ECE537 Advanced and High
Performance Networks
4: Implementation Issues with Mobile
Networking
Professor Richard A. Stanley, P.E.
Spring 2009
© 2000-2009, Richard A. Stanley
ECE537/4 #1
Overview of Tonight’s Class
• Student presentations/discussions on
MANET
• Review of last time
• Issues in mobile networking
implementations
ECE537/4 #2
Last time…
• Mobile networking needs are growing
• Two basic networking needs:
– Mobile nodes in established networks
– Ad hoc networking
• Coexistence of IPv4 and IPv6 complicates
sharing of data across different mobile
protocols
ECE506/4 #3
MANET, etc.
ECE506/4 #4
Mobile Networking Issues
• Virtually all mobile networking involves
wireless hosts and/or clients
• Wireless, in turn, usually means radio links
– Sometimes IR
• Radio links bring their own problems into
the mix, and we cannot ignore the effects of
these Layer 1 issues on Layers 2 & 3
ECE506/4 #5
WiFi: IEEE 802.11x
• The IEEE 802.11 protocol is an extension of
the IEEE 802.3 protocol for wired networks.
• The IEEE 802.3 protocol defines the Media
Access Control (MAC) and physical layers
for a wired network.
ECE506/4 #6
802.3
• The IEEE 802.3 standard is most widely used
standard for wired network which was developed
out of the original work done on Ethernet.
Ethernet was developed by Xerox corporation's
Palo Alto research center (PARC) in the 1970s
and was the technological basis for the IEEE
802.3 specification, which was initially released in
1980. Today, the term Ethernet is often used to
refer to all carrier sense multiple access/collision
detection (CSMA/CD) LAN’s that generally
conform to Ethernet specifications, including
IEEE 802.3.
ECE506/4 #7
802.3 (Continued)
• The 802.3 protocol has many implementations that are
available and to distinguish between them the committee
has developed a concise notation.
• <data rate in Mbps><signaling method><maximum length
in hundreds of meters>
• The defined alternatives for a 10Mbps date rate are
–
–
–
–
10BASE5
10BASE2
10BASE-T (T= Twisted Pair)
10BASE-F (F= Optical Fiber)
ECE506/4 #8
802.3 (Continued)
• The defined alternatives for a 100Mbps data
rate are
– 100BASE-TX (two pairs of data grade twistedpair wire)
– 100BASE-FX (a two-strand optical fiber cable)
– 100BASE-T4 (four pairs of telephone twisted
pair wire)
ECE506/4 #9
802.11 Versus 802.3
The 802.11 standard is similar in most respects to the IEEE 802.3
Ethernet standard. Specifically, the 802.11 standard addresses:
• Functions required for an 802.11 compliant device to operate either in
a peer-to-peer fashion or integrated with an existing wired LAN
• Operation of the 802.11 device within possibly overlapping 802.11
wireless LANs and the mobility of this device between multiple
wireless LANs
• MAC level access control and data delivery services to allow upper
layers of the 802.11 network
• Several physical layer signaling techniques and interfaces
• Privacy and security of user data being transferred over the wireless
media
ECE506/4 #10
802.11 versus 802.3
• There are a number of characteristics that are
unique to the wireless environment (as compared
to a wired LAN) that the 802.11 standard must
take into consideration. The physical
characteristics of a wireless LAN introduce range
limitations, unreliable media and dynamic
topologies where stations move about, interference
from outside sources, and lack of the ability for
every device to ‘hear’ every other device within
the WLAN.
ECE506/4 #11
802.11 versus 802.3
• These limitations force the WLAN standard
to create fundamental definitions for shortrange LANs made up of components that
are within close proximity of each other.
Larger geographic coverage is handled by
building larger LANs from the smaller
fundamental building blocks or by
integrating the smaller WLANs with an
existing wired network.
ECE506/4 #12
Mobility
• Mobility of wireless stations is the most
important feature of a wireless LAN. A
WLAN would not serve much purpose if
stations were not able to move about freely
from location to location either within a
specific WLAN or between different
WLAN ‘segments’.
