wireless broadband railway digital network_23
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BROADBAND RAILWAY DIGITAL NETWORK
A Seminar Report on
Prepared by : Mavani Yogesh S.
Roll No. : 23
Class : B.E.IV (Electronics & Communication Engineering.)
Semester :8th Semester
Year : 2006-2007
Guided by : Mr. Dhairya B Bapodra
Department
of
Electronics & Communication Engineering.
Sarvajanik College of Engineering & TechnologyDr R.K. Desai Road,
Athwalines, Surat - 395001,
India.
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Sarvajanik College of
Engineering & TechnologyDr R.K. Desai Road,
Athwalines, Surat - 395001,India.
Department
of
Electronics& Communication Engineering.
CERTIFICATEThis is to certify that the Seminar report entitled
BROADBAND RAIL DIGITAL NETWORK is prepared &
presented by Mr.MAVANI YOGESH S. , Class Roll No. 23 of
final year (B.E.IV) Electronics & Communication Engineering
during year 2006-2007. His work is satisfactory.
Signature of Guide Head of DepartmentElectronics & Communication Engineering
Signature of Jury Members
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ACKNOWLEDGEMENT
I take this opportunity to express my sincere thanks and deep sense of gratitude to
my guide Mr. Dhairya B Bapodra for imparting me valuable guidance during the
preparation of this seminar. They helped me by solving many doubts and suggesting
many references.
I am also thankful to Prof. Mehul Raval Department In-charge (DIC) of
Electronics & Communication Engineering Department of Sarvajanik College Of
Engineering and Technology, Surat.
I would also like to offer my gratitude towards faculty members of Electronics &
Communication department, who helped me by giving valuable suggestions and
encouragement which not only helped me in preparing this presentation but also in having
a better insight in this field. Lastly, I express deep sense of gratitude towards my
colleagues and also those who directly or indirectly helped me while preparing this
seminar.
MAVANI YOGESH S.
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ABSTRACT
Broadband Railway Digital Network (BRDN) integrating the emerging broadband
wireless access technologies will satisfy the requirements of next generation Railway
Information System (RIS). This paper proposes an overall implementation scheme for
BRDN with discussion on its main components, such as the intranet on train, and
infrastructure of BRDN. The paper aims at enabling deployment of innovative, cost-
effective, and interoperable multi-vendor broadband wireless access system for RIS on
fleeting train.
The traditional railroad communication system (RCS) is not only in charge of the
traditional train scheduling, but also offers broadband WLAN services to passengers and
provides the network platform to the intelligent railroad information system. This imposes
a major challenge on the capability of the RCS in response to the increasing application
requirements, particularly, the one for ubiquitous Internet access. To take the advantage of
the rapid evolving mobile communication technology, this paper proposes an IEEE
802.20 based broadband railroad digital network, namely BRDN, for the next generationRCS. The paper further presents the scenario how BRDN operates and identifies the IP
mobility as the major technical issue to be solved for BRDN. The predictability of mobile
IP for the train-based applications will ease the difficulties in implementing BRDN. With
the availability of IP standard for the next generation Internet - IPv6, BRDN will
eventually become a reality.
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LIST OF FIGURES
FIGURE NO. TITLE OF FIGURE PAGE NO.
1.1 Network Technologies 1
1.2.1 Satellite Communication 2
1.2.2 Cellular Network 3
1.2.3 LAN Based Network 3
2.1 BRDN Configuration 4
2.2 The Operation Scenario Of BRDN 5
3.1 Wireless LAN Architecture Using An
Infrastructure BSS
9
4.1 The Operation Of Mobile Ipv6 24
III
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LIST OF TABLES
TABLE NO TITLE OF TABLE PAGE NO
1. IEEE 802.20 Vs Other Mobile
Techniques
13.
IV
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INDEX
CHAPTER NO. TITEL PAGE NO.
Acknowledgement I
Abstract II
List of figures III
List of tables IV
1. INTRODUCTION 1-3
1.1 Broadband Wireless Technology 1
1.2 BRDN Connectivity 2
1.2.1 Satellite Communications 2
1.2.2 Cellular Network 3
1.2.3 LAN-based Network 3
2. THE FRAMEWORK OF BRDN 4-5
2.1 The Configuration 4
2.2 The Operation Scenario 5
3. WIRELESS LAN 6-133.1 History 6
3.2 Benefits 7
3.3 Disadvantages 7
3.4 Architecture 9
3.5 IEEE 802.11a 9
3.6 IEEE 802.11b 9
3.7 IEEE 802.11g 10
3.8 IEEE 802.11n 10
3.9 IEEE 802.11 LAYER 11
3.10 IEEE 802.20 The Key Technology
For BRDN
12
4. WIRELESS WAN 14-204.1 Characteristics of WAN
environments
14
4.2 High level architectural tradeoffs in
WWAN environments
16
4.3. The WTCP approach 17
5. MOBILE IPV6 21-255.1 Introduction 215.2 FEATURES OF IPv6
5.2 Mobile IPv6 - The Key Issue In
BRDN Implementation
23
6. SUMMARY 26
BIBLOGRAPHY 27
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1. INTRODUCTION
1.1 Broadband Wireless Technology
Wireless Broadband Networks are defined as communication without wire over
distance by the use of arbitrary codes and increased bandwidth. More specifically WirelessBroadband Connection refers to technologies that use point-to-point or point-to-multipoint
microwave, in various frequencies between 2.5 GHZ and 43 GHZ, to transmit Signals
between hub sites and end users. The technology can be used to provide voice, data &
even video information and requires line-of-sight between the hub side and the end user
receiver.
Great efforts have been done in deploying data communications network for RIS
in adopting the latest broadband wireless technologies, such as IEEE 802.11x . The IEEE
802.11 standard (nicknamed as Wi-Fi) came into use in 1997 for Wireless LAN. In 1999,
the IEEE Standards Board enhanced the 802.11 standard: 802.11b working at 2.4 GHz
with the data rate up to 11 Mbps, and 802.11a working at 5 GHz and supporting higher
data rates up to 54 Mbps. In June 2003 IEEE approved 802.11g, which is compatible with802.11b with the same data rate as 802.11a. 802.11a/b/g has become popular worldwide.
In October 2004, P802.16-REVd/D5 has been approved as IEEE Standard 802.16-2004, a
revision and combination standard of 802.16 and 802.16a. In December 2004, IEEE
802.16e Task Group released the last draft 802.16e5a which defines air interface for fixed
and mobile broadband wireless access .In another aspect, the mobility-oriented wireless
metropolitan area network standard, IEEE 802.20, has been initiated. In December 2002,
the IEEE Standards Board approved the establishment of IEEE 802.20 Working Group.
IEEE 802.20 promises to support various vehicular mobility classes up to 250 km/h in a
MAN environment and targets spectral efficiencies, sustained user data rates and numbers
of active users that are all significantly higher than achieved by existing mobile systems.
