Computer Networking Lecture Notes

18 February 2013 • Wired Transmission Media

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Outline

• Wired media.
  • Recordable media.
  • Twisted pair.
  • Coaxial cable.
  • Fiber optic cable.

Transmission Media

• Over what do signals travel?
• Transmission media differ over various characteristics.
  • Bandwidth, delay, cost, installation, maintenance, …
• Wired transmission media provide physically constrained signal propagation.
• Wireless transmission media provide unconstrained signal propagation.

Recordable Media

Removable recordable media: tapes and disks. High bit densities (1-150 Gbit/in2), low cost. The transmission part: write, transport, read.

Bandwidth Example

• A single $40 tape holds 1.5 Tbyte.
• A 2 ft × 2 ft × 2 ft box holds 1,000 tapes.
• Package delivery ships to anywhere in the U.S. in 24 hours.
• Bandwidth:

  (1.5 Tbyte × 1000)/(86,400 sec)
  	= 17 Gbyte/sec
  	> OC-768 (5 Gbytes/sec)

Cost Example

• A tape has a 10 read-write cycle lifetime.
  • $4 per use.
• Package delivery costs $1,000.
• Total cost is $4 × 1,000 + $1,000 = $5,000.
• Cost:

  (1.5 × 1015 bytes)/$5,000
  	= 300 Gbyte/$
  	> OC-768 (43 Gbytes/$)

Twisted Pair

• Removable recordable media has high latency.
  • People will pay to avoid high latency and more convenient delivery.
• Wire transmission is slower and more expensive, but has low(er) latency.
• Twisted pair is the cheapest, oldest, most common wired media.
  • A century of phone company experience.

Twisted Wire

• A pair of 1 mm copper wire covered in plastic insulation.
• Twisting the wires
  • Cancels radiated electromagnetism.
  • Handles external noise better.
• Analog or digital signals can run several km without electronic assist.

  

Category Cabling

• Twisted pairs are bundled into groups called cabling.
• Category 5 cabling (cat 5) is four twisted pair wrapped in a plastic sheath.
  • Cat 3 is cat 5 with fewer twists per inch.
• Cat 6 and 7 are more rigorously specified cables for higher bandwidths.
  • Cat 7 includes shielding on the pairs and the cable.

Cat 5 Example

• Unshielded Twisted Pair (UTP)

Cat 7 Example

• Shielded Twisted Pair (STP)

Bandwidth

• Twisted-pair bandwidth proportional to copper diameter and run length.
  • Low megabits/sec over several km.
• Cat 5 cabling can handle up to 1 Gbps links.
• Cat 6 and 7 cabling can handle up to 10 Gbps links.
• High bandwidths achieved in parallel over several twisted pair.

Transmission Electronics

• A receiver may be a sender, and vice versa.
• A repeater undoes attenuation by
  • amplifying the signal, and
  • possibly reshaping the (usually digital) signal.
• There may be zero or more repeaters.

Transmission Direction

• An endpoint in a channel that’s
  • full-duplex sends and receives at the same time.
    • Simultanious two-way traffic.
  • half-duplex sends or receives but not both at the same time.
    • Alternating two-way traffic.
  • simplex only sends or only receives.
    • One-way traffic.

Coaxial Cable

• Coaxial cable (coax) is single strand, heavy gauge, shielded wire.
  • Shielding and gauge improve over twisted pair.
• Baseband coax transmits digital signals.
• 20–100 Mbit/sec data rates over 0.5–5 km.

Coax Example

A: Plastic outer insulation
B: Copper-clad aluminium braid shield conductor
C: Dielectric
D: Central conductor (copper-clad steel)

Broadband Coax

• Broadband coax transmits analog signals.
  • The “cable” in “cable tv.”
• Up to 150 Mbits/sec up to 100 km.
• Greater distances require electronic assistance (amplifiers, for example).
  • Amplifiers are one-way devices.
  • Amplified signals form simplex channels.

Broadband Networks

• Simplex is fine for cable tv, but data networks require two-way transmissions.
• Dual cable systems have two cables, one in each direction.
• Midsplit systems divide a single cable into several channels.
• In either case the cable company head-end is the router.

Baseband vs Broadband

• Baseband systems are easy and cheap to set up, use, and maintain.
  • Broadband systems are harder and more expensive.
• Broadband systems are more capable over longer distances.
  • Baseband systems are not.
• The choice is straightforward depending on intention.

Fiber Optics

• The wire is a 10–100 μm strand of glass, silicon, or plastic.
  • Human hair ≈ 17 μm.
• Fiber optic signals are light pulses.
  • Lasers or light-emitting diodes (LED) convert from electricity to light.
  • The fiber transmits light.
  • A photo diode light detector converts from light to electricity.

Fiber Example

Light Transmission

• Light refracts (bends) when passing from one medium to another.

  

• A ray at or above the critical angle is internally reflected with (almost) no loss.

Fiber Modes

• A ray’s mode is its incident angle.

  

• Thick (50–100 μm) multimode fiber admits several rays with different modes.
• Thin (8–10 μm) single mode fiber admits a single ray.
• Single mode fiber higher data rates for longer distances than does multimode fiber.

Fiber Advantages

• High data rates (Gbit/sec) over long distances (km) with low error rates (10^-9 BER).
• Hard to snoop signals, tapping difficult.
• Lighter and thinner than metallic wires; much greater cost/benefit ratio.
• Immune to electric interference or corrosion (rust).
• Less support electronics needed less frequently.

Fiber Disadvantages

• Hard to tap and splice.
• Restricted optical end-to-end network topologies.
• Subject to physical trauma (kinking, crushing).
• Simplex channel.
  • Dual-core fiber.

Wire Comparison

Summary

• Wired media constrains transmission in certain directions.
  • Along the wire’s axis.
• Transmission rates are inversely proportional to distance.
  • Error rates proportional to distance.
• Each medium wins in its niche, and fiber wins the future.
  • The future approaches with cheaper optical electronics.

Credits

• FBI DATA REELS (UN-Classified) and unreadable to me! by spike55151 under a Creative Commons BY NC SA license.
• Twisted Pair by Wolf Wings in the public domain.
• CAT5e Cable.jpg by Richard Wheeler under the GNU Free Documentation License.
• Cat7.jpg by Bewareircd in the public domain.
• RG-59.jpg by Arj under a Creative Commons BY SA license.
• Optikai kábel.jpg by Zátonyi Sándor under a Creative Commons BY SA license.
• Speed-vs-length.svg by Alexander Stohr under a Creative Commons BY SA license.

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This page last modified on 2013 February 18.