Реферат: Wireless connectivity
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The
Medium Access Control (MAC) layer.
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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:
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Support
of asynchronous and time-bounded delivery service
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Continuity
of service within extended areas
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Accommodation
of transmission rates between 1 and 20 Mbps
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Support
of most market applications
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Multicast
service
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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:
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Correct
delivery of data to its destination.
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Congestion
control.
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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.
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