Broadband Video over Twisted Pair Cabling. By Paul Kish Director, IBDN Systems & Standards NORDX/CDT

Transcription

1 Broadband Video over Twisted Pair Cabling By Paul Kish Director, IBDN Systems & Standards NORDX/CDT December 2002

2 Introduction Over the last 10 years we have seen some tremendous advances in the manufacturing technology of balanced twisted pair cabling. This is reflected in the impressive improvement in transmission performance for both cables and connecting hardware from the early days of Category 5. The recent publication of the Category 6 standard by TIA marks an important milestone in this cabling system evolution. Category 6 at least doubles the bandwidth (usable frequency range) compared with Category 5/5e cabling. Typically advances in technology lead the standards. When a standard is published, it establishes a minimum performance benchmark in the industry. There are some cabling systems on the market today that go well beyond Category 6 standard, thus providing even more impressive performance and available bandwidth. What does this mean to the end user? Does the end user need all this extra performance? After all, doesn t Gigabit Ethernet run on Category 5e? What applications can take advantage of this additional performance? For this article I will try to answer these questions by defining the benefits of improved cabling performance. One of the best places to start is to try and make sense of all the technical jargon in the cabling specifications. For example, when I do a spell check for this article the built-in dictionary tries to substitute the word crosswalk for crosstalk. Clearly, the parameters used to specify cabling performance are not in common usage in the world of bits and bytes. Cabling Performance We can understand cabling performance by looking at cabling as a black box (see Figure 1). The black box is called a channel and consists of all the components that comprise a cabling system including cables, cords and connectors. A channel is also called a link segment by IEEE and represents the physical link between the local and the remote equipment. The beauty of the black box representation is that we don t need to know much about what is inside the black box to measure its performance. The performance can be determined from a series of measurements at the input and the output of a channel. Figure 1 - Cabling performance Most of the parameters currently External Noise specified in the cabling standards can be classified as either a measure of Signal level or Noise Level. The output Signal level compared to the input Signal level is called Insertion Signal out Signal in Loss, also referred to as Attenuation. It is the most important parameter. Insertion Loss Internal Noise Near End Crosstalk and Far End Crosstalk is a measure of internal noise that is generated between pairs within Channel the same cable or within a connector. Return Loss is a measure of self generated noise on a given pair due to component impedance mismatches or due to impedance variations along the cable. External noise can come from many sources including RFI, power line transients or alien crosstalk between adjacent cables. The important thing to remember is that the overall performance of your network is determined by comparing the Signal level and the total Noise level from all noise sources at the Receiver. This metric is called the Signal-to-Noise Ratio (SNR), expressed in db. There is a direct relationship between the SNR and the Bit Error Rate (BER) performance of digital networks. The higher the Signal-to-Noise Ratio over the available bandwidth, the fewer bit errors are generated and the higher the data throughput. For Ethernet systems, information is

3 transmitted in frames (a sequence of bits with an address header) using TCP/IP protocol. It has been shown that a 1 % frame error rate can slow down the performance of your network from 100% to 20%, which is quite dramatic. In order to meet satisfactory network performance, IEEE specifies a bit error rate objective of less than 1 error in 10 billion (10 10 ) bits of information transmitted. The biggest advantage of Category 6 cabling compared with Category 5/5e is that it provides about 10 db (or ten times) higher Signal-to-Noise Ratio over the bandwidth utilized by existing Ethernet networks. All this means that Category 6 systems are less susceptible to transceiver impairments and other environmental effects such as temperature that can slow down your network. Broadband Video Transmission Now that we understand the benefits of an improved Signal-to-Noise ratio for existing Ethernet applications, what other applications are there that can take full advantage of the higher bandwidth and improved performance of Category 6 cabling? One of the most demanding applications on the market today is broadband video, commonly known as CATV or cable television. It carries a broad range of signals extending from 54 MHz to beyond 600 MHz. Coaxial cable (RG-59 or RG-6) is commonly used for these applications, primarily for home networks. Most people are not aware that high performance twisted pair cabling can also support broadband video. For this purpose, we set out to evaluate the real-time performance of Category 5e and Category 6 cabling for the broadband video application. The video source used for this evaluation was a high-resolution video signal from a commercial DVD player. Figure 2 - Broadband Video Distribution Satelite, Antenna or CATV input Patch Panel Cat 5e / 6 / 6+ UTP BB Video balun DTV & NTSC channels Combiner Distribution Hub Modulator DVD Player Video Camera... Figure 2 illustrates how broadband video can be distributed in a business environment. Potential applications would include Cable TV/ Satellite newscasts, stored or live video broadcast, distance learning, security monitoring and video conferencing. A big advantage of broadband video over twisted pair is that it has the capability to transmit many channels simultaneously using only a single pair in a 4-pair cable for broadcast video or two pairs for bi-directional video transmission. How is broadband video transmitted over the cable? A standard-definition composite video signal is composed of the video information (luminance and chroma) and the audio information. The composite video signal is then modulated on a carrier as shown in Figure 3.

