CS475 – Networks
Lecture 6
Chapter 2: Getting Connected
Assignments
• Reading for Lecture 7: Sections 3.1 – 3.2
2.6 Ethernet
Ethernet is the overwhelmingly most successful local area
Ethernet was developed by Xerox and is based on the Aloha
networking technology.
packet-radio network once used at the University of Hawaii.
Ethernet is a working example of carrier sense, multiple access
Ethernet was standardized as IEEE 802.3
with __________________________ (CSMA/CD) technology.
2.6.1 Physical Properties
Original Ethernet (______________________) used 10Base5
(10 Mbps/Baseband/500 m link) signaling.
Multiple hosts could be connected to the same cable and any
signal was received by all hosts.
Thin-net used 10Base2 (200 m). Twisted pair uses 10 or
100BaseT (100 m) signaling and CAT5 cable.
Fig 2.22 Ethernet transceiver and adaptor
Multiple thick-net segments could be joined together with (no
more than 4) ________________ for a total reach of 2500 m.
Thin-net segments could be joined similarly.
Fig. 2.23 Ethernet repeater
With 10/100BaseT a hub (multiway repeater) is used to connect
hosts and segments.
All hosts on segments connected by hubs or repeaters are in the
same ________________________________________.
Fig. 2.24 Ethernet hub
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2.6.2 Access Protocol: Frame Format
An Ethernet frame has a 64 bit preamble of alternating 0s and 1s Each frame contains at least 46 bytes of data (64 total) and no
that are used for synch.
more than 1500 bytes. A 32 bit CRC is included in each frame.
Ethernet (_________) addresses are 48 bits. Each adaptor has a
unique address. The type field is used for demuxing higher
protocols.
Fig. 2.25 Ethernet frame format
2.6.2 Access Protocol: Addresses
Addresses are 48 bits (6 bytes). They are typically printed in hex A frame with a destination address of all 1s is a
with colons separating the bytes (8:0:2b:e4:b1:2).
_____________________ frame. A destination address with a
Manufacturers have their own prefix to ensure that each adaptor leading 1 (but not a broadcast address) is a multicast frame.
has a unique address.
2.6.2 Access Protocol: Transmitter Algorithm
When an adaptor has a frame to send and it senses that the line is When an adaptor detects that a frame it sent has collided with
idle, it transmits the frame immediately. If the line is busy, it
another frame it stops transmitting and immediately transmits a
waits for the line to go idle and then transmits the frame.
32 bit jamming sequence.
Due to propagation delays, ____________________ can occur.
In the worst case, A and B are widely separated and B starts
Adaptors give up after 16 tries and report an error to the host.
transmitting just as A's frame reaches B.
A's frame must be at least 2 x 2500 m / 3 x 108 m/s = 16.67 μs
long in order to detect the collision. Ethernet specifies a min.
length of 51.2 μs (512 bits or 64 bytes at 10 Mbps.)
Once a collision has been detected the adaptor waits before
transmitting the frame again. ___________________________
is used. It firsts waits either (randomly) 0 or 51.2 μs. If there is
another collision it waits either 0, 51.2, 102.4 or 153.6 μs and so
on.
In general, a k value is randomly selected between 0 and 2n – 1
(n is the number of collisions). The adaptor waits k x 51.2 μs.
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Fig. 2.26 Worst-case collision scenario
2.6.3 Experiences with Ethernet
Multi-access Ethernet has been used primarily in situations
Ethernet characteristics that made it successful
where there are fewer than 200 hosts and distances in the 100s of
•
Original Ethernet easy to administer and maintain - no
meters, since collisions waste network capacity. E.g. in a room
switches, no configuration tables, just add a tap to the
to a hub connected to switch in a wiring closet.
network wire.
•
Inexpensive - coaxial cable is cheap, one network
adapter per host.
Switched networks needed to similar. Resulted in ___________
Ethernet. Most Ethernet links are now point-to-point between
hubs, switches, and routers.
2.7 Wireless
Wireless networks share some common characteristics with
Wireless networks differ from wired networks in several ways
wired networks
•
Bit errors are of great concern
•
Media is inherently multi-access
•
Low power, especially small mobile devices
•
Limits on transmission power and frequency, uses
___________________ to minimize interference from
other devices.
Table 2.4 Overview of Leading Wireless Technologies
Typical link length
Typical BW
Typical use
Wired analog
Bluetooth 802.15.1
10 m
2.1 Mbps (shared)
Peripherals to computer
(PAN)
USB
WiFi 802.11
100 m
54 Mbps (shared)
Laptop to network
3G Cellular
Tens of km
384+ kbps
Cell phone to Internet
Ethernet
DSL
Most wireless links are asymmetric. One node (the
_____________________) is not mobile and has a wired (or
high BW) connection to another network.
Even though radio waves transmitted by one client may be
received by another, all node-to-node comm. is routed through
the base station.
Fig 2.28 A wireless network using a base station
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The base station topology is associated with three levels of
mobility: (1) no mobility (original WiMAX), (2) mobility within
the range of a base (Bluetooth), and (3) mobility between bases
(WiFi, cell phones).
An alternative topology is the _________________________
network in which there is no base station. All nodes are peers
and communicate with neighbors. Messages may be forwarded
along a chain of peer nodes.
Fig 2.29 A wireless mesh or ad hoc network
2.7.1 802.11 / WiFi
WiFi runs over six different physical layer protocols. The original 802.11 defined spread spectrum and direct sequence channels at
2 Mbps. 802.11b defined a new channel operating at 11 Mbps. All operated at 2.4 GHz. 802.11a delivers up to 54 Mbps and
operates at 5 GHz. 802.11g went back to 2.4 GHz and delivers up to __________________________.
Most commercial products support 802.11a, 802.11b, and 802.11g to ensure compatibility with all WiFi devices.
2.7.2 802.11 / WiFi - Collision Avoidance
Due to hidden nodes collision _________________________ is
Clear to Send (CTS) from the receiver. Any adjacent nodes that
used instead of collision detection.
see the CTS do not transmit.
The sender sends a Request to Send (RTS) packet waiting for a
Fig. 2.30 Hidden Node
Fig. 2.31 Exposed Node
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2.7.2 802.11 / WiFi - Scanning
To enable initial connection and mobility between
2) All APs within range send a Probe Response.
______________________________ (APs):
3) The node sends an Assoc. Request to one AP
1) A node sends a Probe.
4) The AP responds with an Assoc. Response frame
Fig. 2.32/2.33 Node Mobility
Fig. 2.32/2.33 Node Mobility
2.7.2 802.11 / WiFi - Frame Format
A WiFi frame can carry up to 2,312 data bytes. The ____________________ field indicates if the frame is an RTS or CTS and also
contains the ToDS and FromDS bits.
If two nodes are directly comm. both DS bits are 0, Addr1 ids the destination and Addr2 the source. In the more complex case
where two nodes use two different APs, both bits are 1 and the AP addresses are included in the frame.
Fig. 2.34 WiFi Frame Format
2.7.2 Bluetooth (802.15.1)
Bluetooth operates in the license exempt band of 2.45 GHz. It
offers speeds up to 2.1 Mbps and low power consumption. It has
a short range of only 10 m.
A Bluetooth _________________________ consists of a master
device and up to seven active slave devices (up to 255 devices
can be inactive or parked). All comm. is through the master.
Bluetooth uses spread spectrum across 79 channels to prevent
interference with other devices operating at 2.45 GHz.
Fig 2.35 A Bluetooth piconet
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