Rede de malha sem fio
Content
História
As redes de rádio de malha sem fio foram originalmente desenvolvidas para aplicações militares, de modo que todo nó pudesse servir dinamicamente como roteador para todos os outros nó. Dessa forma, mesmo no caso de uma falha de alguns nós, os nós restantes poderiam continuar se comunicando e, se necessário, servem como uplinks para os outros nós.
Os nós de rede de malha sem fio anteriores tinham um único rádio semi-duplex que, em qualquer instante, podia transmitir ou receber, mas não ambos ao mesmo tempo. Isso foi acompanhado pelo desenvolvimento de redes de malha compartilhadas. Isso foi posteriormente substituído por hardware de rádio mais complexo que poderia receber pacotes de um nó a montante e transmitir pacotes para um nó a jusante simultaneamente (em uma frequência diferente ou em um canal CDMA diferente). Isso permitiu o desenvolvimento de redes de malha comutadas. À medida que o tamanho, o custo e os requisitos de energia dos rádios diminuíram ainda mais, os nós podem ser equipados com rea forma de custo com vários rádios. Isso, por sua vez, permitiu que cada rádio lidar com uma função diferente, por exemplo, um rádio para acesso ao cliente e outro para serviços de backhaul.
O trabalho nesse campo foi auxiliado pelo uso de métodos da teoria dos jogos para analisar estratégias para a alocação de recursos e o roteamento de pacotes.
Características
Arquitetura
A arquitetura de malha sem fio é o primeiro passo para fornecer uma mobilidade econômica e baixa em uma área de cobertura específica. A infraestrutura de malha sem fio é, na verdade, uma rede de roteadores menos o cabeamento entre nós. Ele é construído de dispositivos de rádio pares que não precisam ser cabelos em uma porta com fio como os pontos de acesso tradicionais da WLAN (AP). A infraestrutura de malha carrega dados em grandes distâncias dividindo a distância em uma série de saltos curtos. Os nós intermediários não apenas aumentam o sinal, mas também passam os dados do ponto A ao ponto B, tomando decisões de encaminhamento com base em seu conhecimento da rede, ou seja, executar o roteamento, primeiro derivando a topologia da rede.
As redes de malha sem fio são uma rede relativamente "estável-topologia", exceto pela falha ocasional dos nós ou pela adição de novos nós. O caminho do tráfego, sendo agregado de um grande número de usuários finais, muda com pouca frequência. Praticamente todo o tráfego em uma rede de malha de infraestrutura é encaminhado para ou para um gateway, enquanto em redes ad hoc ou redes de malha de cliente sem fio Os fluxos de tráfego entre pares arbitrários de nós.
Se a taxa de mobilidade entre os nós for alta, ou seja, as quebras de link acontecem com frequência, as redes de malha sem fio começam a quebrar e têm baixo desempenho de comunicação.
Gestão
Esse tipo de infraestrutura pode ser descentralizado (sem servidor central) ou gerenciado centralmente (com um servidor central). Ambos são relativamente baratos e podem ser muito confiáveis e resilientes, pois cada nó precisa apenas transmitir até o próximo nó. Os nós atuam como roteadores para transmitir dados de nós próximos a pares que estão muito longe para alcançar um único salto, resultando em uma rede que pode abranger distâncias maiores. A topologia de uma rede de malha deve ser relativamente estável, ou seja, não muita mobilidade. Se um nó cair da rede, devido à falha de hardware ou por qualquer outro motivo, seus vizinhos poderão encontrar rapidamente outra rota usando um protocolo de roteamento.
Formulários
As redes de malha podem envolver dispositivos fixos ou móveis. As soluções são tão diversas quanto as necessidades de comunicação, por exemplo, em ambientes difíceis, como situações de emergência, túneis, plataformas de petróleo, vigilância do campo de batalha, aplicações de videoclipe móvel de alta velocidade a bordo do transporte público, telemetria de carros de corrida em tempo real ou auto Organizando o acesso à Internet para as comunidades. Um aplicativo possível possível para redes de malha sem fio é o VoIP. Ao usar um esquema de qualidade de serviço, a malha sem fio pode suportar chamadas telefônicas locais através da malha. A maioria dos aplicativos em redes de malha sem fio é semelhante à das redes ad hoc sem fio.
