Transcript Chapter 2

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
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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
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This is at the link level
See Figure 2.7
Octet is 8-bits (byte is sometimes character size)
Format
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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
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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
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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
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X25NET
Point-To-Point Networks
Dial-Up
Wireless
For Next Time
• Read Chapters 3 and 4