ECE506/4 #13
Mobility (Continued)
•
For compatibility purposes, the 802.11 MAC must appear to the upper
layers of the network as a ‘standard’ 802 LAN. The 802.11 MAC layer is
forced to handle station mobility in a fashion that is transparent to the upper
layers of the 802 LAN stack. This forces functionality into the 802.11 MAC
layer that is typically handled by upper layers.
ECE506/4 #14
802.11 WLAN Architecture
The 802.11 architecture is comprised of several components and
services that interact to provide station mobility transparent to the
higher layers of the network stack.
Wireless LAN Station
The station (STA) is the most basic component of the wireless
network. A station is any device that contains the functionality of the
802.11 protocol, that being MAC, PHY, and a connection to the
wireless media. Typically the 802.11 functions are implemented in the
hardware and software of a network interface card (NIC).
A station could be a laptop PC, handheld device, or an Access Point.
Stations may be mobile, portable, or stationary and all stations support
the 802.11 station services of authentication, de-authentication,
privacy, and data delivery.
ECE506/4 #15
802.11 WLAN Architecture
• Basic Service Set (BSS)
802.11 defines the Basic Service Set (BSS)
as the basic building block of an 802.11
wireless LAN. The BSS consists of a group
of any number of stations.
ECE506/4 #16
802.11 Topologies
• Independent Basic Service Set (IBSS)
The most basic wireless LAN topology is a
set of stations, which have recognized each
other and are connected via the wireless
media in a peer-to-peer fashion. This form
of network topology is referred to as an
Independent Basic Service Set (IBSS) or an
Ad-hoc network.
ECE506/4 #17
IBSS
•
In an IBSS, the mobile stations communicate directly with each other. Every mobile
station may not be able to communicate with every other station due to the range
limitations. There are no relay functions in an IBSS therefore all stations need to be
within range of each other and communicate directly.
Independent Basic Service Set (IBSS)
ECE506/4 #18
Infrastructure Basic Service Set
•
An Infrastructure Basic Service Set is a BSS with a component called an Access Point
(AP). The access point provides a local relay function for the BSS. All stations in the
BSS communicate with the access point and no longer communicate directly. All frames
are relayed between stations by the access point. This local relay function effectively
doubles the range of the IBSS.
The access point may also provide connection to a distribution system.
Distribution System
ECE506/4 #19
Distribution System
• The distribution system (DS) is the means by which an access point
communicates with another access point to exchange frames for
stations in their respective BSSs, forward frames, to follow mobile
stations as they move from one BSS to another, and exchange frames
with a wired network.
• As IEEE 802.11 describes it, the distribution system is not necessarily
a network nor does the standard place any restrictions on how the
distribution system is implemented, only on the services it must
provide. Thus the distribution system may be a wired network like
803.2 or a special purpose box that interconnects the access points and
provides the required distribution services.
ECE506/4 #20
Extended Service Set
• Extending coverage via an Extended Service Set (ESS)
802.11 extends the range of mobility to an arbitrary range
through the ESS. An extended service set is a set of
infrastructure BSS’s, where the access points communicate
amongst themselves to forward traffic from one BSS to
another, to facilitate movement of stations between BSS’s.
The access point performs this communication through the
distribution system. The distribution system is the
backbone of the wireless LAN and may be constructed of
either a wired LAN or wireless network
ECE506/4 #21
ESS
• Typically the distribution system is a thin layer in each
access point that determines the destination for traffic
received from a BSS. The distribution system determines if
traffic should be relayed back to a destination in the same
BSS, forwarded on the distribution system to another
access point, or sent into the wired network to a destination
not in the extended service set. Communications received
by an access point from the distribution system are
transmitted to the BSS to be received by the destination
mobile station.
ECE506/4 #22
ESS
• Network equipment outside of the extended
service set views the ESS and all of its mobile
stations as a single MAC-layer network where all
stations are physically stationary. Thus, the ESS
hides the mobility of the mobile stations from
everything outside the ESS. This level of
indirection provided by the 802.11 architecture
allows existing network protocols that have no
concept of mobility to operate correctly with a
wireless LAN where there is mobility.
