Wireless connectivity
Wireless Connectivity
Content
Abstract
Introduction
Operation
Wireless Application Design
Caching
Prefetching
Data
reduction
Mobile
WWW Browsers
Active documents
Dynamic
URLs
Data over Cellular links
Radio-based wireless
connectivity
IEEE 802.11 protocol
Interworking Units
for wireless connectivity
Internet Mobile Host Protocol
IP
Tunneling
Abstract
Traditional
networking technologies offer tremendous capabilities from an office or home
via the Web. But, limitations to networking through the use of wired-based
systems exist because you cannot utilize these network services unless you are
physically connected to the system. As mobile computing becomes more prevalent,
systems and applications must deal with scarcity of resources such as
bandwidth. Mobile devices and wireless workstations should handle some of the
work that has been traditionally carried-out by the network through techniques
such as document partitioning. Dynamic documents can also be used to cache and
prefetch documents while the network connection is not being utilized fully.
Meanwhile,
the need for higher speed wireless connections is growing with multimedia rich
contents on the World Wide Web (WWW). The IEEE 802.11 protocol and the Medium
Access Control part of the protocol (DFWMAC) will allow wireless networks to
operate at high data rates (1 to 20 Mbps). Furthermore, the 802.11 only effects
the bottom two layers of the OSI's seven layered architecture; hence, through
an access point (Router), wireless packets are routed to the Web.
Introduction
Wireless
LANs will provide the first layer of connectivity between mobile users and the
global information infrastructure. Wireless devices such as Personal Digital
Assistants (PDAs) and Notebooks will be an extension of the Web. The user
should not know nor care whether the information travels over a wire or a radio
frequency. Depending on the power of the transmitters and the sensitivity of
the receivers, wireless devices may become the first truly universal form of
virtual LAN. By mixing the wireless Networks with other wireless communication
technologies such as cellular and satellite, the user can have full
connectivity at all times and more importantly everywhere on the globe.
Wireless
connectivity to the web can also be achieved through the use of existing
cellular telephone links. Using Spread Spectrum Technologies (SST) such as
time-division multiple access (TDMA), code-division multiple access (CDMA) and
extended time-division multiple access (ETDMA) has allowed the cellular links
to carry more information and as a result better suited for data transmission.
Although the overhead in cellular data transmission is somewhat high, but data
reduction techniques, and caching is used to reduce network latency.
With
the Introduction of PDA, people began to see the natural progression of
Wireless technology into these devices. However, the current state of these
devices has obvious limitations. Computational power, storage, communication
bandwidth, display size and power consumption are just a few of these
limitations. Nevertheless, presently such devices are running Web browser, mail
clients and etc.Presently there is a variety of pen based computer systems like
palmtops, notebooks and different versions of what John Sculley, Apple's vice
president in 1992 introduced as a PDA.
Personal
communication is the primary motivation for wireless connectivity, but in addition,
wireless users need access to on-line information in real time. There are three
reasons why users need to be connected to the Web. First, it is often
difficult, if not impossible, to determine the data of interest ahead of time
and download it to the hand-held device. Second, even then, space limitations
may prevent caching of all data. Finally, some data changes dynamically with
time such as weather forecast, or stock market activities. (Watson, 1994)
The
current application environment is ill-suited for the wireless Web, the wired
web squanders bandwidth through unusable information on the client's side. In
the wired world these inefficiencies amount to only milliseconds, but as the
bandwidth is reduced over wireless links, milliseconds can add up to seconds
and perhaps time-outs by the underlying protocols such as TCP/IP. As a result
various groups have proposed new HyperText Mark-up Language (HTML) or new
protocols such as HTTP+. But these avenues of solutions are rigid and the need
for standardization is greater than a temporary increase in throughput for a
particular scheme.
Operation
Similar
to any transmission system, a wireless system needs a transmitter, a receiver
and a transmission medium. In a wireless system, the transmission medium is air
rather than the cables used by conventional wired systems. The use of air as a
transmission medium utilizes two major spectra: infrared and radio frequency.
The
key difference between the use of infrared and radio frequency is the support
of roaming. Infrared is a line-of-sight technology. There has to be a direct
line of sight or at least a surface to bounce the waves from the transmitter to
the receiver. On the other hand, radio frequency systems can penetrate through
objects such as walls and doors in most office buildings; hence their
popularity in present wireless systems. FCC rules allow only small sections of
the electromagnetic spectrum (figure below) to be used for wireless data
networks; thus techniques are needed to avoid interference from other devices
that share the space or perhaps multiple stations using the same frequency.