Algumas aplicações atuais:
U.S. military forces are now using wireless mesh networking to connect their computers, mainly ruggedized laptops, in field operations.[citation needed]Electric smart meters now being deployed on residences, transfer their readings from one to another and eventually to the central office for billing, without the need for human meter readers or the need to connect the meters with cables.The laptops in the One Laptop per Child program use wireless mesh networking to enable students to exchange files and get on the Internet even though they lack wired or cell phone or other physical connections in their area.Smart home devices such as Google Wi-Fi, Google Nest Wi-Fi, and Google OnHub all support Wi-Fi mesh (i.e., Wi-Fi ad hoc) networking. Several manufacturers of Wi-Fi routers began offering mesh routers for home use in the mid-2010s.Some communications satellite constellations operate as a mesh network, with wireless links between adjacent satellites. Calls between two satellite phones are routed through the mesh, from one satellite to another across the constellation, without having to go through an earth station. This makes for a shorter travel distance for the signal, reducing latency, and also allows for the constellation to operate with far fewer earth stations than would be required for an equal number of traditional communications satellites. The Iridium satellite constellation, consists of 66 active satellites in a polar orbit and operates as a mesh network providing global coverage.Operação
O princípio é semelhante à maneira como os pacotes viajam pela Internet com fio - os datos saltos de um dispositivo para outro até que ele chegue ao seu destino. Os algoritmos de roteamento dinâmico implementados em cada dispositivo permitem que isso aconteça. Para implementar esses protocolos de roteamento dinâmico, cada dispositivo precisa comunicar informações de roteamento a outros dispositivos na rede. Cada dispositivo então determina o que fazer com os dados que recebe - passe -os para o próximo dispositivo ou mantê -los, dependendo do protocolo. O algoritmo de roteamento usado deve tentar sempre garantir que os dados seguam a rota mais apropriada (mais rápida) para seu destino.
Malha de rádio multi
A malha multi-radio refere-se a ter rádios diferentes operando em diferentes frequências para interconectar nós em uma malha. Isso significa que há uma frequência única usada para cada salto sem fio e, portanto, um domínio dedicado de colisão CSMA. Com mais bandas de rádio, é provável que a taxa de transferência de comunicação aumente como resultado de mais canais de comunicação mais disponíveis. Isso é semelhante ao fornecimento de caminhos de rádio duplos ou múltiplos para transmitir dados de receber.
Tópicos de pesquisa
Um dos artigos mais frequentemente citados sobre redes de malha sem fio identificou as seguintes áreas como problemas de pesquisa aberta em 2005
New modulation schemeTo achieve higher transmission rate requires new wideband transmission schemes other than OFDM and UWB.Advanced antenna processingAdvanced antenna processing including directional, smart and multiple antenna technologies is further investigated, since their complexity and cost are still too high for wide commercialization.Flexible spectrum managementTremendous efforts on research of frequency-agile techniques are being performed for increased efficiency.Cross-layer optimizationCross-layer research is a popular current research topic where information is shared between different communications layers to increase the knowledge and current state of the network. This could facilitate development of new and more efficient protocols. A joint protocol that addresses various design problems—routing, scheduling, channel assignment etc.—can achieve higher performance since these problems are strongly co-related. Note that careless cross-layer design can lead to code that is difficult to maintain and extend.Software-defined wireless networkingCentralized, distributed, or hybrid? - In a new SDN architecture for WDNs is explored that eliminates the need for multi-hop flooding of route information and therefore enables WDNs to easily expand. The key idea is to split network control and data forwarding by using two separate frequency bands. The forwarding nodes and the SDN controller exchange link-state information and other network control signaling in one of the bands, while actual data forwarding takes place in the other band.SecurityA WMN can be seen as a group of nodes (clients or routers) that cooperate to provide connectivity. Such an open architecture, where clients serve as routers to forward data packets, is exposed to many types of attacks that can interrupt the whole network and cause denial of service (DoS) or Distributed Denial of Service (DDoS).Exemplos