ECE506/4 #23
ESS
ECE506/4 #24
Distribution Services
• Distribution services provide functionality
across a distribution system. Typically,
access points provide distribution services.
The five distribution services and functions
detailed below include: association,
disassociation, re-association, distribution,
and integration.
ECE506/4 #25
Association
• The association service is used to make a logical
connection between a mobile station and an access point.
Each station must become associated with an access point
before it is allowed to send data through the access point
onto the distribution system. The connection is necessary
in order for the distribution system to know where and how
to deliver data to the mobile station.
The mobile station invokes the association service once
and only once, typically when the station enters the BSS.
Each station can associate with one access point though an
access point can associate with multiple stations.
ECE506/4 #26
Disassociation
• Disassociation
The disassociation service is used either to force a mobile station to
eliminate an association with an access point or for a mobile station to
inform an access point that it no longer requires the services of the
distribution system. When a station becomes disassociated, it must
begin a new association to communicate with an access point again.
An access point may force a station or stations to disassociate because
of resource restraints, the access point is shutting down or being
removed from the network for a variety of reasons. When a mobile
station is aware that it will no longer require the services of an access
point, it may invoke the disassociation service to notify the access
point that the logical connection to the services of the access point
from this mobile station is no longer required.
ECE506/4 #27
Disassociation
• Stations should disassociate when they
leave a network, though there is nothing in
the architecture to assure this happens.
Disassociation is a notification and can be
invoked by either associated party. Neither
party can refuse termination of the
association.
ECE506/4 #28
Re-association
• Re-Association enables a station to change its
current association with an access point. The reassociation service is similar to the association
service, with the exception that it includes
information about the access point with which a
mobile station has been previously associated. A
mobile station will use the re-association service
repeatedly as it moves through out the ESS, loses
contact with the access point with which it is
associated, and needs to become associated with a
new access point.
ECE506/4 #29
Re-association
• By using the re-association service, a mobile station
provides information to the access point to which it will be
associated and information pertaining to the access point
which it will be disassociated. This allows the newly
associated access point to contact the previously associated
access point to obtain frames that may be waiting there for
delivery to the mobile station as well as other information
that may be relevant to the new association.
The mobile station always initiates re-association.
ECE506/4 #30
Distribution
• Distribution is the primary service used by an 802.11 station. A station
uses the distribution service every time it sends MAC frames across
the distribution system. The distribution service provides the
distribution with only enough information to determine the proper
destination BSS for the MAC frame.
The three association services (association, re-association, and
disassociation) provide the necessary information for the distribution
service to operate. Distribution within the distribution system does not
necessarily involve any additional features outside of the association
services, though a station must be associated with an access point for
the distribution service to forward frames properly.
ECE506/4 #31
Integration
• The integration service connects the 802.11
WLAN to other LANs, including one or more
wired LANs or 802.11 WLANs. A portal performs
the integration service. The portal is an abstract
architectural concept that typically resides in an
access point though it could be part of a separate
network component entirely.
The integration service translates 802.11 frames to
frames that may traverse another network, and
vice versa.
ECE506/4 #32
802.11 Media Access Control
• The 802.11 MAC layer provides functionality to allow reliable data
delivery for the upper layers over the wireless PHY media. The data
delivery itself is based on an asynchronous, best-effort, connectionless
delivery of MAC layer data. There is no guarantee that the frames will
be delivered successfully.
• The 802.11 MAC provides a controlled access method to the shared
wireless media called Carrier-Sense Multiple Access with Collision
Avoidance (CSMA/CA). CSMA/CA is similar to the collision
detection access method deployed by 802.3 Ethernet LANs.
ECE506/4 #33
802.11 Media Access Control
• Another function of the 802.11 MAC is to
protect the data being delivered by
providing security and privacy services.
Security is provided by the authentication
services and by Wireless Equivalent Privacy
(WEP), which is an encryption service for
data delivered on the WLAN.
