The Interconnected Home - Getting Wired for the Digital Era - Part 2

Ethernet

The best system for connecting all of the digital devices in a home is an Ethernet Local Area Network (LAN, see Part 1). It's best because it is universal-almost all digital devices can connect to it. The following discussion is intended as a guide to understanding the basics of Ethernet LANs. It includes some helpful definitions.

Ethernet Standard

The development of standards for LANs and for the cables used to build them has made it possible for the explosive growth of computer-based communication and control. Ethernet was developed in the 1980s and has remained the major standard for transmitting data. The Ethernet standard was published by a committee of the IEEE as 802.3. It is a series of rules that allow all interconnected digital devices to talk with each other. Its beauty, and the reason for its long survival, is that it can continue to be used, even as equipment gets much more sophisticated and transmission speeds go up by factors of 10.

The IEEE 802.3 Standard follows a structure specified by the OSI (Open Systems Interconnection) model, which reduces every design and communication problem into seven layers, each with assigned functions. Ethernet uses two of the layers: the data link and the physical link.

• Data link layer. The data link layer specifies a system for conveying information with digital bits (1s and 0s). The data link protocol specifies that the basic unit for transmitting digital messages is a group of individual digital bits, called frames. The frame is a sequence of bits in a specified order, signaling the start, MAC address of the message destination, MAC address of the message source, actual message content, plus a few other signals. Therefore a whole series of bits has to be transmitted in order to send one piece of useable information.
• Physical layer. The physical layer specifies the type of cable-four unshielded twisted pairs, the type of connector (RJ45) and pin layout, voltages used for the bits, and some other physical details.
• MAC address. Every device, such as a computer, HDTV, or a lighting controller, connected to an Ethernet network receives a unique Media Access Control (MAC) address. The MAC address of the sender and the MAC address of the receiver are both coded into every frame, so the message will be sent to its intended destination and the device at the receiving end will know where the message came from.

Ethernet Switch

An Ethernet switch is a piece of electronic gear that makes it possible to have many branches connected to the same LAN. It's like a central post office. It has a number of ports, each with an RJ45 connector for a separate branch of the network. When it receives a message at one of the ports, it sends that message to all of the others. When it finds a port with the right destination MAC address, the message is delivered. The switch remembers which MAC address is connected to which port, so the next time a message with that address arrives, the switch will send it directly to the correct port.

Internet

Connections can also be made between any of the devices on the LAN and the Internet. That is done by connecting one of the ports on the Ethernet switch to another type of switch called a router. The difference between these two switches is that routers connect two different types of networks - Ethernet and Internet, for example. Every device connected to the Internet has a unique Internet Protocol (IP) address, so that it can be located. The Internet router sets up communication between any MAC address on the LAN and any IP address on the Internet. It is very common to have an Ethernet switch in the same package as a router. This combination will have several Ethernet ports and one Internet port.

Definitions

• Frequency is one of the basic parameters used to describe cable. Frequency is simply the number of times per second the voltage and current signals reverse themselves from positive to negative and back again. If the frequency becomes too high for the pair of wires, the signal will not reach its destination without distortion.
• Bits . Digital signals are made up of two different voltages standing for 1s and 0s arranged in a pattern that encodes the information being transmitted. These 1s and 0s are called bits. Designers are constantly looking for ways of sending more bits per second by coming up with clever coding schemes that don't raise the frequency.
• Bandwidth. The electrical signals can be transmitted on a cable at a frequency at least up to what's specified in the category standard as the rated bandwidth. The actual frequency, which can be higher than the rated bandwidth depends upon measurements of signal quality taken between the connectors at both ends of the cable. The standard length for measurement of category cable is 100 meters (328 ft.), but for residential settings cable runs are usually no more than 50 meters (164 ft.). The shorter the length of the cable, the higher its actual bandwidth.
• Throughput. Within these limits, there can be significant differences in the rate at which information can be transmitted, since information is encoded in bits. The number of bits per second can vary according to the scheme that is used to encode them. Bit information can be sent using different techniques such as modulating amplitude, frequency, or phase. Bit rate is called throughput and is given in bps (bits per second), for example, 100 Mbps (megabits) is a 100 million bits per second. The bit rate is dependent on the particular system you're using to encode the information - how many bits can be coded into a frame and how many frames per second can be sent without distortion.

