Local Area Networking

An article on the characteristics of network media: desecriptions of LAN wiring, bandwidth rating, max segment length, interference susceptibility, and a discussion contrasting broadband and baseband transmission; comparisons between common Ethernet media: coax, UTP, STP, fibre-optic, and wireless.

Part 7: Local Area Network Media

Cable Characteristics

Bandwidth rating

Bandwidth is the amount of data (measured in Mega or Giga bits) which can be transmitted in a given amount of time.

Max segment length

Different cables can carry a signal further or less far before attenuation makes it necessary to regenerate the signal. The distance they can transport a signal without the need for regeneration is the maximum segment length.

Max number of segments per internetwork

Latency is the delay that an electrical signal undergoes while traveling a given distance. If a signal has to travel too far it will exceed the maximum delay which a particular network can tolerate. Therefore segments must be arranged so that the time it takes for a signal to travel from one end of the network to the other is tolerable.

If segments are laid end to end (in series) latency increases. If they are arranged as a hierarchy (parallel) then the total distance from one end of the network to the other is decreased and latency goes down.

Max number of devices per segment

Each device connected to an electrical circuit introduces resistance which causes the signal strength (voltage) to drop.

Interference susceptibility

The degree to which the signal being carried by a given type of cable is affected by RFI and EMI.

Connection hardware

Type of connectors which can be used with any given cable.

Cable grade

The building and fire code rating.

Bend radius

How far a cable can be bent before it undergoes physical damage.

Baseband and Broadband transmission

Baseband

Baseband networks use digital transmission which represents data as signals which are either on or off. Digital transmission occurs at a fixed frequency and only one digital signal can be sent at a time.

Since only one signal can be sent or received at one time, baseband is said to operate at half-duplex.

Full duplex mode, in which a signal can be transmitted and received simultaneously, requires two cables and two nics for each node.

Broadband

Broadband networks use analog energy to represent data. Analog energy varies constantly in amplitude and its frequency can also be varied.

Broadband can send multiple signals over the same wire because each (analogue) signal occupies its own band of frequencies. Data is represented by a shift in frequency (FM) within the frequency band assigned to that signal.

Broadband transmission can use either of two different methods to achieve full duplex mode:

Mid-split broadband divides the bandwidth of a single cable into two channels, each operating on a different frequency. One channel is used to transmit and the other to receive.

Dual-cable broadband uses two cables, one to transmit and one to receive, just as broadband does.

Broadband is more complicated because the transmitters and receivers must be tuned to the signal.

Why is bandwidth so critical? Because of the increasing demand for greater and greater volumes of data - multimedia, including video and audio as well as binary data.

Cable Types: Comparison of Ethernet Cabling Systems

  Mbps Cable Connector Notes
10Base5 Thicknet 10 Thick coax AUI 15-pin "D" Center wire and metallic braid separated by foam insulation. Max segment=500M.
*10Base2 Thinnet 10 Thin coax BNC with Tee's Sometimes used along with Thicknet. Max segment=180M.
*10BaseT Twisted Pair 10 Unshielded twisted-pair (UTP) RJ-45Two pairs of wires, insulated from each other and twisted together inside a plastic casing. Several grades - CAT5 being the most popular. Max segment=100M. Low grade (telephone wire) not suitable for 10BaseT.
100BaseT Fast Ethernet 100 Shielded Twisted Pair (STP) RJ-45 STP costs more than UTP and Thinnet, but less than Thicknet. A very high grade of UTP is sometimes used for local connections on Fast Ethernet.
100BaseFL/FX Fiber-Optic 10 or 100 Optical fiber (fiber-optic cable) ST or SC fiber-optic connector Uses glass fiber about the diameter of a human hair (2 - 5 mil or roughly .25 thousandths of an inch).

*Two most popular Ethernet cabling systems

Coax

Coaxial because two concentric conductors share the same axis.

Coax is a copper wire inside a plastic insulator and surrounded by either a copy braid or aluminum sheathing.

Comes in two thicknesses:

  • .4 inch - 10Base5, or thicknet
  • .2 inch - 10Base2, or thinnet

Thinnet is cheaper and easier to handle than Thicknet.

Coax shielding is superior to UTP making it a better choice in environments with a lot of RFI (Radio Frequency Interference, electrical noise produced by electric machinery and electronic devices).

Thinnet uses BNC connectors while Thicknet uses AUI 15-pin "D" connectors.

