Because residential networks connect such precise and complicated machines as computers, the networks that connect them must be built to exact standards.
Manufacturers agree upon those standards and every piece of equipment in the network must also conform to the standard requirements. Otherwise, the network will not work. Thus the cabling and associated devices must be installed according to the standard, with very little margin of error.
In this lesson, we'll focus first on the primary network cabling standard, EIA 568 and basic networking concepts. Then, we'll cover the new residential cabling standard EIA 570.
One of the most common terms you'll encounter when designing any type of data network is structured cabling. Structured cabling generally refers to a network cabling system that is designed and installed according to pre-set standards. The expected benefits of structured cabling are the following:
1. Buildings, new or re-furbished, are pre-wired without needing to know future occupants' data communication needs.
2. Future growth and re-configuration accommodated by pre-defined topologies and physical specifications, such as distances.
3. Support of multi-vendor products, including cables, connectors, jacks, plugs, adapters, baluns and patch-panels.
4. Voice, video and all other data transmissions are integrated.
5. Cable plant easily managed and faults readily isolated.
6. All data cabling work can be accomplished while other building work is taking place.
By far the most commonly used standard for structured wiring systems is EIA/TIA-568, or 568A. This standard is important even for residential networks because it is the source document that the Residential Cabling Standard (ANSI/TIA/EIA-570-A) is derived from.
Some of the parameters of EIA/TIA 568 are the following:
90 meter horizontal distance limit between closet and desktop
4 pairs of conductors to each outlet all must be terminated
25-pair cables may not be used (crosstalk problems)
May not use old wiring already in place
Bridge taps and standard telephone wiring schemes may not be used
Requires careful installation procedures
Requires extensive testing procedures
Category-rated cable is used for high-speed data transmission, and is increasingly being used for other communications uses. For several years Cat. 5 cabling was the primary cable being used for networking. Recently, many installers have begun using Cat. 5e (e for extended) and Cat. 6 cabling. There is a standard for Cat. 5e cabling, but at the time of this writing, there isn't a standard for Cat. 6 cabling. (A problem we have covered elsewhere.)
In order to accomplish high-speed transmission, category cabling requires very tight design characteristics. In particular, the conductors in these cables must be very tightly twisted. Furthermore, this tight twist pattern can not be altered during installation. Even at terminations, no more than 13 millimeters (about inch) of conductor can be unwound. While working with a large number of cables terminating at one block, this can be difficult.
The minimum bending radius for Cat. 5 cables is typically four times the cable diameter. This is not especially difficult for installers, although it can be difficult in tight areas. When being pulled into place, not more than 25 pounds of tension can be applied to the cable. One easily missed hazard of Cat. 5 cabling is that the use of tie-wraps can damage the cable's performance. When tie-wraps are cinched down tightly on these cables, they deform the pattern of the twists, and can permanently damage the electronic characteristics of the cable badly enough that it will not handle high frequency signals.
Cat. 5 compliance ensures application independence. It defines transmission performance for all pair combinations for both the RJ45 connector interface (mated plug and jack) and the associated cable. Finally, Cat. 5 provides a platform for implementing new high performance LAN technology regardless of the pair combination required.
Here are a few of the devices commonly used in local area networks:
A repeater receives and then immediately retransmits each bit. It has no memory and does not depend on any particular protocol. It duplicates everything, including the collisions.
A bridge receives the entire message into memory. If the message was damaged by a collision or noise, it is discarded. If the bridge knows that the message was being sent between two stations on the same cable, it discards it. Otherwise, the message is queued up and will be retransmitted on another Ethernet cable. The bridge has no address. Its actions are transparent to the client and server workstations.
A router acts as an agent to receive and forward messages. The router has an address and is known to the client or server machines. Typically, machines directly send messages to each other when they are on the same cable, and they send the router messages addressed to another zone, department or sub-network.
Baseband network. A baseband network is one that provides a single channel for communications across the physical medium (cable), so that only one device can transmit at a time. Devices on a baseband network, such as Ethernet, are permitted to use all the available bandwidth for transmission, and the signals they transmit do not need to multiplexed onto a carrier frequency. An analogy is a single phone line: Only one person can talk at a time if more than one person wants to talk everyone has to take turns.
