Transcript IP, ATM, Wireless

IP, Wireless
The world is the network
From Ethernet up
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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
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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:
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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!