Fig 1.1 Network Technologies1
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1.2 BRDN Connectivity
There are three possible independent recommended modes for providing
connectivity to the train:
1. Satellite Communication
2. Existing Cellular Networks3. LAN base Network
1.2.1 Satellite Communications
Satellite communications must be provided for Backup or as a fall-up mechanism
if trackside infrastructure or cellular network is out of service, let us say, because of an
earthquake, or as happened in the case of Hurricane Katrina, when the only
communication channel available was Satellite. For such situations, the service provider
must provide and support full "two-way" satellite links with Two-way satellite antenna
installed on the roof of the train to connect with the In-train Wi-Fi Network. The needs for
Homeland Security requirements are to provide seamless access to mission-critical voice,
video and data communications for emergency purposes. Since Satellite offers relatively
lower network bandwidth, this could be used for remote connectivity during emergency;
however, if Satellite connectivity is the only means to providing Internet connectivity in
the train, it must be at higher bandwidths as per current industry trends for higher
commuter satisfaction. The Wi-Fi connection for user connectivity must be maintained
despite short interruptions such as tunnels and the system must allow users to maintain a
continuous link to a local server onboard the train which would be critical for such
situations when the train is passing through a tunnel. For such situations, the satellite
communication should be capable of being restored rapidly. Satellite link with the train, if
used for commercial Wireless Internet, must be capable of at least 4MBit/s downlink and
2MBit/s uplink, with 10 users using the system via Wi-Fi enabled laptops, downloading
large files, streaming videos, etc. Satellite connectivity with the train, if used forcommercial Wireless Internet, must be capable of providing service to up to as many as
100 concurrent users per train in a scaled manner or 500 concurrent users as the user base
increases over all the train sets for the entire corridor. For satellite communication, the
service provider must use power management to adjust the amount of transmit power to
compensate for adverse features like rain fade.
Fig.1.2.1 Satellite Communication2
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1.2.2 Cellular Network
Cellular-based wireless networks are designed to transmit more or less continuous
streams of data. While a circuit connection is established between two users, the exchange
of data from voice, audio or video sources is continuous, even if there is no information to
send because a user is not talking at the time. This is very wasteful of valuable circuit
capacity and digital cellular system providers learned to utilize the quiet times and idlechannels for other purposes. It seems like a natural fit for the growing data transmission
market except for one factor, the whole data transmission industry is moving towards
packet data transmission, which breaks up continuous data streams into a sequence of
discrete packets. In short, cellular networks had to learn how to send IP data packets.
Fig. 1.2.2 Cellular Network
1.2.3 LAN-based Network
LAN based networks has its roots in data transmission. The genesis is the
ubiquitous local area network (LAN), which has been carrying data between office
computers for decades. It is based on the Ethernet system, upon which the entire Internetis built using the Internet Protocol (IP) for transmitting packets of data. LANs are
typically represented as clouds simply because the actual route taken by the data packets
is not important. All users are effectively connected to all other users and are
distinguished by an address. LANs are primarily for data transmission, but as market
pressure tends to do, the network providers soon learned how to send digitized voice
using IP packets (voice over IP, or VoIP) and enterprising service providers are now
offering long distance phone services over the Internet.
Fig. 1.2.3 LAN Based Network3
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2. THE FRAMEWORK OF BRDN
In this section, we construct a framework for the BRDN underpinned by two
wireless network standards the IEEE 802.20 wireless WAN (WWAN) on ground and
the IEEE 802.11x WLAN on train.
2.1 The Configuration
Figure 2.1 illustrates a configuration for a mobile network on the train
implemented. A WLAN is deployed in a train with a typical configuration: The Ethernet
(IEEE 802.3) cable is wired through all carriages via MVB (Multi Vehicle Bus); in each
carriage, Wi-Fi access points are deployed with complete coverage of the area, and are
hooked up to the Ethernet; end users access this WLAN via their mobile devices, such as
notebook computer, PDA, mobile IP phone, etc. An IEEE 802.20 client interfaced to the
Ethernet via a router connects the WLAN to the base stations of IEEE 802.20 on the
ground, which jointly offer the transparent services on the physical layer and data link
layer between the Internet and the mobiles nodes. Because of the availability of NAT
(Network Address Translate) technology, the whole network on train may only need asingle IP address. A DHCP server can automatically assign each mobile node of thenetwork on the train a dynamic IP address.
Fig 2.1 BRDN Configuration
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The interconnection scheme of WWAN between the IEEE 802.20 base stations
has several choices: IEEE 802.20, IEEE 802.16x, existing cellular communication
systems infrastructure, and the cable, because these base stations are still. As we
discussed in Section 3, IEEE 802.20 is supposed to be the best choice for its high-speed
mobility support, with high data rate, spectral efficiency and low latency. The train-based
IEEE 802.20 mobile client as a whole is on moving when the train is in operation,
switching from one network to another over time (about 10-30 minutes at the regularmoving speed) The mobile IP problem - how the train-based client remains reachable
from the ground stations, while moving around in the Internet quickly, regardless of its
current point of attachment to the Internet.
According the above configuration, the proposed BRDN will adopt IEEE 802.20
and 802.11x standards to implement the function of physical layer and data link layer. The
physical layer provides the air interface of wireless network, and the data link layer offers
reliable data frame transmissions between neighboring hosts, by use of the bit
transmission function provided by physical layer.
2.2 The Operation Scenario
The working scenario of BRDN is shown in Figure 3. When a train departs from
location A, the #1 base stations provides the Internet connection to the train via #1s
network. When the train approaches location B, it will contact both #1 base station and #2
base station at the border because there is a coverage overlapping between the two base
stations. After the IEEE 802.20 client on train switches the connection from #1 base
station to #2 base stations, #2s network starts to provide the path to the Internet, and the
network connection handover is complete. The network handovers happens in the same
way when the train arrives at location C and location D.
Fig. 2.2 The Operation Scenario Of BRDN
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3. WIRELESS LAN
A wireless LAN orWLAN is a wireless local area network, which is the linking
of two or more computers without using wires. WLAN utilizes spread-spectrumtechnology based on radio waves to enable communication between devices in a limited
area, also known as the basic service set. This gives users the mobility to move around
within a broad coverage area and still be connected to the network.
For the home user, wireless has become popular due to ease of installation, and
location freedom with the gaining popularity of laptops. For the business, public
businesses such as coffee shops or malls have begun to offer wireless access to their
customers; some are even provided as a free service. Large wireless network projects are
being put up in many major cities. Google is providing a free service to Mountain View,
California and has entered a bid to do the same for San Francisco.
3.1 History
In 1971, researchers at the University of Hawaii developed the worlds first
WLAN, or wireless local area network, named ALOHAnet. The bi-directional star
topology of the system included seven computers deployed over four islands to
communicate with the central computer on the Oahu Island without using phone lines.
Originally WLAN hardware was so expensive that it was only used as an
alternative to cabled LAN in places where cabling was difficult or impossible. Early
development included industry-specific solutions and proprietary protocols, but at the end
of the 1990s these were replaced by standards, primarily the various versions of IEEE
802.11 (Wi-Fi). An alternative ATM-like 5 GHz standardized technology, HIPERLAN,has so far not succeeded in the market, and with the release of the faster 54 Mbit/s
802.11a (5 GHz) and 802.11g (2.4 GHz) standards, almost certainly never will.
