steal capacity from another with spare
  capacity.

6
What is MPEG?

• MPEG - Moving Picture Experts Group.
• MPEG1 - compression algorithms for video and
  audio circa 1.5 Mbit/s.
• MPEG2 - compression algorithms for broadcast
  quality video and audio, 1.5 Mbit/s to 9 Mbit/s.
• MPEG3 - obsolete (was intended for HDTV but
  replaced by enhancements to MPEG2).
• MPEG4 - low bit rate compression algorithms for
  multimedia web based services including object
  representation.
• MP3 - MPEG audio compression algorithm

7
Why does MPEG need DVB?

• MPEG is primarily a compression standard.
• Although the output from an MPEG encoder can be
  carried as an MPEG stream in a noise free
  environment, it is not suitable for a noisy
  channel.
• DVB provides error correction, program
  information and conditional access signalling.
• DVB guarantees the delivery of an MPEG stream
  over Satellite (DVB-S), Terrestrial Radio Channel
  (DVB-T), Cable (DVB-C) or Local Microwave
  Distribution Service (DVB-MC).
• DVB-S takes the DVB Transport Stream, adds more
  forward error correction and modulates a carrier
  using QPSK.

8
DVB Packet
The DVB Transport stream consists of a series of
packets 204 bytes long, 188 bytes carry
information and the other 16 bytes carry an outer
Reed-Solomon code. The packet is short and can
survive a noisy channel subject to interference.
188 Bytes
16 Bytes
Information Reed Solomon Parity block
9
DVB Packet
The DVB packet has a 4 byte header carrying a
SYNC byte, a Packet Identifier (PID) as well as
other data. The sequence of packets is known as a
Transport Stream (TS).
4 Bytes
184 Bytes
Header Payload
10
What do the DVB packets carry?

• Two main types of data packets - streams and
  sections.
• Packetised Elementary Stream (PES), a series of
  packets carrying video, audio and data streams.
• Data Section - a block of data within one or more
  TS payloads carrying program information and
  other data.

11
Packetised Elementary Streams

• Carry the video channel and several audio
  channels for one program using one PES per
  channel.
• The PESs for several programs can be carried
  within the Transport Stream.
• The Packet Identifiers (PIDs) are used by the
  receiver for rapid packet de-multiplexing.
• Transport Streams can be multiplexed (active
  multiplexing means re-mapping PID numbers to
  remove duplications).
• Program Time Stamps (PTSs) which are used to lock
  the video and audio to a common Program Clock
  Reference (PCR). Stamps and references are 32 bit
  numbers with a 27 MHz resolution and long
  repetition interval.

12
PES Packet Structure
13
Data Sections

• Carry program service information (SI) within
  tables (for the receiver).
• Carry electronic program guides (EPGs for the
  viewer within data carousels).
• Carry private data - IP packets for Internet
  traffic.

14
Streams Data Sections
PES Packet
Data Section
DVB Transport stream fixed length packets. Some
carry PES packets, other carry data sections. In
general, the higher layer packet or section is
much longer than the TS packet. De-multiplexing
is by the PID number.
15
DVB Tables

• Program Association Table (PAT)
• Received on a well known PID. Tells the
  receiver where to find the PMT Program Map Table
  (PMT), by giving the PMTs PID.
• Program Map Table (PMT)
• Gives the PIDs for all the streams which make up
  a single program, including PCR.
• Network Information Table (NIT).
• Uses a well known PID and tells the receiver
  the medium (satellite, terrestrial, cable),
  frequency, polarisation etc.
• Conditional Access Table (CAT)
• Uses a well known PID and gives the PID of the
  Entitlement Management Message (EMM) for a given
  program.
• Private Tables can exist.

16
Data Section Structure

• Data sections carry
• Public DVB tables,
• Private DVB tables,
• Private user data,
• Program information (electronic program guides).
• They have their own PIDs so the receivers can
  extract the sections they need quickly.
• The following slide shows the format of a private
  data section. The data in the section shown is
  divided into 184 byte blocks, each one of which
  forms the payload of a DVB packet.