Packet radio networks or ALOHA networks were first used in Hawaii to connect the islands. Given the bulky radios, and low data rate, the network is less useful than it was envisioned to be.In 1998–1999, a field implementation of a campus-wide wireless network using 802.11 WaveLAN 2.4 GHz wireless interface on several laptops was successfully completed. Several real applications, mobility and data transmissions were made.Mesh networks were useful for the military market because of the radio capability, and because not all military missions have frequently moving nodes. The Pentagon launched the DoD JTRS program in 1997, with an ambition to use software to control radio functions - such as frequency, bandwidth, modulation and security previously baked into the hardware. This approach would allow the DoD to build a family of radios with a common software core, capable of handling functions that were previously split among separate hardware-based radios: VHF voice radios for infantry units; UHF voice radios for air-to-air and ground-to-air communications; long-range HF radios for ships and ground troops; and a wideband radio capable of transmitting data at megabit speeds across a battlefield. However, JTRS program was shut down in 2012 by the US Army because the radios made by Boeing had a 75% failure rate.Google Home, Google Wi-Fi, and Google OnHub all support Wi-Fi mesh networking.In rural Catalonia, Guifi.net was developed in 2004 as a response to the lack of broadband Internet, where commercial Internet providers weren't providing a connection or a very poor one. Nowadays with more than 30,000 nodes it is only halfway a fully connected network, but following a peer to peer agreement it remained an open, free and neutral network with extensive redundancy.In 2004, TRW Inc. engineers from Carson, California, successfully tested a multi-node mesh wireless network using 802.11a/b/g radios on several high speed laptops running Linux, with new features such as route precedence and preemption capability, adding different priorities to traffic service class during packet scheduling and routing, and quality of service. Their work concluded that data rate can be greatly enhanced using MIMO technology at the radio front end to provide multiple spatial paths.ZigBee digital radios are incorporated into some consumer appliances, including battery-powered appliances. ZigBee radios spontaneously organize a mesh network, using specific routing algorithms; transmission and reception are synchronized. This means the radios can be off much of the time, and thus conserve power. ZigBee is for low power low bandwidth application scenarios.Thread is a consumer wireless networking protocol built on open standards and IPv6/6LoWPAN protocols. Thread's features include a secure and reliable mesh network with no single point of failure, simple connectivity and low power. Thread networks are easy to set up and secure to use with banking-class encryption to close security holes that exist in other wireless protocols. In 2014 Google Inc's Nest Labs announced a working group with the companies Samsung, ARM Holdings, Freescale, Silicon Labs, Big Ass Fans and the lock company Yale to promote Thread.In early 2007, the US-based firm Meraki launched a mini wireless mesh router. The 802.11 radio within the Meraki Mini has been optimized for long-distance communication, providing coverage over 250 metres. In contrast to multi-radio long-range mesh networks with tree-based topologies and their advantages in O(n) routing, the Maraki had only one radio, which it used for both client access as well as backhaul traffic.The Naval Postgraduate School, Monterey CA, demonstrated such wireless mesh networks for border security. In a pilot system, aerial cameras kept aloft by balloons relayed real time high resolution video to ground personnel via a mesh network.SPAWAR, a division of the US Navy, is prototyping and testing a scalable, secure Disruption Tolerant Mesh Network to protect strategic military assets, both stationary and mobile. Machine control applications, running on the mesh nodes, "take over", when Internet connectivity is lost. Use cases include Internet of Things e.g. smart drone swarms.An MIT Media Lab project has developed the XO-1 laptop or "OLPC" (One Laptop per Child) which is intended for disadvantaged schools in developing nations and uses mesh networking (based on the IEEE 802.11s standard) to create a robust and inexpensive infrastructure. The instantaneous connections made by the laptops are claimed by the project to reduce the need for an external infrastructure such as the Internet to reach all areas, because a connected node could share the connection with nodes nearby. A similar concept has also been implemented by Greenpacket with its application called SONbuddy.In Cambridge, UK, on 3 June 2006, mesh networking was used at the “Strawberry Fair” to run mobile live television, radio and Internet services to an estimated 80,000 people.Broadband-Hamnet, a mesh networking project used in amateur radio, is "a high-speed, self-discovering, self-configuring, fault-tolerant, wireless computer network" with very low power consumption and a focus on emergency communication.The Champaign-Urbana Community Wireless Network (CUWiN) project is developing mesh networking software based on open source implementations of the Hazy-Sighted Link State Routing Protocol and Expected Transmission Count metric. Additionally, the