4 Different channels are spaced at 6 MHz intervals. Therefore, depending on the bandwidth of the cabling, a lot of channels can be carried at the same time. A typical allocation would carry from channel 2 to channel 78 using a frequency band from 54 MHz to 550 MHz. The broadband video systems on the market today are designed to be used with 75 Ohm coaxial cables. To transmit these signals over 100 Ohm balanced twisted e.g. Channel 2 Video Carrier pair cabling requires a high quality broadband Figure 3 - Composite Video Bandwidth Allocation balun (Balanced to unbalanced transformer) at either end of the channel. These baluns adapt a 75 Ohm coaxial input using an F-connector to a balanced output using a modular 8-position connector. How far can you transmit video signals over Category 5e and Category 6 cabling? Most video receivers (e.g. TV sets) are designed to accommodate a wide dynamic range of signals. The minimum signal level at the remote television receiver is around 1 mv (-10 dbmv). For weaker signals, the picture is snowy and also much more susceptible to external noise. The maximum output level from the local amplifier is 50 dbmv, giving a dynamic range of up to 60 db for the cabling. It should be noted that the signal level may need to be reduced below 50 dbmv because of radiated emission requirements that can further limit the dynamic range for the application. 6 MHz Luminance 54 MHz 60 MHz 1.25 MHz 3.58 MHz 4.5 MHz Chroma Color Audio Figure 4 - Channel Insertion Loss including balun losses Weak signal snowy picture Cat 5/5e Cat 6 IBDN 4800LX Loss (db) Frequency (MHz) Figure 4 is a graph of the frequency range that can be accommodated for a 100-meter Category 5e, Category 6 and a Category 6+ channel based on a 60 db Insertion Loss limit. The frequency range is 400 MHz for a Category 5e and 650 MHz for Category 6+ channel. The Category 6+ channel is representative of high-end system, such as the NORDX/CDT 4800LX system which has a very low Insertion Loss and about 10 db improved SNR beyond the Category 6 standard. This is the theoretical upper limit based on signal quality. For this graph, it is assumed that the transmission characteristics of the cabling are well behaved beyond the maximum frequency specified for Category 5e (100 MHz) and Category 6 (250 MHz). Other transmission impairments and a restriction on the maximum amplified signal level can further restrict either the maximum reach (100 meters) or the maximum number of channels supported.

5 Picture Quality To simulate a worst-case scenario, we tested three 100-meter channels that were representative of Category 5e, basic Category 6 and beyond Cat 6 IBDN 4800LX cabling. We tested them for two different configurations. The first configuration evaluated the picture quality using a modulated signal at 547 MHz (Channel 78) on pair 4 with no other signals present in the cable. The second configuration evaluated the picture quality using a modulated signal at 67 MHz (Channel 4) on pair 4 and a 100BASE-TX signal on pairs 2 & 3 applied simultaneously on the same cabling. The results for the second configuration are shown in Figure 5*. Similar results were obtained for the first configuration. We observed an appreciable difference in picture quality for the three different channels tested due to the effect of signal interference or due to the effect of signal losses in the cabling. From these results, the benefits of improved cabling are visually apparent. IBDN 4800LX Cat 6 Cat 5 / 5e Figure 5 - The effect of interference and loss on picture quality *Note: All these tests were performed under the same conditions and demonstrate the relative difference in picture quality between different types of cabling. Different results can be achieved in practice depending on the amplifier settings. Future Directions Broadband video is definitely one application that can take full advantage of the improved transmission performance offered by Category 6 cabling and beyond. A low Insertion Loss and a high Signal-to-Noise Ratio are the most important cabling parameters for the broadband video application. Although the composite video signal is an analog signal today, future digital television signals will use the same broadband frequency spectrum and channel allocation but a different digital modulation scheme (MPEG-2 / 8-VSB). Digital television offers the potential to carry up to three standard definition channels or one high definition channel within the same 6 MHz bandwidth allocation. Broadband video can find all kinds of applications in a business environment including specialty news channels, video conferencing, distance learning and security monitoring. There is an advantage to having the video application running on a separate broadband network to prevent network congestion and bottlenecks. High quality video will be the next wave of applications that will have a dramatic impact on your network. Category 6 cabling is well positioned to meet the bandwidth requirements for these applications.

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