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Transcript
Network Technologies Chapter 2 Approaches to Network Communication • Connection-Oriented – Circuit-switched – A dedicated connection (circuit) is formed between two points – Guaranteed capacity at what cost? • Connectionless – Packet-switched – Data is divided into small packets, each packet carries information about destination – Flexible at what cost? • Connectionless networks are very popular – Multiple computers share the network bandwidth – Fewer connections are required • For the rest of the book, networks generally refer to connectionless networks • Wide area networks – called long-haul networks – connected with packet switch computers and lines – to extend, just add another switch • Local area networks – operate at faster speeds, with lower delays – and do not span great distances – each computer has a network interface card connecting it to the network • Tradeoffs: speed and distance Network Hardware Addresses • Each computer attached to a network has a unique address • When a sender sends a message, the address of the destination is included in the packet Ethernet Technology • Packet-switched LAN technology standardized in 1978 by Xerox, Intel and DEC • IEEE standardized in 802.3 • See Figure 2.1 showing 10Base5 using coax cable • To connect, a transceiver connects to the cable and a host interface card plugs into the computer • See Figures 2.3 and 2.4 • Problems: thick wire doesn’t bend well, transceivers are with cable, not computer Thinnet • 10Base2 • Thinner, cheaper, more flexible • Does not provide as much protection from interference, covers shorter distance • Transceiver hardware is replaced with digital circuits as shown in Figure 2.5 • Works well in a small space • Easy to connect • Uses BNC connectors Twisted Pair • Uses unshielded copper wires like telephone wires • 10Base-T • A set of 8 wires (4 pair) connects each computer to an Ethernet hub as in Figure 2.6 • A connection to a hub is similar to a connection to a transceiver • Cable is called category 5 cable (CATV) Fast Ethernet • 100Base-T • Also uses CATV cable • Allows more stations and more total traffic Fiber Optics • • • • 10Base-F Good between buildings Expensive Good noise immunity Collision Detection • When a station sends a message, the signal travels at ~ 70% the speed of light • Transceivers sense that a signal is present and will not send from their connected station • If two signals are on the line at once, neither signal will make sense and a collision occurs • When a collision is detected, the host interface aborts transmission and waits (how long?) for the line to be idle before it sends again; CSMA/CD Ethernet Hardware Addresses • Each computer attached to an Ethernet network has a 48-bit address – called Ethernet address, hardware address, physical address, MAC address (media access), or layer 2 address • The host interface receives a copy of every packet that passes through a hub – it can recognize when a packet has this address as the destination address, it ignores all others • Unicast, Broadcast, Multicast Ethernet Frame Format • • • • This is at the link level See Figure 2.7 Octet is 8-bits (byte is sometimes character size) Format – – – – – Preamble for synchronization Destination address Source address Frame type indicates protocol (IP, IPX, TCP…) Data and CRC Repeaters and Bridges • Both extend a network • A repeater relays electrical signals from one cable to another – Can be used to double the length of a cable – At most two can be placed between two machines. Why? • A bridge receives a complete frame, and then sends it on FDDI • • • • Provides a data rate comparable to Fast Ethernet Uses fiber Data is encoded in pulses of light Uses token ring technology; a station waits for the token before trying to send – Uses two rings connected as in Figure 2.9 to recover from failure – If a station fails, network will bypass the failure as in Figure 2.10 • See Frame format in Figure 2.11 ATM • • • • • • Connection-oriented network technology Used in LANs and WANs Very high data switching speeds Network consists of high-speed switches and fiber Uses fixed-size frames called cells of 53 octets The destination host is first contacted to see if it will establish a connection; if so, cells are sent with a connection identifier; connection terminates Example Networks • ARPANET • NSF Networks: see Figures 2.12, 2.13, 2.14 and 2.15 • vBNS – Replaced ANSNET in 1995 • • • • X25NET Point-To-Point Networks Dial-Up Wireless For Next Time • Read Chapters 3 and 4