The Category System

In the 1990s, the Telecommunications Industry Association (TIA), an offshoot of the Electronic Industries Alliance (EIA), developed a set of standards for Ethernet cables-TIA/EIA-568. One of the many important outcomes of this work was the decision to use a standard cable arrangement, which would consist of four twisted pairs, each pair being twisted separately. This was the start of the category standards for UTP high-speed cable. They also described a standard connector for these eight wires - RJ45.

The purpose of twisting is that the magnetic fields around each wire of the pair cancel each other so that electrical interference from and to outside sources is minimized. Each pair also has a different twist length to limit crosstalk between the pairs. The degree to which this is effective is called balance. Higher throughputs require better balance, which improves with each increase in category rating.

UTP is an inexpensive, easy to install method of providing high-speed data connections between digital devices. The development of UTP cable standards by TIA has paralleled the development of Ethernet standards by IEEE. The initial category rated UTP cable was CAT 3, which was designed to transmit Ethernet data at 10 Mps (megabits per second). Its specified bandwidth is 20 MHz. The corresponding Ethernet protocol was called 10BASE-T. The 10BASE-T Ethernet standard was approved by the IEEE in 1990 and the CAT-3 standard for wiring 10BASE-T LANs was approved by the TR-42 committee in 1991.

Category 3 cable uses two of the four cable pairs and is no longer used in new installations because category 5 was approved in 1995 to transmit data at 10 times the rate of what CAT 3 could handle. 100 Mps (100BASE-T) Ethernet was approved in the same year.

The general trend has been towards developing faster and faster Ethernet systems and cables to match.

What Do the Category Ratings Really Mean?

It is important to understand that if two UTP cables from two different manufacturers both meet the CAT 6 standard, this does not mean that the two cables will have identical performance. It is possible that one may be able to run a particular application but the other may not. However the category rating means that if, say you want to run 1000BASE-T Ethernet, you are guaranteed that CAT 6 cable will do the job-assuming all of the other components in your system, such as plugs and jacks are also rated for category 6 and that the installation is done properly. This means that the wires are untwisted at the connector by no more than the specified length, for example. Also, care has to be taken that the pin assignments at each end of the cable match properly. But it's also possible that some cables rated CAT 5e might work. It's important to understand that the cabling standards assure a minimum level of performance. It's also important to keep in mind that the performance specs in the table are for a cable 100 meters (328 ft.) long. Shorter cables have higher bandwidth - the IEEE 802.3 Ethernet working group recognizes that although category
6 is rated 1 Gbps, short lengths can be run at 10 Gbps. The performance at lengths less than 100 meters, however should be confirmed by testing. There are portable instruments on the market that can perform these tests. Another safe precaution when selecting cable for a particular application, is to check with the application provider to be sure that your cabling system is adequate.

Summarizing

To summarize - the importance of these standards is that you can be sure that when you buy a cable with a category label it will meet the requirements of the Ethernet standard as specified in the table. Not only that, but each cable is backwards compatible so that CAT-6A, for example, can be used for any application that runs on Categories 6, 5e, 5, or 3. Installing category rated cable throughout a home will enable all of the digital devices to easily interact with each other and be coordinated by any connected computer.

As far as best performance now and in the future, it would make sense when rewiring an older home or building a new one, to use category 6A cable since it will be compatible with the high-speed systems that are coming down the road and it will still be capable of running older, slower devices. Some experts think that CAT 6 will good enough for home systems in the near future and that people might find it hard to justify the price difference between 6A and 6. However it might be less expensive in the long run to install 6A now rather than discovering 5 to 10 years from now that CAT 6 won't support the latest high tech equipment.