无线通信研究趋势：从MANET到UWB的影响

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"《无线通信研究趋势》是一本由Tongs. Lee编辑的书籍，由Nova Science Publishers在2007年出版。本书探讨了无线通信领域的最新研究趋势和技术挑战，涵盖了一系列的主题，包括对等网络的拓扑构建、安全匿名路由、移动性管理、能量感知的介质访问控制协议、传感器网络中的分布式数据管理、移动Ad-hoc无线网络的可靠性、微电子机械系统（MEMS）微天线在无线生物医学系统中的应用、混合网络的传输层安全协议、3G与2G系统的合作方式、超宽带（UWB）对宏蜂窝下行链路的影响以及无硬件支持的可靠公共无线局域网等。书中各章节由不同作者撰写，分别深入讨论了各自主题。例如，Wei Ding在第1章中探讨了在自组织网络上构建对等系统拓扑的方法，强调了在动态环境中建立稳定连接的重要性。Lijun Qian等人在第2章中介绍了SARC，这是一种针对基于集群的移动自组织网络（MANET）的安全匿名路由方案，旨在保护用户隐私和数据安全。Christian Makaya和Samuel Pierre在第3章中讨论了IP为基础的下一代无线网络中移动性管理所面临的趋势和挑战，包括如何高效地处理用户移动性和网络资源分配。此外，Jagoba Arias等人在第4章中提出了能量感知的介质访问控制协议，以优化无线网络的能源效率。Stefano Chessa等人在第5章中聚焦于传感器网络的分布式数据管理，探索如何在有限资源下实现高效的数据存储和处理。Jason L. Cook和Jose Emmanuel Ramirez-Marquez在第6章中分析了移动Ad-hoc无线网络的可靠性问题，提出了提高网络稳定性的策略。 P. M. Mendes和J. H. Correia在第7章中讨论了MEMS微天线在无线生物医学系统中的应用，这些微型设备在医疗监测和远程健康照护方面具有巨大潜力。Nikos Komninos在第8章中设计了一种适用于混合网络的传输层安全协议，以确保数据传输的安全性。S-E. Elayoubi等人在第9章中研究了3G系统如何与2G系统协同工作，以提高网络性能和用户体验。 Bazil Taha-Ahmed等人在第10章中分析了超宽带技术对UMTS、CDMA-450、DCS-1800和GSM-900系统宏小区下行链路的影响，揭示了UWB可能带来的干扰和潜在优化方向。最后，Jenn-Wei Lin和Ming-Feng Yang在第11章中提出了一个无需硬件支持的可靠公共无线局域网解决方案，而Yuan-Lin Ko等人在第12章中构建了一个基于移动代理的3G/UMTS动态服务提供平台，以适应不断变化的网络需求。本书提供了丰富的无线通信理论和实践知识，对于研究人员、工程师和学生来说，是理解和探索无线通信领域新发展的宝贵资源。"

Topology Construction for Bootstrapping Peer-to-Peer Systems... 3

routing, resource sharing, advertising, looking up, retrieval, and data dissemination. It is one

of dominant factors that affect primary performance parameters such as efficiency,

robustness, scalability, and feasibility. In fact, topology has broader functionality. For

example, Jelasity and Babaoglu [JB2005] have shown that problems such as clustering and

sorting can be transformed into topology problems and be solved by specific topology

construction.

1.3. Current Status

There are two major approaches for bootstrapping a structured P2P system over wired

networks. One is to jumpstart a network from one or a few predefined nodes, in which the

common way to expand the network is node joining. In wired networks, many structured P2P

systems require manual creation of a “seed” network in bootstrapping. Nodes have to be

booted one by one in a slow, linear manner, which costs long time. In addition, the

jumpstarted network often needs extra long time for stabilization before normal routing could

work. In another approach all nodes cooperate concurrently to construct an overlay topology.

This approach is distributed and decentralized. The concurrency makes it much faster than

joining approach. Furthermore the P2P system could advance to normal working status

immediately after bootstrapping.

Remarkable advance has been recently made in topology construction in wired networks.

Topology generators can construct topologies such as line, ring, mesh, star, and tree. Generic

generator, which could construct any topology if given a mathematical expression, is already

available. [JB2005] However, no such construction tool has been reported in MANETs.

Many problems remain unsolved in this area. In wired networks, existing protocols for

topology construction and maintenance are usually based upon unrealistic assumptions. Most

of them assume the existence of a specific initial topology. Some protocols demand that the

network remains in an ideal topology all the time as a necessary condition for normal

operations. The second problem is the ignorance of network merger and partition. Some

systems even require each node keep and monitor global state of the entire network. Another

problem is: many schemes for topology construction still employ centralized strategy. Some

require central coordinators; some follow a network-wide top-down view in protocol design.