ECE506/4 #34
CSMA/CA
• The fundamental access method of 802.11 is
Carrier Sense Multiple Access with Collision
Avoidance or CSMA/CA. CSMA/CA works by a
"listen before talk scheme". This means that a
station wishing to transmit must first sense the
radio channel to determine if another station is
transmitting. If the medium is not busy, the
transmission may proceed.
ECE506/4 #35
CSMA/CA
• The CSMA/CA protocol avoids collisions among
stations sharing the medium by utilizing a random
backoff time if the station’s physical or logical
sensing mechanism indicates a busy medium. The
period of time immediately following a busy
medium is the highest probability of collisions
occurring, especially under high utilization.
ECE506/4 #36
CSMA/CA
• The CSMA/CA scheme implements a minimum time gap
between frames from a given user. Once a frame has been
sent from a given transmitting station, that station must
wait until the time gap is up to try to transmit again. Once
the time has passed, the station selects a random amount of
time (the backoff interval) to wait before "listening" again
to verify a clear channel on which to transmit. If the
channel is still busy, another backoff interval is selected
that is less than the first. This process is repeated until the
waiting time approaches zero and the station is allowed to
transmit. This type of multiple access ensures judicious
channel sharing while avoiding collisions.
ECE506/4 #37
802.11 “Flavors” - 1
•
•
•
•
•
•
•
•
•
•
•
•
•
IEEE 802.11 - The WLAN standard was originally 1 Mbps and 2 Mbps, 2.4 GHz RF
and infrared [IR] standard (1997), all the others listed below are Amendments to
this standard, except for Recommended Practices 802.11F and 802.11T.
IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
IEEE 802.11c — Bridge operation procedures; included in the IEEE 802.1D standard
(2001)
IEEE 802.11d - International (country-to-country) roaming extensions (2001)
IEEE 802.11e - Enhancements: QoS, including packet bursting (2005)
IEEE 802.11F - Inter-Access Point Protocol (2003) Withdrawn February 2006
IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)
IEEE 802.11i - Enhanced security (2004)
IEEE 802.11j - Extensions for Japan (2004)
IEEE 802.11-2007 - A new release of the standard that includes amendments a, b, d, e,
g, h, i & j. (July 2007)
IEEE 802.11k - Radio resource measurement enhancements (2008)
ECE506/4 #38
802.11 “Flavors” - 2
•
•
•
•
•
•
•
•
•
•
•
•
•
•
IEEE 802.11n - Higher throughput improvements using MIMO (multiple input, multiple output
antennas) (September 2009)
IEEE 802.11p - WAVE — Wireless Access for the Vehicular Environment (such as ambulances and
passenger cars) (working — June 2010)
IEEE 802.11r - Fast roaming Working "Task Group r" - (2008)
IEEE 802.11s - Mesh Networking, Extended Service Set (ESS) (working — September 2010)
IEEE 802.11T — Wireless Performance Prediction (WPP) - test methods and metrics
Recommendation cancelled
IEEE 802.11u - Interworking with non-802 networks (for example, cellular) (working — September
2010)
IEEE 802.11v - Wireless network management (working — June 2010)
IEEE 802.11w - Protected Management Frames (September 2009)
IEEE 802.11y - 3650-3700 MHz Operation in the U.S. (2008)
- Extensions to Direct Link Setup (DLS) (August 2007 - December 2011)
- Robust streaming of Audio Video Transport Streams (March 2008 - June 2011)
IEEE 802.11mb — Maintenance of the standard. Expected to become 802.11-2011. (ongoing)
- Very High Throughput <6GHz (September 2008 - December 2012)
- Extremely High Throughput 60GHz (December 2008 - December 2012)
ECE506/4 #39
802.11 Physical Layer (PHY)
• The 802.11 physical layer (PHY) is the interface
between the MAC and the wireless media where
frames are transmitted and received. The PHY
provides three functions. First, the PHY provides
an interface to exchange frames with the upper
MAC layer for transmission and reception of data.
Secondly, the PHY uses signal carrier and spread
spectrum modulation to transmit data frames over
the media. Thirdly, the PHY provides a carrier
sense indication back to the MAC to verify
activity on the media.
