The zap time is the total duration from the time viewer presses the channel change button, to the point the picture of the new channel is displayed, along with corresponding audio. These kind of delays exist in all television

Television

Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

 systems, but they are greater in digital television

Digital television

Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...

 and systems that use the Internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 like IPTV

IPTV

Internet Protocol television is a system through which television services are delivered using the Internet protocol suite over a packet-switched network such as the Internet, instead of being delivered through traditional terrestrial, satellite signal, and cable television formats.IPTV services...

. Human interaction with the system is completely ignored in these measurements, so zap time is not the same as channel surfing

Channel surfing

Channel surfing is the practice of quickly scanning through different television channels or radio frequencies in order to find something interesting to watch or listen to. Modern viewers, who may have cable or satellite services beaming down dozens if not hundreds or thousands of channels, are...

.

Zap time can be very disturbing for the viewer since it has to wait too much time when decides to switch the channel. For this reason, zap time is an IPTV feature that has to be reduced in an IPTV system.

Factors

The delays in the TV channel change can be caused by different factors. They can be classified according to the systems that cause them. Consequently there are network

Television network

A television network is a telecommunications network for distribution of television program content, whereby a central operation provides programming to many television stations or pay TV providers. Until the mid-1980s, television programming in most countries of the world was dominated by a small...

 factors, MPEG acquisition factors and Set Top Box Buffering/Decode.

Network Factors

• Access network
  Access network
  An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...
  
   latency (processing/propagation delays)

1. STB – IGMP Leave channel X, Join Y
2. DSLAM – Stop X, Start Y
3. DSL FEC/Interleave
4. IGMP features used (version, fast leave, snooping, etc)
5. Availability of the channel (Channel replication point)

• Core/Aggregation network
  Core network
  A core network, or network core, is the central part of a telecommunication network that provides various services to customers who are connected by the access network. One of the main functions is to route telephone calls across the PSTN....
  
   latency

1. Multicast routing
   Routing
   Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
   
    mechanisms used
2. Availability of the channel (Channel replication point)

Actually, these are only a small component of delay in the final time. Usually they take about 50 - 200 ms of the overall zap time. Network quality of service

Quality of service

The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

(QoS) can reduce these time ensuring minimal jitter

Jitter

Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...

, latency and packet drop.

MPEG Acquisition

• Analyze data to locate MPEG Program Specific Information
  Program Specific Information
  Program-specific information is metadata about a program and part of a MPEG transport stream.The PSI data contains five tables:* PAT * CAT * PMT...
  
  

1. Wait for and parse PAT
2. Wait for and parse PMT

• Obtain conditional access
  Conditional access
  Conditional Access is the protection of content by requiring certain criteria to be met before granting access to this content...
  
   keys (ECMs) to decrypt channel: wait for ECMs – part of PMT – 100ms to 500ms

• Obtain MPEG key frame

1. I-frame (MPEG2) or IDR frame (H.264)
2. One Index frame per group of pictures
   Group of pictures
   In Video coding, a group of pictures, or GOP structure, specifies the order in which intra- and inter-frames are arranged. The GOP is a group of successive pictures within a coded video stream. Each coded video stream consists of successive GOPs...
   
    (GOP) – 12 to 30 (IBP) frames
3. Typical frequency of I-frame – 500ms.
4. Long GOP structure (2-4 seconds) saves bandwidth, but can causes significant channel change latency

Set Top Box Buffering/Decode

• MPEG Buffer: Encoder buffer fullness model (typical latency – 750ms to 2s). Wait until the buffer is full

• Decode/Display delay (typically about 50 ms)

Computing Zap Time

The factors that has been explained in the last section don't affect in the same way to the overall zap time. So in the table below there are an example of the zap time in IPTV DSL:

	Channel Change Latency Factor	Device/Location	Typical Latency	Cumulative Latency
1	Send IGMP Leave for channel X	STB	< 10 ms
2	Send IGMP Join for channel Y	STB	< 10 ms
3	DSLAM gets Leave for channel X	DSLAM/Network	< 10 ms
4	DSLAM gets Join for channel Y	DSLAM/Network	< 10 ms	~ 20 - 40 ms
5	DSLAM stops channel X, and sends Channel Y	DSLAM/Network	~ 30 – 50 ms	~ 50 – 90 ms
6	DSL Latency (FEC FEC FEC is an acronym which may refer to:*Foreign exchange certificate, a type of currency*Forward error correction, a system of error control for data transmission*Family entertainment center, a small amusement park or indoor equivalent intended for family fun... /Interleave)	DSLAM/Network	~ 10 ms	~ 60 - 100 ms
7	Core/Agg Network Latency	Router/Network	~ 20 – 60ms	~ 80 – 160ms
8	De-jitter buffer	STB	~ 300 ms	~ 380 - 460 ms
9	Wait for PAT/PMT	STB MPEG buffer	~ 125 ms	~ 500 - 580 ms
10	Wait for ECM/CA	STB MPEG buffer	~ 125 ms	~ 620 - 700 ms
11	Wait for I-frame	STB MPEG buffer	~ 250 ms to 2s	~ 870 ms – 2.7s
12	MPEG buffer	STB MPEG buffer	~ 1s to 2s	~ 1.8s – 4.7s
13	Decode	STB	~ 50ms	~ 1.9s – 4.8s

Examples

In this section some typical values of zap time are shown. Actually, in IPTV television these delays are greater than in other technologies:

• Analog (Cable
  Cable television
  Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
  
  ) ~ 1s
• Analog (off-air
  Off-the-air
  In telecommunication, the term off-air or off-the-air has the following meanings:# In radio communications systems, pertaining to a radio station or television station that is completely shut down, i.e. that is not transmitting any signal, not even an un-modulated carrier wave...
  
  ) ~ 1 – 3s
• MPEG2 over QAM ~ 1.2 – 3s
• MPEG2 over QPSK ~ 2 – 4s
• MPEG2 over IP Multicast ~ 1.5 – 3.5s
• H.264 over IP Multicast ~ 1.7 – 4s

External links

• IPTV Industry
• ITU IPTV Focus Group
• ITU-T. Zap time.