Some have deficiencies in fault-tolerance and recovery. For structured P2P systems over

MANETs, some approaches can not keep up with the rate of change.

1.4. RAN – A New Solution

In this chapter, the Ring Ad-hoc Network (RAN) protocol is introduced. It builds a ring

topology in P2P ID space. To best of author’s knowledge, it is the first successful attempt in

topology construction for MANETs. The ring is used to bootstrap ring-based P2P systems,

such as Chord [SMKKB2001, DBKKMSB2001], Pastry [RD2001], and Virtual Ring Routing

[CCNOR2006], over MANETs. On this ring, ring-based P2P systems could be put into

normal operation immediately without lengthy stabilization. As in many literatures, Chord is

used in demonstration. Three patterns are tested in RAN: distributed exhaustive pattern,

virtual centralized exhaustive pattern, and random pattern. Simulation shows that the

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distributed exhaustive pattern has best overall performance. So this pattern is essentially

representative of RAN.

The rest of this chapter is organized as follows. Section 2 introduces P2P systems,

especially Chord. Section 3 depicts three previous research projects ⎯ T-Man, T-Chord, and

Ring Network. They are most successful approaches for Chord ring construction in wired

networks. Section 4 describes P2P systems in MANETs. Section 5 outlines the RAN protocol

suite. Section 6 gives algorithms of RAN in AP notation. Simulation results are given in

Section 7. Section 8 concludes the chapter.

2. Peer-to-Peer Systems

2.1. Peer-to-Peer Paradigm

P2P overlay systems provide fast, accurate, and scalable resource discovery, resource sharing,

and storage services without a central controller. The concept of P2P systems first appeared in

mid 1990s. As file sharing platforms, especially to distribute music over Internet, P2P

systems became a hot topic in the late 1990s. A traditional P2P system is built upon IP. It

uses IP as the communication platform. An IP capable host can reach anything attached to the

Internet or other IP networks like IEEE 802 family by an IP address. However, IP layer could

not tell a host how and where to find given content or another host. This is done by P2P

overlay systems. The basic task of P2P overlay systems is to connect to other peers and find

out interesting content.

P2P systems are distributed and self-organized. A host is called peer, because all hosts

usually have same status, share same responsibility, and the relationship among them is

characterized by equality. Unused bandwidth, storage, CPU cycles are shared among peers.

Peers enjoy great freedom and privacy. Usually consumers are also producers, so aggregate

resources grow exponentially with utilization. P2P systems have excellent fault tolerance,

because there is no single point of failure in a P2P system.

The emergence of P2P systems was a revolution against long time dominance of

client/server model in computing and communication. In the client/server model, powerful,

reliable servers provide data and services. Clients request data and services from servers. The

client/server model has proved extremely successful by its famous offspring, such as World

Wide Web, database systems, and FTP. However, it has following inherent defects:

 need central controller

 dictation in which clients look like slaves

 presents a single point of failure

 unused resources through out the network

 poor scalability

2.2. Peer-to-Peer Systems

P2P systems address above defects of client/server model. At large P2P computing aims at

sharing and exchanging resources and services between terminals. These resources and

Topology Construction for Bootstrapping Peer-to-Peer Systems... 5

services include information (file or data structure), CPU cycles, storage (memory, cache, and

disk), I/O devices, etc. P2P paradigm takes advantage of superfluous computing capacity,

storage, and network bandwidth, so end users can unite and leverage their collective power to

carry out huge task or achieve mutual benefits.

In a P2P system, all nodes are clients, servers, and routers at same time. All provide and

consume data and services. No centralized data source endangers the system as the single

point of failure. Nodes collaborate directly with each other. Any node can initiate a

connection. All nodes are totally free: they may enter and leave the network arbitrarily and

frequently. It will be “the ultimate form of democracy on the Internet” as well as “the ultimate

threat to copy-right protection on the Internet.” [Kaashoek2003]

P2P systems have following advantages: [Muthusamy2003]

 Efficient use of resources

 Unused bandwidth, storage, CPU cycles at the edge of the network become available

to any user

 Scalability

− Consumers of resources also donate resources. If remarkable consumers turn into

producer, aggregate resources will grow with utilization.

− Self-scaling

 Reliability

− No single point of failure

− Geographic distribution

− Replicas

− Built-in fault tolerance

− Fault tolerance

 Easy administration

− Nodes self organizing

− No need to deploy servers

− Load balancing

Besides file sharing, P2P paradigm could be applied in collaborative Internet (e.g. ICQ,

shared whiteboard), distributed computing and grid computing (e.g. UC Berkley Seti@Home

Project), multiplayer network games (e.g. Doom) and many other fields. However, P2P

systems, especially those for file sharing, remain to be the oldest and most sophisticated P2P

application. In a typical file sharing network, a user makes files (music, video, etc.) on her

computer available to others. Then another user connects to the network, searches for the

files, finds the first user’s computer, and downloads files directly from first user’s computer.