On networks using coax cable nodes are usually connected in a bus topology, with the cable from the previous computer and the cable to the next computer both connecting to the BNC connector on the NIC using a "Tee".

Cabling is more direct so less is used, but adding nodes on the bus topology is less convenient than when using a hub scheme.

The two extremities of the bus end with a 50-ohm terminator. The terminator is a BNC connector with a 50-ohm shunt. The terminator's job is to attenuate the signal when it reaches the end of the cable run. Without attenuation the signal would be reflected back across the network and that "bounce-back effect" would needlessly increase network traffic.

UTP/STP

Shielded (STP) or Unshielded (UTP) Twisted Pair.

This type of cable consists of one or two pairs of copper conductors which are twisted to reduce interference by cancelling out cross-talk.

In shielded TP, the twisted pair are surrounded by a metallic sheath which reduces interference but also increases attenuation because of the inductance and capacitance created by the shielding and the conductors.

Comes in various grades - CAT(egories) 1 and 2 (telephone wiring) are unsuitable for data. CAT 3 - CAT 5 are both suitable but CAT5 is recommended and is the most popular type for Ethernet.

On Ethernet networks UTP is called 10BaseT and RJ-45 connectors are used.

The female connectors are sometimes referred to as 10BaseT ports.

The 10BaseT designation is derived from:

  • "10" for 10Mbps
  • "Base" for baseband
  • "T" for twisted

Fiber-Optic Cable

Data is sent as light (photons) over a cable of glass fibers inside a sheath of Kelvar or other strengthening material.

Two types: single-mode and multi-mode. Multi-mode uses multiple light waves to carry more than one signal at a time, but single-mode signals can transfer data at higher rates because multi-mode creates overlapping light pulses and pulses being delayed because they enter the pipe at different angles which causes non-axial travel (bouncing off the walls) which in turns increases the distance traveled.

Usually used as a high-speed interconnection between LAN's or distant buildings.

Although very expensive (~10x expensive as cable), fiber-optic cable offers important benefits:

  • Very low attenuation over long distances
  • Unaffected by electromagnetic interference, capacitance, or cross-talk
  • Very secure - no radiated energy - any attempt to tap the photons is detectable because it stops them

Light sources can be LED (light emitting diode) or ILD (injection laser diode). ILD is more powerful and achieves higher data rates and over longer distance.

Wireless

Two types of wireless:
  • Infrared (two types: point-to-point and broadcast)
  • Radio frequency
Advantages of wireless:
  • No cables
  • RF can penetrate physical obstructions
Disadvantages of wireless:
  • Infrared requires line-of-sight
  • RF requires free portion of spectrum
  • RF is vulnerable to eaves-dropping
  • Currently, there are multiple RF standards

Five primary technologies for wireless networks: (Tomsho 4th ed, p.93)

  • Infrared
  • Laser
  • Narrowband, single-frequency radio
  • Spread-spectrum radio
  • Microwave p.99

Infrared

Disadvantages

  • Requires line-of-sight for best operation
  • Subject to interference from visible light spectrum

Four primary types of Infrared LANs include: (Tomsho 4th ed, p. 94)

  • Line of sight - line of sight
  • Reflective - nodes (probably wired) to nearby optical transceivers which broadcast to a central hub. See page 92 Tomsho 4th ed.
  • Scatter - bounces off walls, inherent delays reduce bandwidth and limit range to 100 feet.
  • Broadband optical telepoint - high speed and wide bandwidth.

Laser

Requires line of sight. Same limitations as infrared to protect people from radiation. Compared with infrared not as subject to visible light interference.

Narrow-band, single-frequency radio LAN technologies

Spread-spectrum LAN technologies

Bluetooth technology is catching on most rapidly. It uses "frequency-hopping" to overcome the security problem. Packets are blasted out over randomly selected frequencies within a certain spectrum to make it very difficult to intercept enough of the packets to retrieve the information. Fixes interference and security at the cost of bandwidth.

Wi-Fi

The 802.11 family of Wi-Fi standards replaced the PAN (personal area networks) standard beginning in 1997. Wi-Fi is used to connects personal computers, PDA's, laptops, and other wireless devices to networks via wireless routers. The area within the operational range of a wireless router is referred to as a "hot spot".

Mixing and matching Media

10BaseT and 10Base2 stuff can be mixed and matched with no problems as long as the gear has the appropriate connectors. Hubs often have a coax connector in addition to the 10BaseT connectors.


Bruce Miller, 2002, 2014