Broadband network. A baseband network is in many ways the opposite of a baseband network. With broadband, the physical cabling is virtually divided into several different channels; each with its own unique carrier frequency, using a technique called frequency division modulation. These different frequencies are multiplexed onto the network cabling in such a way to allow multiple simultaneous conversations to take place. The effect is similar to having several virtual networks traversing a single piece of wire. Network devices tuned to one frequency can't hear the signal on other frequencies, and visa-versa. Cable TV is the best example of a broadband network, with multiple channels transmitted simultaneously over a single cable.
10Base5, 10BaseT, 10Base2, 10Broad36, etc. These are the IEEE names for the different physical types of Ethernet. The 10 stands for signaling speed: 10MHz. Base means Baseboard; broad means broadband. Initially, the last section as intended to indicate the maximum length of an unreported cable segment in hundreds of meters. This convention was modified with the introduction of 10BaseT, where the T means twisted pair, and 10BaseF where the F means fiber.
10Base2 10MHz Ethernet running over thin, 50 Ohm baseband coaxial cable.
10Base5 10MHz Ethernet running over standard (thick) 50 Ohm baseband coaxial cabling.
10BaseF 10MHz Ethernet running over fiber-optic cabling.
10BaseT 10MHz Ethernet running over unshielded, twisted-pair cabling.
10Broad36 10MHz Ethernet running through a broadband cable.
Driver. The software that allows an Ethernet card in a computer to decode packets and send them to the operating system and encode data from the operating system for transmission by the Ethernet card through the network. By handling the nitty-gritty hardware interface chores, it provides a device-independent interface to the upper layer protocols, thereby making them more universal and easier to develop and use.
In October 1999, the EIA/TIA approved a formal standard for residential networks. The title is ANSI/TIA/EIA-570-A Residential Telecommunications Cabling Standard. As mentioned above, this standard is derived from the usual EIA/TIA 568 standard for structured cabling systems.
Following are several of the EIA/TIA 570 standard's key points:
Daisy chaining of telephone circuits long the standard is out. Instead, each outlet must have its own home run. This is called a star topography.
The 100-meter link length is carried over from EIA/TIA 568.
Two grades of cabling, jacks and distribution devices are specified: Grades 1 and 2.
Grade 1 cabling may be Cat. 3, which will not likely be used. Grade 2 cabling must be Cat. 5, with Cat. 5e recommended.
Grade 2 distributive devices are required to be larger than Grade 1 devices.
Grade 1 outlets terminate one 4-pair UTP cable and one 75-Ohm coax cable. Grade 2 outlets terminate two 4-pair UTP cables, two 75-Ohm coax cables and provide for an optional optical fiber termination.
At least one outlet must be provided in each kitchen, bedroom, family/great room and den/study. It is recommended that one outlet be provided for each 12 feet of unbroken wall space.
The eight-position modular jack is the only UTP jack allowed for the outlet and it shall be wired in the A configuration. The six-position RJ-11 is not allowed. Additionally, splitting of pairs is only allowed with an external adapter; not behind the outlet.
The standard specifies a Distribution Device (DD) for each residence. This device is a panel of sorts, functioning as a type of service entrance panel for the telephone, cable TV and broadband services to the home.
Location, space and electrical power requirements are provided in 570: The DD must be located in a centralized, accessible location in the tenant space, if practical. This is to minimize the length of outlet cables and to allow for easy maintenance and configuration of the DD.
Space allocations for the DD are provided based on grade and number of outlets served. The recommendations are provided based on the spacing between wall studs. A non-switchable 15A duplex outlet is required at the DD for Grade 2 systems and recommended for Grade 1. The standard also makes recommendations for multi-tenant dwellings and backbone-cabling infrastructure.
The required testing for residential networks is not as rigorous as that for commercial networks. Commercial systems go through a difficult testing process called certification: home cabling systems go through a less difficult process called verification. Verification assures that the cabling system is continuous (that is, it has no shorts or open circuits) and that the correct terminations have been made. Verification, unlike certification, does not measure the information-carrying capacity of the link. This is considered unnecessary because residential links are nearly always considerably shorter than commercial links and suffer much less from attenuation a significant factor in a link's capacity. In the shorter links, near-end cross talk (NEXT) and far-end cross talk (FEXT) are a much-reduced concern.
The primary test for residential links is the wiremap test, verifying the pin connections on both ends of the link. This is not to say that there is anything wrong with doing a complete certification with the much more expensive Cat. 5 tester; but it's not necessary for normal residential links.
This discussion of residential LANS, hard wired, will be continued in our next installment of the Iowa State Residential Networking Course.
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