COMS 161
Introduction to Computing
Title: Local Area Networks
Date: September 27, 2004
Lecture Number: 14
1
Announcements
• This material is from chapter 17 in the
book
2
Review
• Connecting to the Digital Domain
3
Outline
• LANs
4
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?
5
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
6
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)
7
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
8
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)
9
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.
10
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
11
Transmission Media
NETWORKS ARE BUILT ON PHYSICAL MEDIA
Type
Uses
Maximum 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
12