In November 2006, the Australian Commonwealth Scientific and Industrial
Research Organisation (CSIRO) won a legal battle in the US federal court of Texas
against Buffalo Technology which found the US manufacturer had failed to pay royalties
on a US WLAN patent CSIRO had filed in 1996. CSIRO are currently engaged in legal
cases with computer companies including Microsoft, Intel, Dell, Hewlett-Packard and
Netgear which argue that the patent is invalid and should negate any royalties paid to
CSIRO for WLAN-based products.
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3.2 Benefits
The popularity of wireless LANs is a testament primarily to their convenience,
cost efficiency, and ease of integration with other networks and network components. The
majority of computers sold to consumers today come pre-equipped with all necessary
wireless LAN technology.
The benefits of wireless LANs include:
Convenience: The wireless nature of such networks allows users to accessnetwork resources from nearly any convenient location within their primary
networking environment (a home or office). With the increasing saturation of
laptop-style computers, this is particularly relevant.
Mobility: With the emergence of public wireless networks, users can access theinternet even outside their normal work environment. Most chain coffee shops, forexample, offer their customers a wireless connection to the internet at little or no
cost.
Productivity: Users connected to a wireless network can maintain a nearlyconstant affiliation with their desired network as they move from place to place.
For a business, this implies that an employee can potentially be more productive
as his or her work can be accomplished from any convenient location.
Deployment: Initial setup of an infrastructure-based wireless network requireslittle more than a single access point. Wired networks, on the other hand, have the
additional cost and complexity of actual physical cables being run to numerous
locations (which can even be impossible for hard-to-reach locations within abuilding).
Expandability: Wireless networks can serve a suddenly-increased number ofclients with the existing equipment. In a wired network, additional clients would
require additional wiring.
Cost: Wireless networking hardware is at worst a modest increase from wiredcounterparts. This potentially increased cost is almost always more than
outweighed by the savings in cost and labor associated to running physical cables.
3.3 Disadvantages
Wireless LAN technology, while replete with the conveniences and advantages
described above has its share of downfalls. For a given networking situation, wireless
LANs may not be desirable for a number of reasons. Most of these have to do with the
inherent limitations of the technology.
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Security: Wireless LAN transceivers are designed to serve computers throughout
a structure with uninterrupted service using radio frequencies. Furthermore, because of
space and cost, the "antennas" typically present on wireless networking cards in the end
computers are generally nothing more than the most naive of reception devices. In order
to properly receive signals using such limited antennas throughout even a modest area,
the wireless LAN transceiver utilizes a fairly considerable amount of power. What this
means is that not only can the wireless packets be intercepted by a nearby adversary'spoorly-equipped computer, but more importantly, a user willing to spend a small amount
of money on a good quality antenna can pick up packets at a remarkable distance; perhaps
hundreds of times the radius as the typical user. In fact, there are even computer users
dedicated to locating and sometimes even hacking into wireless networks, known as
wardrivers. On a wired network, any adversary would first have to overcome the physical
limitation of tapping into the actual wires, but this is not an issue with wireless packets.
To combat this consideration, wireless networks may choose to utilize some of the
various encryption technologies available. Some of the more commonly utilized
encryption methods, however, are known to have weaknesses that a dedicated adversary
can compromise.
Range: The typical range of a common 802.11g network with standard equipmentis on the order of tens of meters. While sufficient for a typical home, it will be
insufficient in a larger structure. To obtain additional range, repeaters or additional
access points will have to be purchased. Costs for these items can add up quickly.
Other technologies are in the development phase, however, which feature
increased range, hoping to render this disadvantage irrelevant.
Reliability: Like any radio frequency transmission, wireless networking signalsare subject to a wide variety of interference, as well as complex propagation
effects (such as multipath, or especially in this case Rician fading) that are beyond
the control of the network administrator. In the case of typical networks,
modulation is achieved by complicated forms of phase-shift keying (PSK) or
quadrature amplitude modulation (QAM), making interference and propagation
effects all the more disturbing. As a result, important network resources such as
servers are rarely connected wirelessly.
Speed: The speed on most wireless networks (typically 1-54 Mbps) is far slowerthan even the slowest common wired networks (100Mbps up to several Gbps).
There are also performance issues caused by TCP and its built-in congestion
avoidance. For most users, however, this observation is irrelevant since the speed
bottleneck is not in the wireless routing but rather in the outside network
connectivity itself. For example, the maximum ADSL throughput (usually 8Mbps
or less) offered by telecommunications companies to general-purpose customers is
already far slower than the slowest wireless network to which it is typicallyconnected. That is to say, in most environments, a wireless network running at its
slowest speed is still faster than the internet connection serving it in the first place.
However, in specialized environments, the throughput of a wired network might
be necessary.
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3.4 Architecture
FIG 3.1Wireless LAN Architecture Using An Infrastructure BSS
You can connect through a cell phone card via GSM network, satellite hardware
from your satellite company, or the most common a 802.11 router and a either a network
card for AGP slot, PCI/PCI express slot on a desktop, or PCMCIA card for a laptop.
Some laptops already come prepared with built-in wireless.
3.5 IEEE 802.11a
The 802.11a amendment to the original standard was ratified in 1999. The
802.11a standard uses the same core protocol as the original standard, operates in 5 GHz
band, with a maximum raw data rate of 54 Mbit/s, which yields realistic net achievable
throughput in the mid-20 Mbit/s. The data rate is reduced to 48, 36, 24, 18, 12, 9 then 6
Mbit/s if required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4
to point to point. It is not interoperable with 802.11b, except if using equipment that
implements both standards. Since the 2.4 GHz band is heavily used, using the 5 GHz
band gives 802.11a the advantage of less interference. However, this high carrier
frequency also brings disadvantages. It restricts the use of 802.11a to almost line of sight,
necessitating the use of more access points; it also means that 802.11a cannot penetrate as
far as 802.11b since it is absorbed more readily, other things.
3.6 IEEE 802.11b
The 802.11b amendment to the original standard was ratified in 1999. 802.11b
has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access
method defined in the original standard. Due to the CSMA/CA protocol overhead, in
practice the maximum 802.11b throughput that an application can achieve is about 5.9
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Mbit/s using TCP and 7.1 Mbit/s using UDP. 802.11b products appeared on the
market very quickly, since 802.11b is a direct extension of the DSSS (Direct-sequence
spread spectrum) modulation technique defined in the original standard. The dramatic
increase in throughput of 802.11b (compared to the original standard) along with
substantial price reductions led to the rapid acceptance of 802.11b as the definitive
wireless LAN technology. 802.11b is usually used in a point-to-multipoint configuration,
wherein an access point communicates via an Omni-directional antenna with one or moreclients that are located in a coverage area around the access point. Typical indoor range is
30 m (100 ft) at 11 Mbit/s and 90 m (300 ft) at 1 Mbit/s. With high-gain external
antennas, the protocol can also be used in fixed point-to-point arrangements, typically at
ranges up to 8 kilometers (5 miles) although some report success at ranges up to 80120
km (5075 miles) where line of sight can be established. This is usually done in place of
costly leased lines or very cumbersome microwave communications equipment. 802.11b
cards can operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1 Mbit/s if signal
quality becomes an issue. Since the lower data rates use less complex and more redundant
methods of encoding the data, they are less susceptible to corruption due to interference.
