Tripping the Video Fantastic
Currently, the most popular service provided by the telephone
companies (telcos) through Asymmetric Digital Subscriber Line (ADSL)
modems is best-effort high-speed Internet access. Due to the hostile
and noisy outside plant environment, however, ADSL modems with a 6
decibel (dB) or better margin are designed to function with a
relatively high Bit Error Rate (BER) in the range of 10E-9 to 10E-10.
By comparison, the BER of fiber multiplexers is in excess of 10E-11.
Just a short time ago, the long reach of ADSL technology enabled
telcos to service customers that were 18 kilo feet (kft) away from a
Central Office (CO) on a single twisted pair. Data errors (which are
measured as Cyclical Redundancy Checks, or CRCs, of the xDSL frame) and
packet loss was acceptable for high-speed Internet access because if an
Ethernet packet was lost, the Transmission Control Protocol/Internet
Protocol (TCP/IP) in the program application of the end user's computer
would simply ask the host to resend the packet. The end user was
completely unaware of the TCP/IP packet resend request. End users
considered high-speed Internet access to be a great improvement over
slow, unreliable dial-up access.
Too soon it all changed. When the cable competitors began providing
high-speed Internet access through cable modems, they could easily add
Voice over Internet Protocol (VoIP) telephone service via cable modem.
Consumers who generally received the existing television broadcast
package provided by cable and satellite TV companies as good could now
also get telephone from the cable guys. "Why not have my cable company
deliver the entire bundle?" they rationalized.
In response, the telcos made a strategic and financial decision to
offer video services to remain competitive in the industry. But to
effectively compete and maintain subscribers, they must exceed the
existing broadcast packages offered by cable and satellite competitors
in terms of quality and content. However, this isn't easy.
IPTV Baselines Require More
Many telcos now offer Internet Protocol Television (IPTV) as the
platform for video delivery. One of the main video delivery protocols
for IPTV is Moving Pictures Expert Group-2 (MPEG-2). Downstream speeds
and IP packet loss for Standard Definition TV (SDTV) on MPEG-2 range
from 3-5 megabits per second (Mbps) with an IP packet loss of
approximately 10E-5. High Definition Television (HDTV) on MPEG-2 has
downstream speeds between 15-18 Mbps with an IP packet loss of
BER and IP packet loss are related but not the same thing. A typical
IP packet contains seven 188-byte MPEG-2 packets. So the loss of a
single IP packet is a loss of over 10,000 bits. To attain an IP packet
loss of 10E–6 requires a BER of 10E-10.
Telcos can achieve these speeds needed through a combination of
Fiber-to-the-Curb (FTTC), Very High Speed Digital Subscriber Line 2
(VDSL2), and upgrading the existing outside plant by removing bridged
Figure 1. FTTC Architecture
FTTC architecture originates with fiber from the CO to the Service
Access Interface (SAI). The subsequent copper local loop from the SAI
electronics to the subscriber is no longer than 3 kft in most cases.
VDSL2 greatly improves the downstream/upstream data rates from the ADSL
speeds. Typically, VDSL2 downstream/upstream speeds for a 3 kft loop
are 29/5 Mbps compared to that of ADSL's speeds of 1.5/.256 Mbps. Some
large North American telcos are also going through the effort of
manually removing bridged taps because of their detrimental impact on
Ring Transient Tripping
The local battery from the telco Central Office provides -48 Volts
direct current (VDC) in a telephone off-hook condition. The ringing
signal from the CO is 90 Volts alternating current root-mean-square
(VAC RMS) @ 20 Hertz (Hz) and the peak voltage of the ringing signal is
actually +127 Volts.
Figure 2. Off-hook and ringing voltages superimposed.
