 |
California Educational Data Processing Association
|
The DataBus - Vol. 36, No. 4
June-July, 1996
|
Network Nomenclature
Terminology: Understanding is essential when working with engineers and architects.
Addison Ching
As I come in contact with various school agencies, I find that talking about network design often brings blank stares. There are so many different terms to describe the various components of the network infrastructure and communication often gets muddy, especially when architects become involved.
It is important to have a thorough understanding of network terminology because it is at the heart of designing and implementing a successful network. Higher bandwidth networks will not operate properly with wire and connectors "thrown together." As much as it might work at Ethernet (10mb.) speeds, it will certainly not work, or at the very least, offer poor performance at higher, 100-mb. Fast Ethernet speeds.
Here, in a nutshell, are some terms that are often used when discussing networks and network design.
Cable Identification
Cable is identified by an A-B-C identification scheme. A represents the speed in Mbits/second, B is either base for baseband or broad for broadband transmission, and C refers to the meters per segment multiplied by 100.
- 10Base-5 - (Thick) coax cable with maximum segment lengths of 500 meters.
- 10Base-2 - (Thin) coax cable (thinnet) with maximum segment lengths of 185 meters or about 600 feet.
- 10Base-T - Twisted-pair cable with maximum segment lengths of 100 meters or about 320 feet.
- 10Base-F - Fiber-optic cable backbone with maximum segment lengths of up to 4 kilometers at 10Mbits/second; approved by EIA/TIA for cross-connects between school campus buildings.
- 100Base-X - A new standard that describes 100Mbits/second throughput using twisted-pair cable.
Data Grades of Cable
- Category 1 - POTS (Plain Old Telephone System) voice-grade cable used in analog telephone systems.
- Category 2 - up to 4mb bandwidth.
- Category 3 - up to 16mb bandwidth.
- Category 4 - up to 20mb bandwidth.
- Category 5 - up to 100mb bandwidth.
EIA/TIA 568
A commercial building wiring standard developed by the Electronic Industries Association (EIA) and the Telecommunications Industries Association (TIA) that addresses how wiring should be installed in commercial buildings. The standard provides for a uniform wiring system and supports products and environments from many vendors. Addressed in the standard are wiring system structures, work areas, schemes for horizontal wiring, telecommunications closets, equipment rooms, backbone wiring, and entrance facilities.
Fiber Optic Cable
A special cable comprising glass or plastic strands or conductors to transmit information using light instead of voltage. Fiber optic cable is not subject to conditions that affect the performance of standard copper cable including signal loss, capacitance which distorts the signal, and crosstalk which leaks signals from one set of wires to another. Fiber optic cable is resistant to outside interference from electromagnetic radiation (ignition noise, diathermy, etc.) and does not radiate signals as do copper conductors. Due to its low signal loss, fiber optic cable can be used for longer distances.
Fiber optic cable comes in various configurations. Multimode fiber is most commonly used due to its lower costs. Multimode fiber has a higher dispersion rating, is normally used in LAN environments, and uses light sources generated typically by light-emitting diodes (LEDs). Since LEDs generate light in a less-direct (dispersed) path, signal loss is greater. However, multimode fiber is easier to install and handle and LED light generators do not require special protective equipment. Single mode fiber has the highest bandwidth and distance ratings. This fiber optic cable uses a higher-precision light source (laser) which can permanently destroy eyesight if improperly handled, and requires more precise installation but can be used for greater distances and bandwidths.
Fiber optic conductors are encased in a protective coating (cladding) surrounded by a plastic buffer and DuPont Kelvar for strength. An outer jacket completes the cable composition. Normally, cables comprise multiple fiber conductors in various combinations. A 6/6 cable has six single mode fiber conductors and six multimode conductors.
Various methods of terminating fiber are utilized, but the ST series of connectors are most commonly employed.
5-4-3 Rule
A design rule that is applied to Ethernet networks when cascading or daisy-chaining hubs. It states that between any two devices there shall be
- No more than 5 segments;
- No more than 4 repeaters; and
- Only 3 of the segments may be populated.
For illustrative purposes, a device is a computer, and a repeater can be either a real repeater or a hub. A segment is a portion of the network between any two active components uch as between hubs or between a hub and a device. The rule states that between any two computers, there can be only four hubs. In the situation where classroom computers are connected to a workgroup hub located within that room which in turn connects to the IDF then the MDF then to an IDF in another building and then to the workgroup hub in another classroom and finally to a client computer in that classroom, the 5-4-3 rule is violated because there are actually five hubs in the data path.