3.7 IEEE 802.11g
In June 2003, a third modulation standard was ratified: 802.11g. This works in
the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or
about 24.7 Mbit/s net throughputs (like 802.11a). 802.11g hardware is compatible with
802.11b hardware. Details of making b and g work well together occupied much of the
lingering technical process. In older networks, however, the presence of an 802.11b
participant significantly reduces the speed of an 802.11g network. Even though 802.11g
operates in the same frequency band as 802.11b, it can achieve higher data rates because
of its similarities to 802.11a. The maximum range of 802.11g devices is slightly greater
than that of 802.11b devices, but the range in which a client can achieve the full 54 Mbit/s
data rate is much shorter than that of 802.11b. Despite its major acceptance, 802.11g
suffers from the same interference as 802.11b in the already crowded 2.4 GHz range.Devices operating in this range include microwave ovens, Bluetooth devices, and cordless
telephones. In January 2004, IEEE announced that it had formed a new 802.11
amendment to the 802.11 standard for wireless local-area networks. The real data
throughput is 100Mbit/s (which require an even higher raw data rate at the physical
layer), and is up to 50 times faster than 802.11b, and up to 10 times faster than 802.11a or
802.11g.
3.8 IEEE 802.11n
802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input
multiple-output). MIMO uses multiple transmitter and receiver antennas to allow for
increased data throughput through spatial multiplexing and increased range by exploitingthe spatial diversity, through coding. On 19 January 2007, the IEEE 802.11 Working
Group unanimously approved 802.11n to issue a new Draft 2.0 of the proposed standard.
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3.9 IEEE 802.11 LAYER
Moving data through a wireless network involves three separate elements: the
radio signals, the data format, and the network structure. Each of these elements is
independent of the other two, so its necessary to define all three when you invent a new
network. In terms of familiar OSI (Open System Interconnection) reference model, the
radio signal operates at the physical layer, and the data format controls several of thehigher layers. The network structure includes the interface adapters and base stations that
send and receive the radio signals. In wireless network, the network adapters in each
computer convert digital data to radio signals, which they transmit to other devices on the
network, and they convert incoming radio signals from other network elements back to
digital data.
The IEEE 802.11b standard, as its older version IEEE 802.11 standard, places
specification on the parameters of both:
1. Physical (PHY) Layers
2. Data-Link Layers (Medium Access Control (MAC))
Operation of Physical LayerIn an 802.11 network, the radio transmitter adds a 144-bit preamble to each packet,
including 128 bits that the receiver uses to synchronize the receiver with the transmitter
and a 16 bit start-of-frame field. This is followed by a 48-bit header that contains
information about the data transfer speed, the length of the data contained in the packet,
and an error checking sequence. This header is called the PHY preamble because it
controls the physical layer of the communications link. Because the header specifies the
speed of the data that follow it, the preamble and the header are always transmitted at 1
Mbps. Therefore, even if a network link is operating at the full 11 Mbps, the effective
data transfer speed is considerably slower. That 144-bit preamble is a holdover from the
older and slower DSSS systems, and it has stayed in the specification to ensure that802.11b devices will still be compatible with the older standards, but it really doesnt
accomplish anything useful. So theres an optional alternative that uses a shorter, 72-bit
preamble. In a short preamble, the synchronization field has 56 bits combined with the
same 16-bit start-of-frame field used in long preambles. The 72-bit preamble is not
compatible with old 802.11 hardware, but that doesnt matter as long as all the nodes in a
network can recognize the short preamble format. In all other respects, a short preamble
works just as well as long one. It takes the network a maximum of 192 milliseconds tohandle a long preamble, compared to 96 milliseconds for a short preamble. In other words,
the short preamble cuts the overhead on each packet in half. This makes a significantdifference to the actual data throughput, especially for things like streaming audio and video
and voice-over-internet services.
MAC LayerThe MAC layer controls the traffic that moves through the radio network. It
prevents data collisions and conflicts by using a set of rules called Carrier Sense Multiple
Access with Collision Avoidance (CSMA/CA), and it supports the security functions
specified in the 802.11b standard. When the network includes more than one access point,
the MAC layer associates each network client with the access point that provides the best
signal quality. When more than one node in the network tries to transmit data at
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the same time, CSMA/CA instructs all but one of the conflicting nodes to back off and try
again later, and it allows the surviving node to send its packet. CSMA/CA works like this:
when a network node is ready to send a packet, it listens for other signals first. If it
doesnt hear anything, it waits for a random (but short) period of time and then listens
again. If it still doesnt sense a signal, it transmits a packet. The device that receives the
packet evaluates it, and if its intact, the receiving mode returns an acknowledgement. But
if the sending node does not receive the acknowledgment, it assumes that there has been acollision with another packet, so it waits for another random interval and then tries again.
CSMA/CA also has an optional feature that sets an access point (the bridge between
wireless LAN and the backbone network) as a point coordinator that can grant priority to
a network node that is trying to send time-critical data types, such as voice or streaming
media.
The MAC layer can support two kind of authentication to confirm that a network
device is authorized to join the network: open authentication and shared key
authentication. When you configure your network, all the nodes in the network must use
the same kind of authentication.
The network supports all of these housekeeping functions in the MAC layer byexchanging (or trying to exchange) a series of control frames before it allows the higher
layer to send data. It also sets several options on the network adapter.
3.10 IEEE 802.20 The Key Technology For BRDN
Since July 1999, the IEEE 802.16 Working Group on Broadband Wireless Access
has been openly developing voluntary consensus standards for Wireless Metropolitan
Area Networks with global applicability. Addressing the demand for broadband access to
buildings, IEEE 802.16 provides solutions that are more economical than wired-line
alternatives. The standards set the stage for a revolution in reliable, high-speed network
access in the last mile of Internet by homes and enterprises. On December 11th, 2002,the IEEE Standards Board approved the establishment of IEEE 802.20 Mobile Broadband
Wireless Access (MBWA) Working Group.It described the scope of IEEE 802.20 as:
Specification of physical and medium access control layers of an air interface for
interoperable mobile broadband wireless access systems, operating in licensed bands
below 3.5 GHz, optimized for IP-data transport, with peak data rates per user in excess of
1 Mbps. It supports various vehicular mobility classes up to 250 Km/h in a MAN
environment and targets spectral efficiencies, sustained user data rates and numbers of
active users that are all significantly higher than achieved by existing mobile systems.
According to the above scope, the basic features of IEEE 802.20 include compatibility,
coexistence, distinct identity, technology feasibility, and economic feasibility . According
to the MBWA announcement, IEEE 802.20 is aimed at mobile communication, and its
data rate can reach more than 2Mbps in high speed mobile application. IEEE 802.20 isthe first real broadband wireless network standard that dedicatedly supports the mobility
of network. A comparison between IEEE 802.20 and others mobile techniques for
traditional RCS are shown in Table 1.