ECE506/4 #40
802.11 Physical Layer (PHY)
• 802.11 provides three different PHY
definitions: Both Frequency Hopping
Spread Spectrum (FHSS) and Direct
Sequence Spread Spectrum (DSSS) support
1 and 2 Mbps data rates.
ECE506/4 #41
802.11b
• Operating in the 2.4GHz frequency range, 802.11b (aka
Wi-Fi) has a nominal maximum data rate of 11Mbps, with
the potential of three simultaneous channels. 802.11b has a
great advantage in that it is accepted worldwide. One of the
more significant disadvantages of 802.11b is that the
frequency band is crowded, and subject to interference
from other networking technologies, microwave ovens,
2.4GHz cordless phones (a huge market), and Bluetooth.
There are drawbacks to 802.11b, including lack of
interoperability with voice devices, and no QoS provisions
for multimedia content. Interference and other limitations
aside, 802.11b is the clear leader in business and
institutional wireless networking and is gaining share for
home applications as well.
ECE506/4 #42
802.11a
• Much faster than 802.11b
– 54Mbps maximum data rate (actually increased to 72Mbps
or 108Mbps in a non-standard double-speed mode
depending on the chipset vendor and component
manufacturer)
– Operates in the 5GHz frequency range and allows eight
simultaneous channels. One big advantage to 802.11a is
that it isn't subject to interference from Bluetooth or any of
the other 2.4GHz frequency denizens.
ECE506/4 #43
802.11a
• One big disadvantage is that it is not directly compatible
with 802.11b, and requires new bridging products that can
support both types of networks, although if you don't mind
spending the money for access points for both 11a and
11b, you can plug them into hubs or better yet, switches on
your network and they'll work just fine. Other clear
disadvantages are that 802.11a is only available in half the
bandwidth in Japan (for a maximum of four channels), and
it isn't approved for use in Europe, where HiperLAN2 is
the standard.
• Like 802.11b, 802.11a has no provisions to optimize voice
or multimedia content.
ECE506/4 #44
802.11g
• Operates in the 2.4GHz frequency band with mandatory compatibility
with 802.11b but with a maximum data rate of 54Mbps
• Uses a minimum of two modes (both mandatory) with two optional
modes.
– The mandatory modulation/access modes are the same CCK
(Complementary Code Keying) mode used by 802.11b (hence the
compatibility with Wi-Fi) and the OFDM (Orthogonal Frequency Division
Multiplexing) mode used by 802.11a (but in this case in the 2.4GHz
frequency band). The mandatory CCK mode supports 11Mbps and the
OFDM mode has a maximum of 54Mbps.
– There are also two modes that use different methods to attain a 22Mbps
data rate--TI's PBCC-22 (Packet Binary Convolutional Coding, rated for 6
to 54Mbps) and Intersil's CCK-OFDM mode (with a rated max of
33Mbps).
ECE506/4 #45
802.11g
• The obvious advantage of 802.11g is that it maintains
compatibility with 802.11b (and 802.11b's worldwide
acceptance) and also offers faster data rates comparable
with 802.11a. The number of channels available, however,
is not increased, since channels are a function of
bandwidth, not radio signal modulation and on that score,
802.11a wins with its eight channels, compared to the three
channels available with either 802.11b or 802.11g. Another
disadvantage of 802.11g is that the 2.4GHz frequency will
get even more crowded
ECE506/4 #46
Wireless Networking Challenges –
Disconnection
• User moves out of range, or obstacle comes
in between
• Techniques to cope with this:
– Operate asynchronously: lazy-write-back,
prefetching
– Expose disconnection to the user
ECE506/4 #47
Wireless Networking Challenges –
Low Bandwidth
• Result of shared channel, high attenuation
• Techniques to cope with this:
–
–
–
–
–
More spectrum (but this is a limited resource)
Smaller cells
Compression
Pre-fetching, lazy write-back
Intelligent scheduling
ECE506/4 #48
Wireless Networking Challenges –
Variable Bandwidth
• Sources of variability:
– Moving from wired to wireless
– Moving from one wireless network to another
– Changing location
• Techniques to cope with this:
– Application has to adapt to changing bandwidth
availability
ECE506/4 #49
Wireless Networking Challenges –
Security Risks
• Problem: broadcast medium!