Also, when cascading or daisy-chaining hubs, only three such cascades can exist.
Hubs
A hub is like a network junction box, allowing multiple devices to be connected together without interfering with each other. Hubs normally support a fixed number of connections, typically in groups of eight, twelve, sixteen or twenty-four, depending upon the manufacturer. Hewlett-Packard manufactures hubs that support fifteen ports.
A workgroup hub is a small device that provides only the basic services of a hub. Workgroup hubs normally support eight connections; some have a ninth connection that can be used to cascade or daisy-chain to another workgroup hub. Typical placement of workgroup hubs are in workgroupsÐclassrooms, offices, or libraries, where a small number of computers need to be clustered together.
A SNMP-capable hub is a much larger device, typically rack-mounted, with added electronics and intelligence toallow it to communicate using SNMP commands to a central administrative program on the network.
A stackable hub is normally SNMP-capable and includes the connection configurations of typical hubs. Stackable hubs also have a special hub interconnect that allows multiple hubs to be literally "stacked" or connected in parallel to provide up to 260 or more individual connections without violating the 5-4-3 rule. Stackable hubs are normally located in IDF facilities.
A segmented hub is a hub that can be specially configured by dividing the hub's bus into segments joined by common connections (backplane.) Modules are plugged into the backplane and connect with any other modules over the bus to form logical LAN segments.
A switched hub has the characteristics of a segmented hub except that it allows LAN microsegmenting to reduce contention. For example, a file server could be placed on its own LAN segment by itself to increase the file serving throughput. Switched hubs allow configuration by logical or common use and "switch" or bridge network traffic between segments with communication between devices on different segments becomes necessary. Switched hubs also allow the switch's bandwidth to be portioned to individual segments. Switched hubs are sometimes used at the MDF to segment network traffic and to ensure that the 5-4-3 interconnect rule is not violated because of excess hubs in the segment.
Interduct
A special protective conduit for fiber optic cable, normally recognized by its bright orange color.
Intermediate Distribution Frame (IDF)
The Intermediate Distribution Frame is a satellite distribution point located in a wiring closet of each network segment. Typically, separate IDFs serve each building. IDFs comprise the backbone feed from the MDF, a distribution hub, and a patch panel. Classroom feeds from each classroom within the building converge at the IDF.
Local Area Network (LAN)
A network that resides in a local area, usually at a school site or administrative campus.
Main Distribution Frame (MDF)
This is the main distribution facility of a network, often described in other terms such as "main hub" or "central hub." This is the starting point of the site network, typically where the site router, outside connections, and the site network backbone all converge.
Patch Panel
As the name implies, a panel with multiple jacks that typically connect to wiring that feeds individual terminations in classrooms. The patch panel acts as a network switchboard; only those connections that are active are "patched" using station cables to the IDF hub's ports.
SNMP
This acronym stands for Simple Network Management Protocol, a subset of TCP/IP commands which allow network devices to be remotely administered on the network. For example, any equipment supporting this feature can be automatically detected as malfunctioning, be isolated from the network, diagnosed remotely, and the problem fixed or the faulty equipment removed from service, all from a remote location. Typical use of SNMP in a school district setting would comprise a monitoring control program at the network central administrative site that would continuously monitor all SNMP devices in the entire district network. All SNMP devices are typically represented by green icons that turn red when a device detects a problem.
Station Cable
An interconnecting cable with RJ-45 connectors on each end, in lengths from 3 to 15 feet or more, and used to connect computers to wall terminations or patch panels to hubs.
Termination
A specially-designed jack used for network connections, that "terminates" the cable segment from the IDF to the classroom or work facility. These terminations are usually modular and snap into special face plates that mount on a standard electrical outlet box. Systems such as the AT&T "M" series of connectors and those manufactured by Allen-Tel, AMP and Seimon are most commonly used.
Topology
Topology refers to a network's layout. Three network layouts, defining how cables will be run to connect devices, are:
- Linear - consists of a single cable that connects one computer to another in a daisy-chain fashion. The ends of the cable are connected to a terminating resistor. Any break in the chain disables all devices connected to the cable. Used in coax cable networks.
- Star - cable wires branch from a central point such as a file server or wiring closet. Each device requires its own cable. A break in the cable connecting a single device does not affect other devices connected to the star. Used in twisted-pair networks.
- Ring - the network cable loops back to itself and signals travel in a loop or ring.
Wide Area Network (WAN)
A wide area network normally refers to the districtwide or enterprise network of an agency that interconnects several site local area networks. A district's wide area network would typically include all school site local area networks and the administrative offices in an all-inclusive network.
Return to June-July index