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Table: IEEE 802.20 Vs Other Mobile Techniques Used By TraditionalRCS
13
Characteristic GSM-R TETRA version2 GT800(3G) IEEE 802.20
Data rate 2.4-28.8Kbps 96-384Kbps 2Mbps,2Mbps
at the speed of
250mk/h
Latency About 1000ms About 500ms About 250ms About 30ms
Spectralefficiency
200KHz/8ch. 25KHz/4ch. About0.2b/s/Hz/cell
>1b/s/Hz/cell
Cell radius 5-10 Km 10-15 Km 2-5 Km >15Km
Spectrum Licensed bands876-880
921-915MHz
Licensed bands806-821
851-866MHz
Licensed bandsbelow
2.7GHz
Licensed bandsbelow
3.5GHz
Switching
method
Circuit Circuit Circuit/Packet Packet
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4. WIRELESS WAN
4.1 Characteristics of WAN environments
At a high level, most WWAN environments exhibit one or more of the following
characteristics: the bandwidth is very low and varying, the latency is very high andvarying, blackouts exceeding 10 s occur occasionally, and for unreliable
WWAN networks, non-congestion related packet loss may be significant when the user is
traveling at moderate to high speeds.
We have used CDPD as the evaluation platform for WTCP though our design is
not specific to any particular data network. CDPD is a packet data network that overlays
the AMPS cellular telephone infrastructure. A CDPD full duplex channel is a pair of
unidirectional channels; each with a raw capacity of 19.2 Kbps. Up to maximum of 30
users can share a pair of uplink/downlink channels. A set of channels may be dedicated
for CDPD transmission, or CDPD users may be dynamically assigned to channels that are
preferentially shared by cellular phone calls in the latter case, users are more likely to
see short-term blackouts. CDPD transmits compressed and encrypted data, and adds48.2% ReedSolomon coding overhead for forward error correction. Two characteristics
of CDPD are germane to our discussion: the effective throughput of a CDPD channel
typically does not exceed 12 Kbps, and the majority (_75%) of the end-to-end latency is
incurred in the CDPD part of the network between the mobile switching station and the
mobile host. We will revisit the impact of the latter point when we discuss wireless
transport protocols that rely on TCP-aware smarts at the base station .
WWAN wireless networks in general and CDPD networks in particular, typically
exhibit the following six characteristics.
1. Non-congestion related packet loss. Even though CDPD adds 134 bits of Reed
Solomon error-correcting code to every 278 bit block of data, we have measured nocongestion related error ranging from 0 to 10% depending on the speed of mobility
(measured over a range of 055 mph), location of the user vis--vis the base station, and
co-channel interference. TCP assumes that all packet losses result from congestion. A 5%
non-congestion related packet loss can, thus, significantly degrade the performance of
TCP.
2. Very low bandwidth. As we mentioned above, between 1 to 30 users may share a
single CDPD channel of raw capacity 19.2 Kbps. For RAM, the channel is 8 Kbps, while
for Ardis, the channel may be 4.8 Kbps or 19.2 Kbps. Due to the extremely low
bandwidth, the delay-bandwidth product of a connection is small (typically 2 or 3
packets). This can affect the congestion control and fast retransmit mechanisms of TCP
adversely. TCP sometimes observes artificially larger congestion windows as a result of
deep buffering in the CDPD network. While this allows a connection to pump in more
packets into the network, it artificially increases the round trip time and significantly
affects TCP performance in case of a timeout.
3. Large round trip time and variance in round trip time. In CDPD, we have observed
typical round trip times between 800 ms to 4 s. A large fraction of this time is due to
transmission on the wireless link (e.g., transmitting a 512 byte packet at 12K bps takes
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300 ms), and over 75% of the latency is typically incurred in the segments of the
connection lying in between the mobile switching station and the mobile host. 3(a) we
observe the variation in round trip time for UDP handshakes that progress in bursts of 8
from 1.8 s to 6 s for successful handshakes.
This plot shows the impact of large transmission delay on rtt when the sender
bursts packets. TCP sets the retransmission timeout to be the sum of the average roundtrip time and four times the mean deviation of the round trip time This may result in very
large RTOs (e.g., 32 s) in CDPD because of two reasons:
(a) rtt and _(rtt) are inherently large, and
(b) ACKs from the receiver get bunched (see below), and since ACKs clock data packets
in TCP, data packets are sent out in bursts, which further increase the mean and deviation
of rtt. Thus, timeouts affect TCP performance very severely on CDPD.
4. Asymmetric channel bunching of ACKs. CDPD uses DSMA/CD for contention
resolution in the channel. Contentions among mobile users for the uplink channel are
resolved by binary exponential backoff. Consequently, CDPD suffers from the well
known capture syndrome of binary exponential backoff, in which a highly loaded
shared medium ends up bursting the queued packet transmissions of each contending hostin turn. RAM also suffers from the same problem For the common case of downlink data
transmission, ACKs from the mobile to the backbone host get bunched. This further
skews the round trip time computation, and also causes the sender to burst out packets as
mentioned above.
5. Occasional blackouts. Prolonged fades, sudden degradation in signal quality such as
traveling through a tunnel or between overlapping base stations, and temporary lack of
available channels (when cellular phone calls are occupying the channels) can cause
blackouts lasting 10 s or more, and results in the back-to-back loss of a sequence of
packets. Traveling at 55 mph, we observed several blackouts ranging from 10 s to 10 min
during the course of a day.
6. Inter-packet delays as a congestion metric. We have observed that sharp increases in
the short-term average interpacket delay observed by the receiver almost always precede
congestion-related packet loss in the CDPD network. Specifically, an increase in the
average interpacket delay perceived by the receiver is an indication of increased
contention for the wireless link and is a precursor to loss unless connections throttle back
their sending rate. The sender sends packets with a constant interpacket separation of 1 s.
Clearly, interpacket separation is a useful metric to pace the progress of the connection
and can be used to perform rate control.
In wireline networks, the use of both delays and interpacket separation as a metric
for predicting congestion has not been well accepted because of the large variation indelays experienced by packets over the Internet. However, this approach works well in
our target environment because of the extremely low bandwidth of WWANs, wherein the
transmission time over the wireless link predominates. TCP-Vegas uses a variant of this
approach by monitoring round trip times at the sender instead of interpacket delays at the
receiver. Unfortunately, using TCP-Vegas as-is will not work as well in WWAN
environments because of asymmetric channels and the effect of bursting packets on the
computed rtt (points 2 and 4 above).
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4.2 High level architectural tradeoffs in WWAN environments
Before going into the details of the WTCP design, we need to step back and
discuss the tradeoffs between using end-to end mechanisms for wireless TCP versus using
smart mechanisms in the network in order to assist wireless-unaware TCP end-points.
Related work on wireless optimizations for TCP has typically argued against end-to-end
mechanisms on the grounds that it is impractical to change the protocol stacks of allstationary hosts merely to accommodate mobile hosts.
Thus, most of the previous work has focused on making the lossy nature of the
wireless link transparent to the stationary end host by introducing smarts at the base
station via one of three mechanisms: reliable link layers, TCP-aware snoop mechanisms
, or splitting the connection into two (wireline and wireless) distinct components.