17
Private Data Section Structure
18
IP Packets within DVB

• Early methods carried IP packets within a PES.
• This was efficient however all methods were
  proprietary.
• Now there is a DVB open standard called Multi
  Protocol Encapsulation (MPE) which is universally
  adopted by all service providers.
• Not so efficient as the overhead is higher.
• The MPE standard uses a Digital Storage Media,
  Command Control (DSM-CC) data section a future
  mechanism for controlling a remote streaming
  source.

19
DSM-CC Section for MPE
20
Data comms using Internet Protocols

• Transport Layer
• TCP Transmission Control Protocol.
• UDP User Datagram Protocol.
• Network Layer
• IP version 4 Existing Internet Protocol.
• IP version 6 New Internet Protocol.
• Medium Access Layer
• Ethernet MAC packets (for cable).
• DVB DSM-CC MPE data sections.
• Return channel packets.

21
Layered Data Communications

• The transport layer provides
• Reliable communication with TCP using TCP packets
  and ACKs,
• Less reliable communication with UDP datagrams
  (packets) without ACKs.
• The network layer handles routing using IP
  packets.
• The data link or medium access control layer
  handles the characteristics of the given
  transmission medium.
• The headers are analogous to envelopes.
• TCP is used for traffic where guaranteed delivery
  is vital.
• UDP is used for video and audio streams where
  lost data is not so troublesome and for web
  screen pictures.
• A return channel (Reachback) is needed for TCP.

22
Layered Data Communications

• Layered data communication follows the classical
  seven layer ISO model.
• Nowadays, the layered model is usually compressed
  to the four layers shown in the next slide, some
  of the theoretical layers having proved redundant
  in the light of experience.
• In the model, physical communication takes place
  at the lowest data link layer (cable or radio
  link) whereas there is virtual communication at
  the higher layers, known as peer-to-peer
  communication.

23
Tasks of each Layer

• Each layer is responsible for a major aspect of
  the communications link
• The data link layer determines the physical link,
  power levels, impedance, frequency, time-slots,
  etc.
• The network layer determines the routing of the
  data.
• The transport layer handles flow control using
  acknowledgements.
• The application layer is the users application,
  e-mail, web browsing etc.

24
Layered Data Communications
application
transport
TH
TH
network
NH
NH
NH
data link
DH
DH
DH
destination
source
router
25
IP Security

• IP Security or IPSec is at the heart of
  e-commerce.
• Without IPSec, IP packets are inherently
  insecure.
• There are 2 IPSec algorithms, Authentication
  Header (AH) and Encapsulating Security Payload
  (ESP).
• IP payloads are encrypted using a block cipher.
• Keys are exchanged using the Diffie-Hellman
  algorithm.
• There are additional features such as
  authentication, digital signatures and
  non-repudiation or anti-replay (proof that a
  transaction happened between 2 parties at a given
  time).

26
IP Security

• The IP packet contains the TCP or UDP packet
  within its payload.
• The IP packet has a 20 byte header (IP version 4)
  added with routing data and other control data.
• The header must stay in the clear so public
  network routers can interpret them.
• Two modes for IPSec - TRANSPORT mode and TUNNEL
  mode.
• TRANSPORT mode protects the TCP or UDP packet.
  The cryptographic end-point is also the TCP or
  UDP end-point.
• TUNNEL mode protects the entire IP packet. The
  cryptographic end-point does not need to be the
  TCP or UDP end-point.

27
IP Security
TCP header
data
IP header
Original IP packet
TCP header
data
IPSec header
IP header
TRANSPORT mode
TCP header
data
IPSec header
IP header
IP header
TUNNEL mode
28
IP Security

• TUNNEL mode allows Virtual Private Networks
  (VPNs).
• The TCP or UDP data is wrapped in an inner
  envelope with a private VPN IP address
  corresponding to the end-user.
• The complete IP packet is then wrapped in a
  second outer envelope with a public IP address
  for the VPN/Public network gateway.
• (Note that tunnelling is also useful for a
  DVB/PSTN hybrid system to ensure that forward
  traffic is routed to the satellite rather than
  the open PSTN channel).