A
technique developed by the military in the 1970s to help secure transmissions
offers a way around this problem. This technique is called Spread Spectrum
Technology (SST). It involves spreading transmissions across a range of
frequencies, rather than transmitting on one frequency all the time.
One
approach known as Frequency-Hopping Spread Spectrum (FHSS) involves dividing a
range of the radio spectrum into individual channels, each on a specific
frequency. A transmitter can hop from one channel to the next and if the
receiver is aware of the hopping pattern of the transmitter, it can follow the
pattern and receive the information. The second method of spread spectrum is
Direct Sequence Spread Spectrum (DSSS). The source data to be transmitted is first
exclusive ORed with a pseudorandom binary sequence. The bits making up the sequence
are random but the same sequence is made much larger than the source data rate.
When this data is modulated and transmitted it occupies a wider frequency band
than the original source data bandwidth. This would make the signal appear as
noise to any other devices using the same frequency spectrum. All the members
of this wireless system know the binary sequence being used .(Halsall, 1996) Thus, all receivers first
search for the known preamble sequence, once it has been recognized, the
receivers start to interpret the bit stream.
FCC
rules for DSSS transmission requires 10 or more redundant data bits to be added
to each signal. This limits the maximum throughput of DSSS transmitters to
approximately 2 Mbps when using the 902-MHz band, and approximately 8 Mbps in
the 2.4-GHz band.
Wireless Application Design
Designing
a web application for a wireless node is different from designing a web application
for a workstation. Bandwidth is a precious resource in the wireless domain and
it must be utilized in the most efficient fashion. Research focuses on
streamlining applications to make the best use of the available bandwidth.
These options include using dynamic documents which use the resources of the
mobile node itself to generate parts of a document or partitioning the application
between a client and the server.
Dynamic
documents can address the variable resources requirement of mobile computers
accessing the Web. Dynamic documents are programs executed by programs such as
Web browsers in order to generate the actual information displayed to the user.
Execution of a dynamic document cause the client to perform any number of
actions in order to generate a final presentation to the user. Dynamic
documents are flexible enough to address many mobile computing resource
constraints. Documents can be customized at the client depending on available
resources. (Kaashoek, 1994)
Application
partitioning can also be used over a wireless link for more effective use of
the wireless link. Much like a client/sever system, applications and their
functionality can be divided into different parts. the boundaries between how
much of the application should be run on the client side vs. the server side
can be determined dynamically and based upon the availability of the bandwidth.
The data and their functions are packaged into hyperobjects. The purpose of
hyperobjects is to expose a certain level of application structure and semantics
to the system in a uniform and manageable way. The system will use this
hyperobject structure, along with observations of access patterns to make
informed decisions. (Watson, 1995)
Partitioning
documents are combined with several other well-known techniques to increase the
effectiveness of wireless clients such as browsers.
Caching
Applications
specify the caching attributes of an object or a number of objects. The default
is to optimistically replicate objects on the mobile device. Explicit
synchronization can be used to make the cache consistent with the wired network
if the wireless link is up. (Watson, 1995)
Prefetching
As
a document is loaded and displayed on a mobile device, the links in a hiarchial
fashion are used to prefetch the relevant documents and cached. In a
hyperobject application the system will use its knowledge of the relevancy and
the position of various objects in order to anticipate and prefetch other
objects. Prefetching can only be done if the system resources allow it. For
example, as a user is viewing the first page of a document, the relevant
objects for that document are being prefetched into the cache, given the
wireless link is up and functioning. Prefetching hides the latency of the link,
and it will also filter the burstiness by spreading the traffic over a longer
time.
Data reduction
Data
reduction can be dynamically decided by the user for various high bandwidth
applications such as video transmissions. A video stream delivers certain
number of frames per constant unit of time. As the number of frames are
reduced, it adversely affects the quality of the video, but the bandwidth
needed is also reduced; hence, the user can dynamically find a balance between
what the available resources and the desired video quality. The same principal
can be applied to the sound, and also any real-time stream of data over the
wireless link.