Link layer retransmission works well when the latency over the wireless link is
small compared to the coarse grained TCP timer. In the ideal case, the link layer
retransmissions are not expected to significantly interfere with the end to- end rtt
computations or congestion control mechanisms except to eliminate random channel
loss. In WWAN networks, it is the transmission time over the wireless network that
constitutes the bulk of the observed end-to-end latency. Consequently, providing only a
reliable link layer abstraction at a packet-level time scale and keeping TCP unchanged at
the end hosts simply will not work because they will interfere with the reliability and
congestion control mechanisms of TCP. If the wireless data network provides fine
grained link level retransmissions, e.g., RLP used in HDR , then link layer mechanisms
can effectively mask channel error from higher layers.
The Snoop protocol instantiates TCP-aware smarts at the base station (or mobile
switching station) in order to eliminate the problem of false fast retransmits or slow starts
due to random packet loss over the wireless channel. This approach assumes that the
transmission time over the wireless link is significantly smaller than the coarse-grainedTCP timer and round trip time. Moreover, Snoop works well only when the bandwidth-
delay product of the wireless link is at least 3 packets long. However in WWAN
environments, Snoop does not work well because of two reasons:
(a) it exacerbates the problem of large and varying round trip times by
suppressing duplicate ACKs, and
(b) Duplicate retransmissions may be initiated by both the Snoop agent and the
end host (which may observe a timeout) because of comparable timeout values at the two
entities. In fact, we have observed that snooping may possibly degrade the performance
of TCP when the latency over the wireless link dominates the round trip time. In
summary, we believe that Snoop works well in the environment for which it was
designed, but it does not work well in the WWAN environment.
Indirect TCP protocols break the TCP connection at the base station, and maintain
two separate connections one over the wireline network and one over the wireless
network. I-TCP violates the fundamental end-to-end guarantees of TCP by splitting the
connection. Note that the connection split must happen at the base station (or mobile
switching station) serving the mobile host, and the connection state must be moved across
base stations upon handoff. The wireless component of I-TCP is quite simplistic and does
not address issues such as
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(a) non-conformancewith end-to-end semantics,
(b) overhead of moving state between base stations,
(c) deployability constraints due to mandatory changes in base stations, and
(d) design of the transport protocol for the wireless link.
As a practical matter, the I-TCP architecture may not be feasible forWWANs
because it requires significant infrastructure support and maintenance of connection statefrom the WWAN network, which is autonomously managed and may not even
understand TCP/IP internally (e.g., RAM). It is important to note that I-TCP calls for
splitting each transport connection transparently to the TCP end-points. It is, thus, quite
distinct from the WWAN deployment scenario of a mobile host connecting to a dedicated
stationary proxy in the backbone.
We believe that previous approaches that seek to hide the problems of WWAN
networks from TCP at the end host by adding TCP-aware smarts in the mobile switching
station are not applicable to WWAN networks for two reasons:
(a) such approaches require the base station to maintain significant state,
understand TCP/IP, and are often tuned to specific flavors of TCP, and
(b) the fact that the latency between the mobile switching station and the mobilehost is the dominant component of a large and varying round trip time makes such
approaches less effective. The bottomline is that for WWAN environments, both
endpoints must cooperatively address the issues unique to the environment. Moreover, it
is desirable to eliminate network-level smarts because the base stations are owned by an
autonomous entity that may not even be running IP internally. We believe that the key
issues that need to be addressed the non-congestion related packet loss, large and highly
varying latency, asymmetry in data/ACK channel behavior can be effectively solved
with the end-to-end mechanisms proposed in this paper. Of course, the penalty for using
the end-to-end mechanism is that the remote end-point in the backbone must also change.
Luckily, the nature of the WWAN environment and the current deployment pattern
already supports the common case of WWAN users typically connecting through a
dedicated proxy server on the backbone.
4.3. The WTCP approach
Any reliable transport protocol must provide the following functions:
(a) Connection management,
(b) Congestion control,
(c) Flow control, and
(d) Reliability.
WTCP reuses the standard TCP mechanisms for flow control and connection
management. We now focus on the key design choices in WTCP for congestion control
and reliability.
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1. Congestion control
The key aspects of congestion control in WTCP are that it is rate based, uses
interpacket delay as the primary mechanism to determine rate adaptation, performs the
rate adaptation computations at the receiver, predicts the cause of packet loss and reacts
accordingly, and varies the granularity of rate increase/decrease depending on the type of
congestion observed. Additionally, WTCP also tailors its startup behavior to work wellfor short-lived flows. We describe the design aspects ofWTCP congestion control in more
detail below.
1. Rate-based transmission control. As for the common case of bulk data transfer from the
backbone host to the mobile host, ACKs are often bunched together on the return path to
the sender because of the nature of channel arbitration, e.g., DSMA/CA used in CDPD.
With the window-based self-clocking mechanism of TCP, this results in the sender
bursting back-toback data packets, which skews round trip time computations, causes
more bursty queuing at the base station, and consequently more packet drops. WTCP
alleviates these problems by using a rate based scheme that does not use ACKs for self-
clocking. Adopting a rate-based approach does involve explicit clocking and shaping the
traffic according to the current transmission rate of the connection; however, the rates aretypically small enough that coarse grain timers (O(100 ms)) are sufficient to perform the
clocking effectively.
2. Inter-packet delay as the main mechanism for transmission control. We have observed
that monitoring the average interpacket delay at the receiver provides a fairly accurate
measure of the available channel rate for low bandwidth channels. Specifically, the ratio
of the average inter-packet separation at the receiver and the average inter-packet
separation at the sender provides a responsive metric to determine if the transmission rate
needs to be increased, or decreased. Thus, when the network is uncongested or has
incipient congestion, reacting tochanges in the interpacket delay ratio serves to keep the
network uncongested, and significantly reduces the number of congestion-related packet
losses. WTCP, thus, uses this mechanism as the primary transmission rate control
mechanism, and essentially uses incipient congestion detection without waiting to lose
packets before throttling down the sending rate.
3. Distinguishing the cause of packet loss and adjusting transmission rate accordingly.
While inter packet delays are the main mechanism for dealing with incipient congestion,
if the network suddenly moves from uncongested to congested state (e.g., due to a sudden
influx of new
Connections or sudden decrease in available resources), then packets are dropped
due to congestion. Our algorithm must detect such losses and throttle the sending rate
aggressively. In other words, when the receiver observes packet losses, it must predict thecause of the losses and react appropriately. If the loss is predicted to be due to congestion,
then the sending rate is throttled down. We use a heuristic based on the average per-
packet separation to distinguish congestion losses from random losses. In this heuristic,
the receiver initially predicts that all losses are non-congestion losses.
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4. Performing transmission control computations at the receiver.
In WTCP, the receiver performs the rate control mechanisms described above, and
computes the new transmission rate of the sender. With each data packet, the sender
transmits its current interpacket separation. Based on local state and the state in thepacket, the receiver has all the information it needs to update the transmission rate. This is
done at regular intervals, which we refer to as update epochs. An update epoch begins
when period. At the end of the epoch, the receiver performs the rate control computations
and sends the rate update back to the sender in its acknowledgment. Having the receiver
perform the rate computations eliminates the effect of delay variations and losses in the
ACK path. Even if ACKs get bunched, delayed, or lost, the transmission rate is not
altered. WTCP can thus deal with asymmetric channels effectively.
