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COMS 161Introduction to Computing

Title: Local Area Networks

Date: September 27, 2004

Lecture Number: 14

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Announcements

• This material is from chapter 17 in the book

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Review

• Connecting to the Digital Domain

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Outline

• LANs

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Differentiating LANs

• Transmission media– What are the actual hardware connections

between nodes (computers) made from?

• Topologies– In what way are the various nodes arranged and

interconnected?

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Transmission Media

• Bounded media– Coaxial cable (like a TV cable)– “Twisted-pair” cable (copper wires)– Optical fiber cable

• Unbounded media (wireless networking)– RF (radio frequency)– IR (infrared)– Cellular modem

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Transmission Media

• Bounded media– Coaxial cable (like a TV cable)

• Original LAN installations were coax• Now almost never used (cost)

– “Twisted-pair” cable (copper wires)• Generally limited to about 100 meters max (330 ft)• Telephone wire (“CAT-3”, or “category 3”)• CAT-5 wiring (up to 10 Mbps – 10 million bits per

second)• CAT-5e wiring (up to 100 Mbps)• CAT-6 wiring (emerging “gigabit” standard – up to 1

Gbps)

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Transmission Media

• Bounded media (cont’d)– Optical fiber cable

• Signal is composed of pulses of laser light, not electricity

• Extremely thin glass strand transmits the light pulse• Lower error rates and high data bandwidth (>2 Gbps)• Becoming very cost-effective for high speed data needs

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Transmission Media

• Unbounded media (wireless networking)– RF (radio frequency)

• Becoming very common

• Speeds of 11 Mbps now common (“802.11b”)

• Faster speeds becoming available (54 Mbps, 108 Mbps)

• Public access points (“hotspots”) becoming common

– Various areas on campus– Downtown areas, such as Manhattan– Airports, hotels, coffee shops, etc. (free or pay)

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Transmission Media

• Unbounded media (cont’d)– IR (infrared)

• An early wireless technique, now mostly unused as RF wireless has advanced

• Required line-of-sight – works well only within enclosed spaces

• Still used for some simple ad hoc networking tasks, such as

– Laptop-to-PDA– PDA-to-printer– Digital camera uploads– Etc.

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Transmission Media

• Unbounded media (cont’d)– Cellular modem

• Wireless connectivity anywhere that there is appropriate digital cellular service

– Various cellular companies are competing– Coverage is still spotty, but improving

• Up to 120 Kbps – better than dialup

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Transmission Media

NETWORKS ARE BUILT ON PHYSICAL MEDIA

Type UsesMaximum Operating Principal Distance (without amplification)

Cost

Twisted pair Small LANs 300 feet Low

Coaxial cable Large LANs 600–2,500 feet Medium

Fiber optic Network backbones; WANs 1–25 miles High

Wireless/infrared LANs 3–1,000 feet (line of sight) Medium

Wireless/radio Connecting things that move Varies considerably High