Mobile WWW Browsers
Web
infrastructure as it exists today can not easily accommodate mobile clients,
because of the fact that almost all information resides statically in HTML
documents. The dynamic information that the Web supports is returned to the
client without incorporating any user context, or is incorporated explicitly
using forms-based interfaces that require user input on the client. Extensions
to the Web have been created to include:
·
A
network server that maintains mobile computing contexts within a client-specific
domain.
·
An
asynchronous callback mechanism to notify Web clients when a user's dynamic
computing environment changes.
·
A
syntax for referencing dynamic information in URLs and documents. (Voelker)
Active documents
Active
documents are HTML documents that allow the Web client to automatically react
to changes in mobile computing environment. If the information in an active document
that the client is displaying becomes invalid, then the client can be notified
of that change so that more relevant information can be displayed. Variables
such as location can be updated as the mobile user roams from one cell area to
the next. Active documents are written just like any other HTML file with only
a minor addition. A subscribe command is embedded in an HTML comment line. By
having the subscribe command embedded in a comment line, backward compatibility
can be preserved, thus allowing regular Web browsers to view the documents. (Voelker)
Dynamic URLs
Ordinarily
URLs are links to set static documents on the Web. Dynamic URLs will reference
a user to a different document based upon other variables, such as the location
variable. Dynamic URLs exist in active documents in order to receive the
variables from the client. When a user selects a dynamic URL in a document, the
client browser is responsible for resolving all references to dynamic variables
within the URL. When all variable references have been resolved, the result is
a standard URL that the client then sends to the server. (Voelker)
Data over Cellular links
The
analog cellular telephone system uses FM (Frequency Modulation) radio waves to
transmit voice grade signals. To accommodate mobility, this cellular system
switches radio connection from one cell to another as the mobile user moves
from one cell to another (roaming). Every cell within the network has a
transmission tower that links mobile callers to a Mobile Telephone Switching
Office (MTSO). The MTSO, which is owned and operated by the cellular carrier in
each area provides a connection to the public switched telephone network. The
public telephone networks acts also as gateways to the Internet.
Most
modems that operate over wireline telephone services will also interface and
interoperate with cellular phones; however, modem software optimized to work with
cellular phones minimizes battery usage. There are problems with modem
communication over cellular links. The first problems occurring were the
hand-off problems or roaming. As a mobile user moves from one service area to
the next, a hand-off occurs from one service area to the next. The hand-off
would disrupt the call for 100 to 200 ms. This is just enough to disrupt the
carrier detect (CD) cycle; hence, the modem assumes that one of the callers has
disconnected, and it hangs up. This problem can be overcome similar to fax
modems over cellular links. The modem will delay 400 ms before hanging up,
giving the hand-off enough time to take place. Some data might be affected, but
error detection, and error correction procedures (CRCs) will detect and correct
the data bits that have been corrupted. But, all these techniques lower the
effective throughput of our communication system and the effective throughputs
achieved with cellular modems hover around 19200 bits/s. (Bates, Gregory, 1995)
To
establish a dedicated wireless data network for mobile users, a consortium of
companies in the United States developed the Cellular Digital Packet Data
(CDPD) standard. CDPD overlays the conventional analog cellular telephone
system, using a channel hopping technique (previous section) to transmit data
in short bursts during idle times in cellular channels. CDPD operates full
duplex, meaning simultaneous transmission in both directions in the 800 and 900
MHz frequency bands. The main advantage of the analog cellular system is
widespread coverage. Since CDPD piggybacks on this system, it will also provide
nearly worldwide coverage. The main advantage with CDPD is that, it uses
digital signals, making it possible to enhance the transmission of data. With
digital signaling, it is possible to encrypt the data stream and provide easier
error control. CDPD is a robust protocol that is connectionless and corrects
errors at the receiver side without asking the source to retransmit the errored
packet.
Other
digital techniques presently being tested and utilized by the carrier companies
are:
·
Time-division
multiple access (TDMA)
·
Extended
time-division multiple access (ETDMA)
·
Code-division
multiple access (CDMA)
·
Narrowband
advanced mobile phone service (N-AMPS)
In
the case of ETDMA the bandwidth can be increased by a factor of 15, making it
much more acceptable for today's application needs.