P2P systems fall into two categories: unstructured P2P systems and structured P2P

systems. [Muthusamy2003] An unstructured P2P system does not have a fixed topology for

routing. By the existence of central index servers, unstructured P2P systems are divided into

three subgroups: centralized with a central index server, like Napster; semi-centralized with

local index servers, like KaZaA; decentralized without any index server, like Gnutella. [Clip2,

Ivkovic2001]

Structured P2P systems use fixed topologies like ring or grid for routing. They impose

specific local relationships between peers, which finally generate global structures. These

Wei Ding 6

topology structures can be used for efficient data placement, search, and retrieval. They have

guaranteed scalability ⎯ hops in routing is not linear with number of nodes. Most of them

could reach the logarithm. They are self-organized, fault-tolerant, and they support load

balancing. Structured P2P systems are usually implemented via Distributed Hash Table

(DHT). Typical systems include Chord [DBKKMSB2001, SMKKB2001], Pastry [RD2001],

CAN [RFHKS2001], BitTorrent [Cohen2003I, Cohen2003B], and Virtual Ring

[CCNOR2006].

2.3. Unstructured P2P Systems

Napster was devoted to sharing music files on Internet. Providers upload their list of files and

IP addresses to Napster server. Downloaders send queries to Napster server for files of their

interest in the format of keyword search. Keywords could be artist, song, album, even bit rate.

Napster server replies with IP address of users with matching files. Downloaders connect

directly to the provider’s computer to download file. Using a central directory/index server

and a central query database, Napster guarantees correct results. At same time, the central

server forms a single point of failure and bottleneck for scalability. Napster is Susceptible to

denial of service attack and mischief from malicious users.

Figure 1. Centralized architecture of Napster

Gnutella enables sharing any type of files, not just MP3. It employs decentralized search.

In Gnutella a user A asks her neighbors for files of interest, those neighbors ask their

neighbors, and so on. Finally either users with matching files reply to A’s query, or the packet

is destroyed after a preset Time To Live (TTL). Each message has a parameter which sets the

max number of hops the packet can “live”. Search is distributed by the means of queries

flooding. Comparing to Napster, Gnutella is decentralized and robust to denial of service

attacks, for it has no single point of failure. Nevertheless, it can not guarantee correct results

for every query. Gnutella is still not scalable. [Clip2, Ivkovic2001]

KaZaA is a hybrid of centralized and decentralized structures, where super-peers act as

local central nodes and local search hubs. Each super-peer is similar to a Napster server in a

smaller scale. Super-peers are automatically chosen by the system based on their capacities

Topology Construction for Bootstrapping Peer-to-Peer Systems... 7

(storage, bandwidth, etc.) and availability (connection time). Users upload their list of files to

a super-peer, which periodically exchange file lists with neighbor super-peers. When the

query reaches a super-peer for files of interest, the file is transferred back to requesting node

following the reverse path. [Muthusamy2003]

Figure 2. Flooding style search in Gnutella

2.4. Structured P2P Systems

Structured P2P systems are also called second generation P2P overlay networks.

[Muthusamy2003] They are self-organized and fault-tolerant with balanced load. Scalability

is guaranteed on numbers of hops to answer a query. One frequently cited difference with

unstructured P2P systems their DHT interface.

A DHT stores (key, value) pairs. Each peer stores a subset of (key, value) pairs. Core

functions of DHT API include insert, lookup, and delete. Insert function stores a (key, value)

pair at the node responsible for the key. Lookup function returns value associated with a key

from the host peer of the pair. Basic operation is to find node responsible for a key. A key

need to be mapped to a node before insert, lookup, or delete functions could be used for this

node. DHT maps Keys evenly to all nodes in the network. Each node maintains information

about only a few other nodes. Messages can be routed to a node efficiently. Arrival or

departure of one node only affects a few nodes.

Many services can be built on top of a DHT interface, like file sharing, archival storage,

databases, naming, service discovery, chat, rendezvous-based communication, publish and

subscribe. There are several implementations of DHT generic interface, for instance, Chord

from MIT, Pastry from Microsoft Research in UK and Rice University, Tapestry from UC

Berkeley, Content Addressable Network (CAN) also from UC Berkeley, SkipNet from

Microsoft Research and University of Washington, Kademlia from New York University,

Viceroy from Israel government and UC Berkeley, P-Grid from EPFL in Switzerland, Freenet

developed by Ian Clarke. These systems are also called P2P routing substrates.

Routing in Chord is based upon a ring, on which nodes are organized according to their

node IDs. Keys are assigned to their successor node in the ring. The consistent hash function

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无线通信研究趋势：从MANET到UWB的影响

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