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Transcript
IP, Wireless The world is the network From Ethernet up • • • • • Ethernet uses 6 byte addresses Source, destination, data, and control stuff Local networks only Not all networks are IP Not all IP is ethernet IP today • Present networks are based on IPv4 • Packets (called datagrams) – 4 bytes describing IP version and other control, including TOS (type of service, often used to control speed and loss of packets) – 4 bytes: Numbering system for splitting information into different fragments – 4 bytes describing information and protocol used. More on the IP packet – – – – 4 bytes source IP number 4 bytes destination IP number Variable bytes describing other options Data! Variable number of bytes IP numbers and networks • 4 bytes = 32 bits. • Usually recorded as the 4 numbers corresponding to each byte: 192.75.245.2 • Part of the number is the number of the network, part the number of the host. • Subnet mask tells us which bits are network number. Subnets • Mask 255.255.255.0 = 1111 1111 1111 1111 1111 1111 0000 0000 – top 3 bytes network, lower byte address – So, 192.75.245.2 would be host number 2 in network 192.75.245.0 • Can have different sized networks: 255.255.0.0 would have two bytes for the host. Quick overview of routing and the Internet • Internetworking is just moving packets from one network to another – When we transmit a packet, if it’s on our network, we just send it to a local machine – If it’s on another network, we send it to a gateway. • Gateways are computers on our network and on another network • Provide link between networks Gateways and Bridges • Gateway or router is a computer/box on a IP network, with two or more IP addresses – One IP on one side, one on another – Communicates at the IP level. Internetworking Protocol! • Bridges connect two segments of the same IP or ethernet network – Act like one network, share IP range. – Can be purely ethernet. Ethernet vs IP • Ethernet is one kind of link-level network. • IP is a more general form of network • IP packets are payload of an ethernet packet in an IP over ethernet network (most modern nets) – Those nets have packets with both ethernet and IP addresses in packet. – Locally, communication uses ethernet addresses, send to other nets via IP addresses. Ports • Two main protocols: UDP and TCP • Have a concept of ports: place a piece of information goes to on a given computer • Ports: two bytes, 0 to 65535. • Well-known ports: – Mail (SMTP): port 25 – Web: port 80 • Packets come from a destination port to make sure they go to the right place. UDP/IP vs. TCP/IP • UDP is just ports on top of IP. Video and more. • TCP is far more complex: – – – – Provides mechanisms to adjust data speed Automatically resends lost packets Keeps data in order Based on two-way handshake: packets flow back from destination to source to control everything. Slows things down. • UDP fast, TCP reliable. Both are IP. Wireless networking systems • Transmit information over radio waves – Can be based on a constantly open link, like a radio version of a serial wire. Cellphones. – Can be based on transmitting packets: ethernet, IP, and more. • Signals are transmitted over a band of frequencies centered at some frequency – Higher data rates = higher bandwidth = more power Wireless Bridges • Modern large wireless systems are ethernet bridges • Work like ethernet transmitted over radio waves instead of wires • Correct for errors: – Interference (spread-spectrum) – Noise (error detection and correction) – Multipathing: bouncing around and interfering with own signal (OFDM) Radio waves • Lower frequencies – Move over obstructions easily – Long range – Slow data rates • Higher frequencies – Need line of sight, or even more! – Very high data rates – Need special antennae to get long range. Bouncing! Range and data rate • power fades as 1/distance^2 • power needed goes as distance squared • power needed proportional to data rate – Higher speed signals don’t go as far – More cells needed, or more power needed • more range causes more problems with multipath, though Antennae • Concentrate the radio waves in one direction – more power in that direction – less or nearly no power in others • Increases range and data speed possible • “Effective Radiated Power” = real power * gain of antenna • Increases signal received for a given local power level from a remote transmitter. Antenna types • Omni: spreads signal around evenly, but may reduce amount it goes up or down. Great for point to multipoint, but requires more power. • Dish: gives a round, pencil-like, beam. Great for point to point connections. • Sectoral: transmits in a pie-like area. Can handle height differences well, and is used for cellphone systems, or point to multipoint. Urban areas • Buildings block signals • Buildings bounce signals around • Lots of interferences from people on same frequencies • Special transmission systems needed to fight problems. • Big area, lots of cells. • Lots of people though-can be cheap per person! Remote areas • Mountains, Trees, can block signals • Rocks bounce signals around, even ground bounces signals if you go far enough • Less interference • Not many people-can’t afford many cells!