– No well defined boundary
• Techniques to cope with this:
– Design system with security in mind
• Problem: device can be stolen!
• Techniques to cope with this
– Protect data in the device (e.g. using PIN)
ECE506/4 #50
Wireless Networking Challenges –
Mobility
• Network address has to change!
• Techniques to cope with this:
– Decouple identity from location
– Need to keep track of user location
– Paging mechanism
ECE506/4 #51
Wireless Networking Challenges –
Power Consumption
• Portable devices cannot have large batteries
• Techniques to cope with this:
– Design system with power in mind
– All protocols and applications must be poweraware
ECE506/4 #52
Wireless Networking Challenges –
User Interface
• Wireless applications cannot expect a
sophisticated interface:
– Form factor & capability of device may be
limited
• Techniques to cope with this:
– Application specific
– Clever UI design (e.g. voice recognition)
ECE506/4 #53
RF Propagation
Questions:
– How does RF propagate with distance?
– Behavior under different environments
– How to quantify these?
Goals:
–
–
–
–
Estimate coverage area, link
performance
Use:
Determine network design parameters
• Locations of transmitters
• Transmit power
• Types of antennae
ECE506/4 #54
Three Basic Propagation Phenomena
ECE506/4 #55
The Electromagnetic Spectrum
ECE506/4 #56
Path Loss in Decibels (dB)
ECE506/4 #57
Absolute Power in dBm
ECE506/4 #58
Putting it Together
ECE506/4 #59
Estimating Path Loss
ECE506/4 #60
Frii’s Free-Space Equation
ECE506/4 #61
Path Loss Example
ECE506/4 #62
Remarks
• Free space path loss is idealistic
• In reality, there is more path loss
– Proportional to d^3 or higher
– Particularly true for moving terminals
• Several path loss models are available
– For indoor environments, outdoor metropolitan,
etc.
ECE506/4 #63
Some Path Loss Models
•
•
•
•
Ground reflection (two-ray) model
Knife-edge diffraction model
Outdoor propagation models
Indoor propagation models (site-specific)
– Partition losses (measured)
– Depends on material
ECE506/4 #64
Received Signal Strength
ECE506/4 #65
Fading and Multipath
ECE506/4 #66
Short- and Long-Term Fading
ECE506/4 #67
Channel Impulse Response
ECE506/4 #68
Power Delay Spread
ECE506/4 #69
ECE506/4 #70
RMS Delay Spread Examples
ECE506/4 #71
Delay Spread Observations
• RMS delay spread is a good measure of multipath
– Urban environments: 2-10 µs
– Indoors: 10-500 ns
• Symbol time: time to transmit a bit (0/1)
• Symbol time ~ RMS delay spread ==> Inter-Symbol
Interference (ISI)
– Equalization required
– Generally, ISI results when symbol time < 10 x RMS-delay-spread
• Thus, delay spread puts an upper bound on network
signaling speed
ECE506/4 #72
Summary
• Wireless networking is growing rapidly in
importance
• There are many “special” considerations for
wireless networking
• Unlike most wired networking, physical
layer effects play a large in proper design of
a network and its protocols
ECE506/4 #73
Homework
• Choose a wireless network which you can discuss
in class. Analyze the interaction(s) between
network design, protocol(s), physical layer
constraints, and performance bounds. Prepare a
paper of approximately 1100 words describing
your findings.
• Be prepared to discuss your findings with the class
for 5-10 minutes next week. You may use slides
if you desire.
Spring 2009
© 2000-2009, Richard A. Stanley
ECE506/4 #74
Disclaimer
Parts of the lecture slides contain original
work of the Indian Institute of Technology
Kanpur and remain copyrighted materials
by the original owner(s). The slides are
intended for the sole purpose of instruction
of computer networks at Worcester
Polytechnic Institute.
Spring 2009
© 2000-2009, Richard A. Stanley
ECE506/4 #75