Samenvatting Computer Networks A Top Down Approach Hoofdstuk 7, 8 en 9 door Kurose en Ross, Bachelor Informatica, UvA
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Course
Networks and Network Security (5062NENS6Y)
Institution
Universiteit Van Amsterdam (UvA)
Book
Computer Networking
Samenvatting van het boek Computer Networks A Top Down Approach door Kurose en Ross Hoofdstuk 7, 8 en 9, voor het vak Networks and Network Security in het 2de jaar van de bachelor Informatica aan de Universiteit van Amsterdam. Daarnaast zijn ook aantekeningen van alle hoorcolleges aangevuld.
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Samenvatting Computer Networks A Top Down Approach Hoofdstuk 5 en 6 door Kurose en Ross, Bachelor Informatica, UvA
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Network and Network Security Summary CHoogteijling
7 Wireless and Mobile Networks
7.1 Introduction
Elements in a wireless network:
• Wireless host: the end-system devices that run applications, the hosts themselves do
not have to be mobile.
• Wireless link: connects hosts at the edge of the network into the larger network in-
frastructure. Can also be used within a network to connect routers and switches.
• Base station: is responsible for sending and receiving data to and from a wireless host
that is associated with that base station.
– Associated wireless host is a host that is within the wireless communication dis-
tance of the base station and uses that base station to relay data between int and
the larger network.
– Infrastructure mode (of a base station): all traditional network services are pro-
vided by the network to which a host is connected via the base station.
– Ad hoc network: a wireless host does not have infrastructure to connect to, they
themselves have to provide for services such as routing and address assignment.
– Handoff: a host moves beyond the range of one base station and into the range
of another and changes its point of attachment into the larger network.
– Cell towers and access points are base stations.
• Network infrastructure: the larger network with which a wireless host communicates.
Taxonomy of wireless networks:
• Single-hop, infrastructure based: base station is connected to a larger wired network
and all communication between the base and a wireless host are over a single wireless
hop. Most used network.
• Single-hop, infrastructure-less: no base station that is connected to a wireless network,
one node may coordinate the transmissions of the other nodes. Bluetooth network.
• Multi-hop, infrastructure based: base station is connected to a larger wired network
but hosts may have to communicate with the base station through other wireless nodes.
Wireless mesh network.
• Multi-hop, infrastructure-less: no base station and nodes may have to communicate
through other nodes in order to reach the destination.
– A mobile ad hoc network (MANET) is a multi-hop, infrastructure-less network
in which nodes are mobile and connectivity among nodes changes.
– A vehicular ad hoc network (VANET) is a MANET where the nodes are vehicles.
7.2 Wireless Links and Network Characteristics
Differences between a wired link and a wireless link:
• Decreasing signal strength.
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,Network and Network Security Summary CHoogteijling
• Interference from other sources.
• Multipath propagation: occurs when portions of the electromagnetic wave reflect off
objects and the ground.
Wireless links have a higher bit error rate (BER) than wired links, so the protocols have
powerful CRC error detection codes and link-level reliable-data-transfer that retransmit
corrupted frames.
The signal-to-noise ratio (SNR) is a relative measure of strength of the received signal and
this noise. It is measured in decibels (dB) and makes it easier for the receiver to extract the
transmitted signal from the background noise. The SNR is 20 times the ratio of the base-10
logarithm of the amplitude of the received signal to the amplitude of the noise.
Physical-layer characteristics and BER:
• For a given modulation scheme, the higher the SNR, the lower the BER. By increasing
the transmission power the SNR is increased, so the BER is decreased. The disadvan-
tage of this is the increasing cost in energy, which is a concern for battery-powered
mobile users.
• For a given SNR, a modulation technique with a higher bit transmission rate will have
a higher BER.
• Dynamic selection of the physical-layer modulation technique can be used to adapt
the modulation technique to channel conditions.
Wireless networks suffer from the hidden terminal problem caused by physical obstructions
in the environment or two senders fading eachothers signal strength.
7.2.1 CDMA
Code division multiple access (CDMA) is a channel partitioning protocol. Each bit is en-
coded by multiplying the bit by a signal that changes with the chipping rate. The chipping
rate is the faster rate of changing than the original sequence of data bits.
Simplified the formula for the sender is: Zi,m = di ∗ cm , and for the receiver: di =
PM
m=1 =Zi,m ∗cm
M . Where m is the mini time-slot, di is the bit-transmission and Zi,m the
output of the CDMA encoder.
CDMA assumes that the interfering transmitted bit signals are additive. The encoder for-
mula is Pthe same as in the simplified version and before decoding there is an extra step:
∗ N s
Zi,m = s=1 Zi,m . Where s is the sender and N the sum of the transmitted bits from all
senders during that mini-slot. After which the simplified decoder formula can be used to
recover the data.
For CDMA receivers to be able to extract a particular sender’s signal, the CDMA codes
must be carefully chosen. Besides that signal strengths from various senders are not the
same, although that is assumed above. This complicates CDMA.
7.3 WiFi: 802.11 Wireless LANs
The different 802.11 standards for wireless LAN technology have the following common
characteristics:
• The CSMA/CA access protocol.
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, Network and Network Security Summary CHoogteijling
• The same frame structure for link-layer frames.
• The ability to reduce their transmission rate to reach out over greater distances.
• They are backwards compatible.
They have some major differences, like frequency ranges and multiple-output (MIMO) an-
tennas or ’smart’ antennas.
7.3.1 The 802.11 Architecture
See figure 7.7 IEEE 802.11 LAN architecture, in the book Computer Networking A Top
Down Approach by Kurose and Ross 7th edition.
The basic service set (BSS) contains one or more wireless stations and a central access point
(AP). Each wireless station, AP and wireless interface has an unique 6-byte MAC address.
This is an infrastructure wireless LAN. In an ad hoc network, stations group themselves
together without an AP, without connection to anything outside the group.
Channels and Association
Each wireless station needs to associate with an AP before it can send or receive network-
layer data. Associating means the wireless device creates a virtual wire between itself and
the AP. The AP will only send the dataframes to the associated device and the device will
only send frames to the AP.
A administrator assigns a Service Set Identifier (SSID) and a channel number to the AP
when installing it. The channel numbers are in the frequency range of the 802.11 standard.
A WiFi jungle is any physical location where a wireless station receives a strong signal from
two or more APs.
An AP periodically sends beacon frames, that includes the AP’s SSID and MAC address.
The 802.11 standard does not specify an algorithm for selecting which of the available APs
to associate with. The designers of the firmware and software in the wireless device deside
that with an algorithm.
Passive scanning is the process of scanning channels and listening for beacon frames.
Active scanning is the process of broadcasting a probe frame that will be received by all
APs within range. The APs respond with a probe response frame and the wireless device
chooses an AP. Then the device sens an association request frame and the AP responds with
an association response frame to complete the handshake.
Sometimes the wireless device has to authenticate itself to the AP. The AP communicates
with an authentication server, relaying information between the wireless device and the
authentication server using a protocol.
7.3.2 The 802.11 MAC Protocol
The three classes of multiple access protocols:
• Channel partitioning.
• Random access.
• Taking turns.
, page 59 of 83
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