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CS 351
Modeling and Simulation
Technologies
Spring 2012
Communications
Network Models
Dr. Jim Holten
Overview
• Physical Layers
• Internet
• Telephones
• Satellite Comm
• Other Comm
• Reliability
CSE 351
Physical Layer
• Links and Nodes (locations)
– “Wired” (wire or fiber)
– Fixed wireless (nodes immobile, interferences)
– Mobile wireless (nodes move, links change,
interferences, varying distances, intermittent)
• Paths
– Dedicated links
– Alternate paths (routes)
– Aggregated links (multiplexed, bundled cables)
CSE 351
Internet (digital network)
• Computers, routers, ISP’s, backbone, AS’s
• Packet-based (mostly TCP/IP)
– Singleton packets, messages, and streams
• Routing (BGP between AS’s)
– Packets may follow different paths
– Messages reassemble packets in original order
– Streams reassemble somewhat, drop some
– Dedicated path streams drop some
CSE 351
Telephones
(continuous analog, digital text, ...)
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Mobile phones, fixed “towers”
Tower “regions” (cells) and channels
Phone motion causes “handoff” between cells
Limited cell capacities (data rates and channels)
Towers on fixed infrastructure (some with
connections to internet)
• Databases for “tracking” phones (to route
responses)
• Analog “digitized” (in the phone), then passed on
CSE 351
Satellite Comm
• “Stationary” satellites
– Synchronous orbit, 23000 miles away
– Relays to/via “ground station”
– ~½ second delay (2 trips plus answer 2 trips)
• Low orbit “comm” satellites
– ~300 miles above ground level (closest point)
– ~300 to ~1500 actual distances (~20 ms delay)
– ~90 minutes per orbit (often more than one
satellite)
CSE 351
Other Comm?
• “Sneaker Net”
– Hand-carry (tapes, removable disks, thumb drives, ...)
– Courier (bicycle, car, passenger on other
transportation)
– Specialized transportation (terabyte drives on a jet,
pony express)
• Other
– Signal lights, flags, semaphores
– Smoke signals
– Drums
CSE 351
Reliability?
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Error detection/correction codes (parity)
Error rates for the links
Retransmissions? (TCP/IP)
Consequences of passing bad data?
– Arbitrary errors give garbled messages, make
no sense
– Key errors can give alternate meanings,
possibly valid but wrong information
CSE 351