Radio-based wireless connectivity
The
most widely sold wireless LAN products use radio waves as a medium between
computers and the WEB or each other. An advantage of radio waves over other
forms of wireless connectivity such as infrared and microwaves is that they
propagate through walls and other obstructions with little attenuation. Even
though several walls might separate the user from the server or an access point
to the Web, users can maintain connections to the network, thus supporting true
mobility. The disadvantage for radio frequencies is that governments manage the
region and not all the spectrum can be used everywhere; hence, techniques such
as FHSS and DSSS (as described ) must be used.
There
are three regions of the E-M spectrum utilized by these waves:
·
902-928
MHz
·
2.4-2.484
GHz
·
5.725-5.850
GHz
Presently
Metricom is operating a two way radio based multi-user data communications
system is San Francisco called Ricochet. The architecture is shown below:
The
concept is to use wireless access points and network radio relays approximately
one half mile apart to facilitate connectivity between users. The radios
operate in the license-free 902-928 portion of the radio spectrum using FHSS.
The underlying network protocol is TCP/IP, allowing it to interact with the
Internet seamlessly.
An
important goal for wireless communications has been to make the application
layer transparent to the underlying protocol (TCP/IP) in order to have more
acceptability by the Web users. To understand the kind of standards developed
for wireless networks, it helps to see the affected layers in an OSI (Open
System Interconnect) model. The bottom two layers are the ones of interest to
us. At the very bottom is the Physical layer. This layer defines the electrical
characteristics of the actual connection between network nodes. For wired
networks, it covers topics like voltage levels and type of cabling. But for
wireless networks, it addresses areas such as frequencies used and modulation
techniques, including spread-spectrum technologies.
The
next layer up is the Data Link Layer. It deals with how the network is shared between
nodes. The Data Link Layer defines rules such as who can talk on the network,
how long they can occupy network resources. This layer can be further divided
into two separate layers (shown below).
·
The
Medium Access Control (MAC) layer.
·
The
Logical Link Control (LLC) layer.
The
first five layers of the OSI model remains unchanged; hence, TCP and IP can be
implemented in their respective layers.
IEEE 802.11 protocol
The
wireless network interface manages the use of air through the operation of a
communications protocol. For synchronization, wireless networks employ a
carrier sense protocol similar to the common Ethernet standard. This protocol
enables a group of wireless computers to share the same frequency and space.
The
lack of standards has been a significant issue with wireless networking. In
response to this problem, the Institute for Electrical and Electronic Engineers
(IEEE) has been involved in the development of wireless LAN standards for the
last seven years. This effort is nearly complete, and the final standard (IEEE
802.11) will be ready by May of 1997.
As
with other 802 standards such as Ethernet and token ring, the primary service
of the 802.11 standard is to deliver MSDUs (MAC Service Data Units) between LLC
(Logical Link Control) connections to the network. In other words, the 802.11
standard will define a method of transferring data frames between network
adapters without wires. In addition, the 802.11 standard will include:
·
Support
of asynchronous and time-bounded delivery service
·
Continuity
of service within extended areas
·
Accommodation
of transmission rates between 1 and 20 Mbps
·
Support
of most market applications
·
Multicast
service
·
Network
management services, Registration and authentication services
The
IEEE 802.11 standard supports operation in two separate modes, a distributed
coordination (DCF) and a centralized point-coordination mode (PCF). The IEEE
802.11 MAC is called DFWMAC (Distributed Foundation Wireless MAC), and the
access mechanism is based upon the principal of CSMA/CA (Collision Sense Medium
Access with Collision Avoidance), which is another adaptation of CSMA/CD used
by Ethernet networks.
Under
CSMA/CD, when a station has data to send, it first listens to determine whether
any other station on the network is occupying the medium. If the channel is
busy, the station will wait until it becomes idle before transmitting data.
Since it is possible for two stations to listen at the same time and discover
an idle channel, it is also possible that two stations could then transmit at
the same time. When this occurs a collision will take place, and then a jamming
signal is sent throughout the network in order to notify all stations of the
collision. The stations will then wait for a random period of time before
re-transmitting their respective frames.