If a subscriber picks up the phone at the negative peak of the
ringing voltage an instantaneous voltage peak of -175 Volts (-48 from
the CO battery + -127 Volts from the ringer) is received at the
splitter. This condition is known as ring trip because the
ringing signal is being tripped or stopped when the end user picks up
the telephone when it rings. Performance of the VDSL2 modems, and
therefore IPTV quality, greatly depend on how well the splitter
performs during the ring trip event.
Conventional splitters can saturate and/or allow high-voltage and
high-frequency voltage transients to enter into the VDSL2 spectrum sent
to the xDSL modem, creating data corruption (i.e., CRCs) and packet
loss. This was not a problem for high speed Internet access because, in
the event of packet loss due to ring trip, the remote end would request
a TCP/IP retransmit request from the host end. However, with streaming
video, there is no TCP/IP packet retransmit request. The video frame
has come and gone, and has already been viewed by the end user. This
bursty, versus streaming, bit rate is a critical difference between
data and video over broadband.
To accommodate packet loss, the IPTV set-top box may insert blank
data which has the effect of macroblocking or pixelization of the
video. The magnitude of this distortion depends on such factors as the
type of screen (CRT, LCD, plasma, etc.), the size of the screen, and
image resolution. Viewers of digital broadcasts provided by the
cable/satellite TV companies may have seen this phenomenon from time to
Telcos may contend that the corrupted data is corrected by the
Physical Layer Forward Error Correction (FEC) provided by VDSL2 modems.
However, the physical layer FEC only corrects isolated bit/byte errors
and impulse noise, and there is a practical limit that can easily be
exceeded with any lengthy amount of impulse noise. A fully compliant
American National Standards Institute (ANSI) T1.413 splitter will
exhibit excessive degradation of video, or worse, may cause the VDSL2
modem to resynchronize entirely. It can be demonstrated in a lab that
there is a marked negative effect on splitter performance in the
presence of ring trip.
Clearly this is an unacceptable condition for delivering video
services. Additional higher layer levels of FEC protection (i.e., Link,
Network, Transport, Application) add significant complexity (a.k.a.
cost) to the set-top box and underlying application. More important,
they add excessive delay to the received signal. An end user is
unlikely to tolerate a 500 millisecond delay when switching between
television stations due to additional, upper layers of FEC.
Furthermore, the benefits from additional layers of FEC are needed only
on an occasional basis, yet the delay and cost of FEC is incurred all
of the time.
Splitting the Solution
A more practical and effective solution for a service provider (telco)
and system provider Digital Supplier Line Access Multiplexer (DSLAM)
manufacturer is to use high-quality video-grade splitters that
significantly reduce the transient effects from ring trip and pass the
error correction on to more complex upper layer protocols. The splitter
works in conjunction with the DSL chipset in negating the ring trip
Two types of splitters are needed: one outside the home in the
Network Interface Device (NID), and the other in the CO. A video-grade
splitter is needed to determine the system level test requirements.
Many system designers don't realize they have a problem because they
have no way to test for it, and currently there is no standard or
specification for testing the system to see how it's functioning. It
isn't until the video severely degrades or dies that they realize
there's a problem. If the problem can be identified, the service
provider must design around it.
Although some off-the-shelf products are available, most splitters
have to be custom ordered due to the variation and specific
requirements in CO splitter board dimensions. There is no standard for
DSL splitter boards so each CO splitter board is unique. Because
splitter manufacturers deal with these problems daily, they are
experienced in selecting the best splitter and providing a
configuration scheme to avoid ring trip issues.
For telcos to successfully compete with cable companies in providing
video to the home, they will have to guarantee clear and uninterrupted
programming. Before this can happen, they will have to deal with the
ring trip issue. Industry standards committees are presently working to
develop a way to test for voltage surge problems. In the meantime, the
small investment for video grade splitters can save a system provider
significant development and operational expenses, such as truck rolls,
when deploying video services over VDSL2.
As companies look for new splitters to accommodate VDSL2, they
should consider securing ones that will also alleviate a small but
nasty ring trip problem.