Transcript ppt
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, ...) • • • • • 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? • • • • 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