CSMA/CA
is a modified version of the CSMA/CD access system. Under the CSMA/CA
technique, as before stations are listening to the medium at all times. A
station that is ready to transmit a frame will sense the medium, if the medium
is busy, it will wait for an additional predetermined time period of DIFS (DCF
Interframe Space) length and then, based upon a random calculation, picks a
time slot within a contention window to transmit its frame. If there were no
other transmissions before this time slot has arrived, it will start transmitting
its frame. On the other hand if there were transmissions by other stations
during this back-off time period, the station will freeze its counter and will
pick-up the count where it left off after the other station has completed its
frame transmission. The collisions can now occur only when two or more stations
select the same time slot to transmit. These stations will have to reenter the
contention procedure to select new time slots to transmit the collided frames.
The figure below illustrates DFWMAC access scheme.
Interworking
Units for wireless connectivity
Just
as in wired networks, the interworking unit (IWU) provides the protocol manipulation
to connect networks with different protocols together. The IWUs act as access
points between wireless stations and the Web. They address issues such as:
·
Correct
delivery of data to its destination.
·
Congestion
control.
·
Differences
in maximum PDU sizes.
To
connect a wireless network that is using the 802.11 protocol to the Internet,
IWUs are needed at access points. Access points are nodes that allow traffic
flow in and out of the Wireless network. Alternatively, IWUs (IP Routers)
control the traffic in and out of the Internet; thus routing wireless packets into
and out of the Internet as shown below:
The
802.11 protocol can support data rates of 20 Mbps, thus making it an attractive
wireless protocol for Internet connectivity. Companies such as Proxim that have
been involved with the development of 802.11, are migrating rapidly to the new
standard.
Internet Mobile Host Protocol
An
important part of wireless connectivity is mobility. Mobile computers must be
able to move between adjacent cells or across multiple network domains without
disturbing the application level process. Mobile users and mobile protocols
must not make any changes to the existing TCP/IP Internet protocol to insure
connectivity and usability of the Internet as it exists today.
A
mobile host is the Internet Mobile Host Protocol (IMHP) entity that roams
through the Internet. Each mobile host has a home agent on its home network.
Each home agent maintains a list known as a home list. The home list is a list
of mobile hosts that the home network will serve and it also maintains the
location of each mobile host as the network becomes aware of their locations.
As mobile hosts roam from one network to the next, they have to register with
foreign agents on new subnets as they try to connect to that network. Foreign
agents are much like a home agent except they interact with visiting home
agents from other networks. Each foreign agent maintains a list known as the
visitor list, which identifies the mobile hosts that are currently registered
with it. The combination of the foreign agents address for a particular home
agent (care-of-address) along with its home address is known as a binding. A
binding defines where to send packets for a particular home agent at any given
time. (Perkins, Myles, and Johnson, 1994)
The
registration protocol which is part of the IMHP management protocol notifies
all the concerned parties of the new mobile host's location. Those include the
previous foreign agents and the host's home agent. It is the responsibility of the
IMHP management protocol to keep a forwarding pointer from the previous foreign
agents until all information about the new location has been updated with the
new network and the home network. Time stamps are used to keep visitor lists
current and to delete the home agents that have left the network. Figure below
shows the registration process for a home agent through a foreign agent and the
notification process.
Any
node may function as a cache agent by caching the bindings of one or more
mobile hosts. All of these cache agents are under the umbrella of the IMHP
management protocol which is running on all IMHP agents as long as they are not
on their home networks. The IMHP management protocol manages the cache agents
in a distributed fashion. This will allow packets to travel to their
destinations without having to be routed to a home networks first. Cache agents
actively attempt to reconform bindings in their location caches using the IMHP
management protocol, and also periodically notifications are send out by the
protocol to update caches when agents move in and out of networks.
IP Tunneling
IMHP
entities direct and send packets to a mobile host's current location using a
tunneling technique. Tunneling in IMHP management protocol takes the form of
encapsulation. The protocol will add 8 bytes to each packet sent to a mobile
host if the sender has a location cache entry for the destination mobile host,
otherwise it adds 12 bytes to each packet. The tunneling header is inserted
into the packet immediately following the existing IP header. In the IP header,
the protocol number is set to indicate the IMHP encapsulation tunneling
protocol, and the destination address is set to the mobile host's
care-of-address, and finally the source address is set to the IP address of the
encapsulating agent. (Perkins, Myles, and Johnson,
1994)
This
tunneling procedure will inssure packet delivery throughout the Internet as it
exists today, since the intermediate routers will see a normal IP packet. It is
only the IMHP network that can recognize the packets by seeing the protocol
number and deliver them to their final destination.
|