29
DVB Conditional Access

• DVB data is scrambled using a confidential key or
  control word as specified in the Common
  Scrambling Algorithm.
• The receiver must know the current control word
  to de-code the DVB data. The control word is
  transmitted regularly within an Entitlement
  Control Message (ECM) which is part of the
  optional TS packet adaptation field encrypted
  with a public key.
• The corresponding private key is stored on the
  subscribers smart card.
• The private key is activated over-the-air by
  means of a Entitlement Management Message (EMM)
  addressed to an individual subscriber.
• The control word can be changed every few seconds
  if required.

30
DVB Conditional Access

• The next two slides show
• how a PES packet is encrypted within a series of
  DVB packets,
• how a data section is encrypted within a series
  of DVB packets.
• One of the features of a DVB packet is to set an
  Adaptation Field (AF). This is a block of data
  (variable length) which can eat into the 184
  bytes of payload. It is used to enable a PES
  packet which is not a multiple of 184 bytes to be
  carried within a series of DVB packets.
• The data section is formatted slightly
  differently and is padded out with 0xFF bytes
  (binary ones).

31
DVB Conditional Access
Encrypted PES payload data
32
DVB Conditional Access
An encrypted Data Section
33
Reliable Multicast File Transfer

• Satellites have an inherent multicast or
  broadcast capability.
• Proprietary products are available which can take
  advantage of this property.
• Files can be transmitted in blocks using UDP/IP
  without ACKs at the transport level.
• Receivers await the end of the transmission and
  then individually request block re-transmissions
  if they were received in error (protected with a
  CRC).
• One example - StarBurst Multicast has 3
  components the proprietary protocol engine,
  server, and client. The protocol engine and the
  server or client software must be installed on
  each PC in the system.

34
DVB Forward Link Protocol Stack
TCP
TCP
IP
IP
IP
MPE
MPE
DVB TS
DVB TS
QPSK
QPSK
Hub
Client
Content ISP
35
PSTN/ISDN Return Link Protocol
TCP
TCP
IP
IP
IP
IP
IP
IP
PPP
PPP
Dial-up/ISDN
Dial-up/ISDN
Hub
Client
Content ISP
Relay ISP
36
DVB RCS Return Link Protocol Stack
TCP
TCP
IP
IP
IP
AAL5
AAL5
ATM cell
ATM cell
MF-TDMA
MF-TDMA
Content ISP
Hub
Client
37
Quality of Service

• Quality of Service (QoS) means giving priority to
  some services at the expense of others.
• With IPv4 there is no real QoS available from
  Internet services.
• IPv6 will offer QoS.
• ATM offers QoS.
• It may be necessary to ensure the PDBS network
  has some QoS implemented by an appropriate
  queuing strategy.

38
DVB Return Channel

• Although DVB is essentially a broadcast delivery
  system associated standards define several
  Interaction (or return) return channel (RC)
  options
• Terrestrial Wired
• PSTN/ISDN
• Cable
• SMATV
• Terrestrial Wireless
• GSM
• DECT
• LMDS
• Satellite
• Interactive Channel for Satellite Distribution
  System

39
DVB Return Channel Options

• Interaction channel ETSI Blue Book
• PSTN/ISDN ETS 300 801 A022 Feb 1997
• GSM ETS 301 195 Rev 1.1.1 A043 Jun 1999
• LMDS EN 301 199 Rev 1.2.1 A032 Jun 1999
• DECT EN 301 193 Rev 1.1.1 A030 Mar 1998
• Cable ETS 300 800 A023 Feb 1997
• SMATV TR 101 201 A043 Mar 1998
• General Protocols ETS 300 802 A021 Feb 1997

• Interaction Channel for Satellite Distribution
  Systems DVB-RCS001 Rev 14 (April 2000),
  supporting return channel via satellite.