Wireless Networking Group in the University of Illinois at Urbana-Champaign are developing a multichannel, multi-radio wireless mesh testbed, called Net-X as a proof of concept implementation of some of the multichannel protocols being developed in that group. The implementations are based on an architecture that allows some of the radios to switch channels to maintain network connectivity, and includes protocols for channel allocation and routing.FabFi is an open-source, city-scale, wireless mesh networking system originally developed in 2009 in Jalalabad, Afghanistan to provide high-speed Internet to parts of the city and designed for high performance across multiple hops. It is an inexpensive framework for sharing wireless Internet from a central provider across a town or city. A second larger implementation followed a year later near Nairobi, Kenya with a freemium pay model to support network growth. Both projects were undertaken by the Fablab users of the respective cities.SMesh is an 802.11 multi-hop wireless mesh network developed by the Distributed System and Networks Lab at Johns Hopkins University. A fast handoff scheme allows mobile clients to roam in the network without interruption in connectivity, a feature suitable for real-time applications, such as VoIP.Many mesh networks operate across multiple radio bands. For example, Firetide and Wave Relay mesh networks have the option to communicate node to node on 5.2 GHz or 5.8 GHz, but communicate node to client on 2.4 GHz (802.11). This is accomplished using software-defined radio (SDR).The SolarMESH project examined the potential of powering 802.11-based mesh networks using solar power and rechargeable batteries. Legacy 802.11 access points were found to be inadequate due to the requirement that they be continuously powered. The IEEE 802.11s standardization efforts are considering power save options, but solar-powered applications might involve single radio nodes where relay-link power saving will be inapplicable.The WING project (sponsored by the Italian Ministry of University and Research and led by CREATE-NET and Technion) developed a set of novel algorithms and protocols for enabling wireless mesh networks as the standard access architecture for next generation Internet. Particular focus has been given to interference and traffic-aware channel assignment, multi-radio/multi-interface support, and opportunistic scheduling and traffic aggregation in highly volatile environments.WiBACK Wireless Backhaul Technology has been developed by the Fraunhofer Institute for Open Communication Systems (FOKUS) in Berlin. Powered by solar cells and designed to support all existing wireless technologies, networks are due to be rolled out to several countries in sub-Saharan Africa in summer 2012.Recent standards for wired communications have also incorporated concepts from Mesh Networking. An example is ITU-T G.hn, a standard that specifies a high-speed (up to 1 Gbit/s) local area network using existing home wiring (power lines, phone lines and coaxial cables). In noisy environments such as power lines (where signals can be heavily attenuated and corrupted by noise), it is common that mutual visibility between devices in a network is not complete. In those situations, one of the nodes has to act as a relay and forward messages between those nodes that cannot communicate directly, effectively creating a "relaying" network. In G.hn, relaying is performed at the Data Link Layer.Protocolos
Protocolos de roteamento
Existem mais de 70 esquemas concorrentes para rotear pacotes em redes de malha. Alguns deles incluem:
Associativity-Based Routing (ABR)AODV (Ad hoc On-Demand Distance Vector)B.A.T.M.A.N. (Better Approach To Mobile Adhoc Networking)Babel (protocol) (a distance-vector routing protocol for IPv6 and IPv4 with fast convergence properties)Dynamic NIx-Vector Routing|DNVRDSDV (Destination-Sequenced Distance-Vector Routing)DSR (Dynamic Source Routing)HSLS (Hazy-Sighted Link State)HWMP (Hybrid Wireless Mesh Protocol, the default mandatory routing protocol of IEEE 802.11s)Infrastructure Wireless Mesh Protocol (IWMP) for Infrastructure Mesh Networks by GRECO UFPB-BrazilODMRP (On-Demand Multicast Routing Protocol)OLSR (Optimized Link State Routing protocol)OORP (OrderOne Routing Protocol) (OrderOne Networks Routing Protocol)OSPF (Open Shortest Path First Routing)Routing Protocol for Low-Power and Lossy Networks (IETF ROLL RPL protocol, RFC 6550)PWRP (Predictive Wireless Routing Protocol)TORA (Temporally-Ordered Routing Algorithm)ZRP (Zone Routing Protocol)O IEEE desenvolveu um conjunto de padrões sob o título 802.11s.
Uma lista menos completa pode ser encontrada na lista de protocolos de roteamento ad hoc.
Protocolos defiguração de autoconas
Wikimedia Commons has media related to Mesh network.Protocolos padrão de configuração de autocon, como a autoconfiguração sem estado DHCP ou IPv6, podem ser usados em redes de malha.
Rede de malha Protocolos específicos de auto -figuração incluem:
Ad Hoc Configuration Protocol (AHCP)Proactive Autoconfiguration (Proactive Autoconfiguration Protocol)Dynamic WMN Configuration Protocol (DWCP)