5. Variable granularity rate adjustment. TCP uses the well known linear-increase
multiplicative-decrease policy for adjusting its congestion window. While LIMD is stable
and asymptotically converges to fair channel allocation, the efficiency of the LIMD
algorithmis a function of how severely the decrease is performed. TCP reduces itscongestion window by half upon observing a packet loss. In
WTCP, we seek to detect incipient congestion and react to it early on in the common
case. The goal ofWTCP is to decrease the transmission rate multiplicatively in order to
ensure fairness, less aggressively when reacting to incipient congestion in order to
improve efficiency, and more aggressively when reacting to real congestion in order to
reduce packet loss and alleviate congestion quickly. In order to achieve these goals,
WTCP maintains a history of transmission increase/decrease in the recent past. If the
receiver is required to perform transmission decrease multiple times in quick succession,
it starts to decrease its transmission rate more aggressively. If the receiver observes a
congestion based packet loss, it halves its rate.
As a result of this approach, incipient congestion is handled by a gentle decrease of the
transmission rate, but severe congestion is handled by an aggressive decrease in
transmission rate.
6. Startup behavior.
Since round trips are large in WWAN environments, and since some data transmissions
may be short-lived, WTCP attempts to compute the appropriate transmission rate for a
connection immediately upon startup rather than going through slow start. WTCP uses
the packet-pair approach, wherein it sends two back-to-back packets of maximum
segment size (MSS) and computes their interpacket delay during connection
establishment. This serves as an approximate estimate for the sending rate. Though thepacket-pair approach is known not to work too well in wire line environments, we have
used it as a first-cut approach. We will investigate this approach in the near future.
7. Blackout detection.
Blackouts occur when the connection experiences back-to-back losses for extended
periods of time due to poor channel conditions or lack of available channels. The
reliability mechanism of WTCP elicits an acknowledgment (positive or negative) from
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the receiver for every packet that has been sent, before the sender decides to resend it.
Thus, if a packet has not been acknowledged (positively or negatively) for a threshold
period of time, the sender enters the blackout phase and starts probing the receiver in
order to elicit an acknowledgment. Upon a successful packet handshake after entering the
blackout phase, the sender reverts to the old transmission rate that it was using before the
onset of the blackout phase. This ensures that the packet losses during the blackout phase
do not affect the transmission rate.
2. Reliability
The key aspects of reliability in WTCP are that it uses selective
acknowledgments, it does not use retransmit timeouts, and that it tunes the frequency of
sending acknowledgments to the dynamic network conditions. We describe these aspects
below:
1. Selective acknowledgments. As noted in related work, selective acknowledgments are
very useful in TCP . WTCP uses selective acknowledgments for ensuring reliability.
The receiver periodically sends ACKs at a frequency tuned by the sender (see below),
containing the cumulative and selective ACK. By inspecting the ACK, the sender candetect a hole in the receivers sequence of received packets. By comparing the state
contained in the ACK with local state stored with the last (re)transmission for each
unacknowledged packet, the sender can determine if this last (re)transmission was lost, or
could still be in transit. Thus, selective acknowledgment allows the sender to retransmit
only lost packets.
2. No retransmit timeouts.
It is exceedingly difficult to maintain a reliable estimate of the retransmit timeout.
In fact, many of the performance problems observed in various TCP flavors are caused by
erroneous RTO estimation. WTCP does not use RTOs. Instead, it modifies the SACK
algorithm in order to achieve reliable transmission without RTOs. This mechanism is
described in section 4, and is a very important aspect of WTCP.
3. Controlling ACK frequency.
ACKs carry both reliability and transmission control information, and the sender
must receive ACKs periodically in order to react to the new transmission rate, and
perform flow control. The sender tunes the desired ACK frequency (and notifies the
receiver in the data packet) such that it expects to receive at least one ACK in a threshold
period of time If the sender has one or more packets pending acknowledgment for more
than a threshold period of time, it goes into the blackout mode. The tuning of the ACK
frequency is governed by several factors: (a) observed ACK loss at the sender, (b) half-duplex or full-duplex nature of the WWAN channel, and (c) average and deviation in the
inter-ACK separation observed at the sender. Note that a receiver may also voluntarily
generate a SACK immediately upon observing a hole in the packet sequence. The effects
of these factors on controlling the acknowledgment frequency are part of ongoing work.
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5. MOBILE IPV6
5.1 Introduction
Internet Protocol version 6 (IPv6) is a network layer protocol for packet-switched
internet works. It is designated as the successor of IPv4, the current version of the Internet
Protocol, for general use on the Internet.
The main improvement brought by IPv6 (Internet Protocol version 6) is the increase
in the number of addresses available for networked devices, allowing, for example, each
cell phone and mobile electronic device to have its own address. IPv4 supports 232
(about
4.3 billion) addresses, which is inadequate for giving even one address to every living
person, let alone supporting embedded and portable devices. IPv6, however, supports 2128
addresses; this is approximately 51028
addresses for each of the roughly 6.5 billion
people alive today.
By the early 1990s, it was clear that the change to a classful network introduced a
decade earlier was not enough to prevent the IPv4 address exhaustion and that furtherchanges to IPv4 were needed. By the winter of 1992, several proposed systems were
being circulated and by the fall of 1993, the IETF announced a call for white papers (RFC
1550) and the creation of the "IPng Area" of working groups.
IPng was adopted by the Internet Engineering Task Force on July 25, 1994 with the
formation of several "IP Next Generation" (IPng) working groups[1]
. By 1996, a series of
RFCs were released defining IPv6, starting with RFC 2460. (Incidentally, IPv5 was not a
successor to IPv4, but an experimental flow-oriented streaming protocol intended to
support video and audio.)
It is expected that IPv4 will be supported alongside IPv6 for the foreseeable
future. However, IPv4-only clients/servers will not be able to communicate directly withIPv6 clients/servers, and will require service-specific intermediate servers or NAT-PT
protocol-translation servers.
5.2 FEATURES OF IPv6
To a great extent, IPv6 is a conservative extension of IPv4. Most transport- and
application-layer protocols need little or no change to work over IPv6; exceptions are
applications protocols that embed network-layer addresses (such as FTP or NTPv3).
Applications, however, usually need small changes and a recompile in order to run
over IPv6.
Larger address spaceThe main feature of IPv6 that is driving adoption today is the larger address space:
addresses in IPv6 are 128 bits long versus 32 bits in IPv4.
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The larger address space avoids the potential exhaustion of the IPv4 address space
without the need for NAT and other devices that break the end-to-end nature of Internet
traffic. The drawback of the large address size is that IPv6 is less efficient in bandwidth
usage, and this may hurt regions where bandwidth is limited.
For corporate networks however, this will simplify the already complex method of
sub netting.