40
Main features of satellite RC

• IP data carried within an ATM like cell.
• IP encapsulation follows ATM AAL5.
• ATM cell encapsulated within burst with added
  preamble and outer Reed Solomon FEC.
• Entire burst is inner coded with ½ rate FEC and
  is QPSK modulated.
• Burst is transmitted within an MF-TDMA structure.
• Frequency and time slot assigned within private
  DVB table.
• Power and frequency corrections controlled by
  private DVB table.

41
Fielded Interactive DVB systems

• Globecast - brand name Digicast - transmitting
  with Ku band DVB carriers from Brookmans Park,
  UK with PSTN/ISDN return channel (operational
  now).
• SES - brand name BBI (Broadband Interactive) with
  Ku band DVB carriers from Betzdorf, Luxembourg
  with Ka band return channels using Astra 1H and
  1K. (operational mid 2001).
• Hughes Network Systems - brand name DirecPC -
  proprietary system offering 400 kbit/s to the
  user with PSTN (GSM?) return channel
  (operational).

42
SES BBI System
43
HNS DirecPC system
INTERNET
DirecPC NOC
WEBSITE
Client Computer
44
More on MPEG and TCP/IP

• Brief overview of how MPEG works.
• Overview of TCP operating over Long Fat Networks
  (LFNs).
• LFNs (pronounced elephants) are high capacity
  long distance fibre links and satellite links.
• The product of bandwidth and transmission delay
  equates to the amount of data in the pipe.
• When the amount is too large problems arise.
• Remember that the TCP protocol was designed 30
  years ago for smaller, lower capacity networks
  than exist today.

45
How does MPEG work?

• The picture is sent as a series of frames.
• I (Interframe) frames gives a complete picture (1
  in every 12).
• P (Prediction) frames carry information on moving
  blocks (ignoring static blocks).
• B (Bi-directional) frames carry interpolated
  information on moving blocks (based on future and
  previous P frames).
• Pictorial data is divided into blocks and
  transformed using the DCT. Lower frequency
  components are allocated more bits than higher
  frequency components.
• Audio compression splits the band into several
  sub-bands and allocates bits to those with most
  energy.

46
Problems with TCP over satellites

• Problems worse with Geo-stationary satellites
• Slow start algorithm.
• Receive window size.
• Large bandwidth - delay product (LFN).
• Congestion control algorithms.
• Wrapped sequence numbers.
• End-to-end host machines control protocol.

47
Solutions for TCP over satellites

• Slow start algorithm
• its mandatory but should be eliminated.
• Receive window size
• can be increased with window scaling option.
• Congestion control algorithms
• can be eliminated or improved with, say,
  selective acknowledgements (SACK).
• Wrapped sequence numbers
• where two TCP packets with the same sequence
  number are in the pipe at the same time.
• time stamping can help.
• Conclusion - use spoofing.

48
TCP Bulk flow
1..1500
1501..3000
3001..4500
4501..4096
1..1500
1501..3000
3001..4500
4501..4096
Tx
4096
1..1500
1501..3000
3001..4500
4501..4096
Rx
49
TCP Bulk flow

• Here rx window size is 4096 bytes
• Transmitter can send one windows worth per round
  trip - OK for Ethernet but on satellite max
  throughput is

50
TCP ACKs
1..1500
1501..3000
3001..4500
1..1500
1501..3000
3001..4500
Tx
1501
3001
1..1500
1501..3000
Rx
51
TCP ACKs
1..1500
1501..3000
3001..4500
1..1500
1501..3000
1501..3000
Tx
1501
1501
1..1500
1501..3000
Rx
52
TCP slow start algorithm

• Transmit window (cwnd) starts at 1 segment
• cwnd is increased by one segment for each ACK
  received
• 1, 2, 4, 8, 16, 32
• exponential increase from very low value

53
TCP slow start algorithm
Tx
Rx
54
Cwnd versus time
55
Throughput versus time
56
Congestion Avoidance