Κεφάλαιο 8 – Δίκτυα Υπολογιστών

Πλεονεκτήματα δικτύων υπολογιστών 1.Επικοινωνία 2.Διαμοιρασμός εξοπλισμού 3.Υψηλή αξιοπιστία 4.Ευκολία επέκτασης 5.Εξοικονόμηση χρημάτων

Δίκτυα Βάσει Φυσικού Μέσου Μετάδοσης Που χρησιμοποιούνται; ΠλεονεκτήματαΜειονεκτήματα Αέρας (Ασύρματα) Συνεστραμμένο ζεύγος Ομοαξονικό Οπτικές Ίνες

Δίκτυα Βάσει Γεωγραφικής Έκτασης Περιοχές που καλύπτουνΠαράδειγμα LAN WAN

Τοπολογίες Δικτύων Περιγραφή τοπολογίας ΠλεονεκτήματαΜειονεκτήματα Αρτηρία (Bus) Αστέρας (Star) Δακτύλιος (Ring)

6 Networks and Communications  Computer network - a group of computers connected together to communicate, exchange data, and share resources in real time  Computer communications - the process in which two or more computers or devices transfer data or instructions by way of a medium Jean Wang / CS1102 – Lec08

7 Communication Channel  Communication channel - the transmission media that are capable of carrying signals  Analog signal : continuous  Digital signal: discrete (either high or low)  The speed at which signal can change from high to low is called signal frequency  Bandwidth is the amount of data that can transmit over the channel (data transfer rate) per unit of time  Usually measured in Kbps, Mbps or Gbps (bits per second)  Low bandwidth is 56 Kbps  High bandwidth is over 1.5 Mbps (also called broadband)  Latency is the time it takes a signal to travel from one place to another  Transmission media can be wired or wireless: Jean Wang / CS1102 – Lec08 Wired (with cables)Wireless Twisted-pair cablesInfrared rays Coaxial cablesRadio waves Fiber optic cablesMicrowaves

 Twisted-pair cable  Most common LAN cables (similar to telephone lines)  Consisting of pairs of twisted copper wires - twisted to prevent wires from electromagnetic interference  Transfer rate up to 100 Mbps ~ 1 Gbps  Coaxial cable  Similar to wires used for cable TV  Can be laid underground or underwater for both short (in office) or long distance  Transfer rate up to 1 Gbps ~10 Gbps  Fiber-optic cable  Bundles of hair-thin strands of glass that uses light beams to transmit signals  Faster than coaxial and twisted-pair  Transfer rate approaching 100Gbps 8 Cable Transmission Media Details in reference [1]

Wireless Transmission Media  Infrared Ray (IR)  Signals carried in infrared light waves  Travel in straight line with no obstructions  Transmission distance up to 30 meters  Transfer rate up to 4Mbps  Radio Frequency  Signals carried in electromagnetic waves  Signals broadcast in omni-directions (can reflect back & forth on obstacles)  Bluetooth  Transmission distance up to 10 meters  Transfer rate up to 2Mbps  Cellular phone radio  Transmission distance up to 10 kilometers  Transfer rate up to 15Mbps (4G)  WiFi  Transmission distance up to 180 meters outdoor  Transfer rate up to 108Mbps 9 Jean Wang / CS1102 – Lec08

10 Wireless Transmission Media  Micro Waves  Signals carried in high frequency electromagnetic waves  Signals broadcast in omni-directions  As the earth is round, we need many microwave stations to relay data over a long distance  The earth-based reflective dishes are built at a distance of around every 30 miles (48 kilometers)  Communication satellite  Space station that receives microwave signals from earth-based stations, amplifies the signals, and broadcasts them back to other stations on earth Jean Wang / CS1102 – Lec08

Transmission Rates 11 Jean Wang / CS1102 – Lec08

12 Types of Computer Networks  Common types of networks  LAN (local area network)  Networks that are in local geographical areas, such as homes or office buildings  connected using cables or wireless media  E.g., home network, CS department network in CityU  WAN (wide area network)  Networks that cover a wide geographic area  Using long distance transmission media to link computers separated by a few or thousands of miles  Internet is the world's largest WAN Jean Wang / CS1102 – Lec08

13 Network Classification Summary CATEGORYDESCRIPTIONEXAMPLES GeographicalArea where network devices are located LAN, WAN StructuralHierarchy of devicesClient/Server, P2P Transmission MediaTechnologies for cables and signals that carry data twisted-pair, coaxial, or fiber-optic cable; radio frequency; microwaves; infrared BandwidthCapacity of networkBroadband, narrowband Jean Wang / CS1102 – Lec08

Έξτρα υλικό 1. /3534,14522/extras/presentations/Kef1_4_school_local_netwo rk/Kef1_4_school_local_network.htmlhttp://ebooks.edu.gr/modules/ebook/show.php/DSB101/535 /3534,14522/extras/presentations/Kef1_4_school_local_netwo rk/Kef1_4_school_local_network.html 2. ict.com/gcse_new/networks/hardware/resources/NWB_SIM.s wf