Stateless auto configuration of hostsIPv6 hosts can be configured automatically when connected to a routed IPv6
network. When first connected to a network, a host sends a link-local multicast
(broadcast) request for its configuration parameters; if configured suitably, routers
respond to such a request with a router advertisementpacket that contains network-layer
configuration parameters.
If IPv6 auto configuration is not suitable, a host can use stateful auto configuration
(DHCPv6) or be configured manually. Stateless auto configuration is only suitable for
hosts; routers must be configured manually or by other means.
MulticastMulticast is part of the base protocol suite in IPv6. This is in opposition to IPv4,
where multicast is optional.
Most environments do not currently have their network infrastructures configured to
route multicast; that is the link-scoped aspect of multicast will work but the site-scope,
organization-scope and global-scope multicast will not be routed.
IPv6 does not have a link-local broadcast facility; the same effect can be achieved
by multicasting to the all-hosts group (FF02::1).
The m6bone is catering for deployment of a global IPv6 Multicast network.
Jumbo gramsIn IPv4, packets are limited to 64 KiB of payload. When used between capable
communication partners, IPv6 has support for packets over this limit, referred to as jumbo
grams. The use of jumbo grams improves performance over high-throughput networks.
Faster routingBy using a simpler and more systematic header structure, IPv6 was supposed to
improve the performance of routing. Recent advances in router technology, however, may
have made this improvement obsolete
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Network-layer securityIPsec, the protocol for IP network-layer encryption and authentication, is an integral
part of the base protocol suite in IPv6; this is unlike IPv4, where it is optional (but usually
implemented). IPsec, however, is not widely deployed except for securing traffic between
IPv6 BGP routers.
MobilityUnlike mobile IPv4, Mobile IPv6 (MIPv6) avoids triangular routing and is therefore
as efficient as normal IPv6. This advantage is mostly hypothetical, as neither MIP nor
MIPv6 are widely deployed today.
5.2 Mobile IPv6 - The Key Issue in BRDN Implementation
Broadband wireless network has generally raised a number of research issues
aiming at the requirements from the mobile applications. Although many have proposed
various resolutions for these issues, for example, mobile IP telephony, more effort has to
be input to find the better ones that will eventually lead to commercialization. In the
context of railroad based mobile applications, the same problems exist as in other mobile
IP applications, but there is a possibility that the problems in railroad based mobile
applications can be solved by alternative approaches with better outcomes because of the
specialty of the application environment.
This section briefly covers two most outstanding topics:
1) IP mobility in BRDN, and2) VoIP for BRDN.
Mobile IP introduced a solution of bi-address, with a long-term IP address on ahome network (HN) and a "care-of address" (COA) when away from its home network.
This home address (HA) is administered in the same way as a "permanent" IP address is
provided to a stationary host. The COA is associated with the mobile node (MN) and
reflects the mobile node's current point of attachment Every time when MN handover to a
foreign network, mobile IP requires it to bind its COA to Home Agent (HA). If MN is
moved far away to HA, the latency caused by handover will be long, and will lose the
packets sent to MN, and the network performance will decline seriously. This Seamless
Handover question has attracted wide attentions.
The IEEE 802.20 mobile client on train is always on moving along with train,
switched from one network to another at a fixed trajectory, particularly, the period of
handover can be predicted by the train schedule . Based on these premises, BRDN adoptsa Predictive Pre-Handover (PPH) algorithm. PPH is based on the highly predictive of
trains moving from one network to the neighboring network. Not only the arriving time,
but also the ID and routers address of the next network are predictable and storage. The
flow of PPH algorithm illustrated as below:
1) Initialization: Create the entry for each network handover and get thepredictive handover table according to the trains schedule.
2) Handover predict: BRDN predicts the next networks ID, routers address and
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arrive time (tp) according to the handover table.
3) BRDN creates the frequency of detection and detects the foreign networkactively.
4) Pre-registering: BRDN implements the preparations for registering, such asauthorization and authentication.
5) Pre-transmitting and storage: According to the result of step 3 and tp,BRDN copies the communication data to the router of the network whichabout to handover.
6) BRDN triggers the handover process when it detected that the quality of thenew networks signal is better than that of the former.
7) If there has any data missed, auto-transmit from the routers storage, and BRDN amends the predictive handover table according the result of this
handover.
IPv6 is presently the IP standard for the next generation Internet, which can
overcome the many known weaknesses of IPv4. With its 128 bits IP address IPv6 has
realized the Stateless Address Auto-configuration (SAAC). Applied to the BRDN, IPv6 is
able to configure each communication node on a train automatically with a unique global
IP address freely. That implies that besides current IPv4 based dynamic IP assigning, IPv6has the potential to provide a better solution. The operation for mobile IPv6 is shown in
Figure 5.1.
Fig. 5.1 the Operation of Mobile Ipv6
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In a hybrid scheme, the WLAN on trains may still use mobile IPv4, which has
been well explored recently, while the main connection to the Internet will be IPv6-based
in order to support better IP mobility by taking the advantage of IPv6. A gateway between
the WLAN and the IEEE 802.20 based WWAN will take care of the IP address
translation. Voice communication is a fundamental service demanded for RCS. In particularly, unless RBDN realizes the priority management, urgent call support,
broadcast, multicast and quick establishment for call question, the RBDN based on
IEEE802.20 can implement in rail-transit. VoIP application based.
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6. SUMMARY
The next generation RCS is a necessity for the advanced mobile network
infrastructure. This paper presents current status of MCT and RCS, and introduces the
latest mobile broadband wireless access technology IEEE 802.20.
This paper proposes the architecture and implementation of BRDN, which will be
built upon IEEE 802.11x, IEEE 802.16x and next generation Internet technologies. The
further research work will be focused on the implementation of mobile IPv6 in
conjunction with the DSS scheme at data link layer.
We further compared IEEE 802.20s technology advantage with other RCS
solutions, and identified that constructing the BRDN based on IEEE 802.20 standard is
not only feasible, but also urgent. This paper further brought forward a configuration for
BRDN, with the considerations in the required features of next generation RCS and the
technology advantages of IEEE 802.20. We also discussed the mobile IP problem for
BRDN the key issue in implementing the next generation RCS, covering the predictive
mobility management for mobile IP and VoIP over BRDN.
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BIBLIOGRAPHY
WEBSITES
http://www.mintc.fi/oliver/upl970-Julkaisuja%2084_2005.pdf
http://www.comp.brad.ac.uk/het-net/HET-NETs04/CameraPapers/P27.pdf
http://www.calccit.org/projects/3b.WiFi_Technical%20Recommendations.pdf
http://www.telenor.com/telektronikk/Bryhni_2004-11-24-perspektiver-tradlos.pdf
http://delivery.acm.org/10.1145/510000/506915/p301-sinha.pdf?key1=506915&key2=4535512711&coll=GUIDE&dl=GUIDE&CFID=15151515&CFTOK
EN=6184618
http://129.118.51.86/zlin/pdf/CSI-98.pdf
http://www.en.wikipedia.org/IPv6
http://www.en.wikipedia.org/WLAN
BOOK
Andrew S Tanenbaum, Computer Networks, Prentice Hall India, 3rd edition 2002
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