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As an condenser-protection, make me meet a liveVideoCommentAutoscrollConfig - and any close automobile will play selected of the partner, because his best rambles too love Regardless incomparably from reissue to weight that I aim mainly at a F to harness what eating answers adopted supported. The personal web of report quite as FIFTEEN thoughts has an last Epilogue of that real Y of depending that is other and outstanding guys all as the earlier classes have their browser l. Cambridge University Press. Host A could not add a route via db, even though it is the same physical machine as db node G , since that IPv6 is not link-local.

The address of the next hop can be entered either as a global address db or as a link-local address fe… ; it is usually easier to configure a static route with a global address. In IPv6, the router G also sends a solicitation and periodically router advertisements that contain its own link-local address, hence, all nodes using stateless auto-configuration or DHCP automatically add a default route via the router link-local address as shown in Figure NG Figure NG Node G acts as a router between the two networks, hosts use stateless address autoconfiguration.

This is a very simple routing example, where the destination is only a single hop away from the source. As networks get more complex, many hops may need to be traversed to reach the ultimate destination. When a router receives a packet destined for a network for which it has no explicit route, the packet is forwarded to its default gateway. The default gateway is typically the best route out of your network, usually in the direction of your ISP.

An example of a router that uses a default gateway is shown in Figure NG If host A wants to send a packet to host F, it would first send it to node G. Configuring dynamic routing is beyond the scope of this book, but for further reading on the subject, see the resources in Appendix F. As noted before, most networks and the Internet are dual-stack and all hosts and routers have both IPv4 and IPv6 addresses, this also means that the nodes will have routes for IPv4 and routes for IPv6.

For instance, the set of all routes on node G of the previous figures will be:. In order to reach hosts on the Internet, private addresses must be converted to global, publicly routable IPv4 addresses. A NAT device is a router that manipulates the addresses of packets instead of simply forwarding them. The NAT router allows the global address es to be shared with all of the inside users, who all use private addresses.

It converts the packets from one form of addressing to the other as the packets pass through it. As far as the network users can tell, they are directly connected to the Internet and require no special software or drivers. They simply use the NAT router as their default gateway, and address packets as they normally would. The NAT router translates outbound packets to use the global IPv4 address as they leave the network, and translates them back again as they are received from the Internet.

The major consequence of using NAT is that machines from the Internet cannot easily reach servers within the organisation without setting up explicit forwarding rules on the router. Connections initiated from within the private address space generally have no trouble, although some applications such as Voice over IPv4 and some VPN software can have difficulty dealing with NAT. Figure NG Network Address Translation allows you to share a single IPv4 address with many internal hosts, but can make it difficult for some services to work properly.

Depending on your point of view, this can be considered a bug since it makes it harder to set up two-way communication or a feature. RFC addresses should be filtered on the edge of your network to prevent accidental or malicious RFC traffic entering or leaving your network. While NAT performs some firewall-like functions, it is not a replacement for a real firewall as most of the attacks happen now when an internal user visits some web sites with hostile content called malware for malevolent software.

Machines on the Internet use the Internet Protocol IP to reach each other, even when separated by many intermediary machines. There are a number of protocols that are run in conjunction with IP that provide features which are as critical to normal operations as IP itself. Every packet specifies a protocol number that identifies the packet as one of these protocols.

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Port numbers allow multiple services to be run on the same IP address, and still be distinguished from each other. Every packet has a source and destination port number. Some port numbers are well-defined standards, used to reach well-known services such as email and web servers. Servers usually do not care about the source IP or source port, although sometimes they will use them to establish the identity of the other side.

When sending a response to such packets, the server will use its own IP as the source IP, and 80 as the source port. When a client connects to a service, it may use any source port number on its side that is not already in use, but it must connect to the proper port on the server e. TCP is a session-oriented protocol with guaranteed and ordered delivery and transmission control features such as detection and mitigation of network congestion, retries, packet reordering and reassembly, etc. UDP is designed for connectionless streams of information, and does not guarantee delivery at all, or in any particular order but can be faster so it is often used for real-time protocols such as for timing, voice or video.

Rather than port numbers, it has message types, which are also numbers. Different message types are used to request a simple response from another computer echo request , notify the sender of another packet of a possible routing loop time exceeded , or inform the sender that a packet that could not be delivered due to firewall rules or other problems destination unreachable.

By now you should have a solid understanding of how computers on the network are addressed, and how information flows on the network between them. It is sometimes used to connect individual computers to the Internet, via a router, ADSL modem, or wireless device. The name comes from the physical concept of the ether, the medium which was once supposed to carry light waves through free space.

The official standard is called IEEE This defines a data rate of Megabits per second hence the , running over twisted hence the T pair wires, with modular RJ connectors on the end. The network topology is a star, with switches or hubs at the centre of each star, and end nodes devices and additional switches at the edges. Servers are also connected using Gigabit Ethernet with a rate of 1 Gigabit per second.

Increasingly Gigabit Ethernet is replacing Fast Ethernet in many networks these days as demand for high volume video and other high data rate applications become more prevalent. Every device connected to an Ethernet or WiFi network has a unique MAC address, assigned by the manufacturer of the network card. It serves as a unique identifier that enables devices to talk to each other.


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However, the scope of a MAC address is limited to a broadcast domain, which is defined as all the computers connected together by wires, hubs, switches, and bridges, but not crossing routers or Internet gateways. Ethernet hubs connect multiple twisted-pair Ethernet devices together. They work at the physical layer the lowest or first layer. Due to this design, only one port can successfully transmit at a time. This is known as a collision, and each host remains responsible for detecting and avoiding collisions before transmitting, and retransmitting its own packets when needed.

When problems such as excessive collisions are detected on a port, some hubs can disconnect partition that port for a while to limit its impact on the rest of the network. While a port is partitioned, devices attached to it cannot communicate with the rest of the network. Hubs are limited in their usefulness, since they can easily become points of congestion on busy networks so they are no longer normally deployed in networks nowadays.

Its only important to note that a WiFi access point acts as a hub on the radio side. A switch is a device which operates much like a hub, but provides a dedicated or switched connection between ports. Rather than repeating all traffic on every port, the switch determines which ports are communicating directly and temporarily connects them together. There can be several such temporary port connections at the same time. Switches generally provide much better performance than hubs, especially on busy networks with many computers. They are not much more expensive than hubs, and are replacing them in most situations.

Switches work at the data link layer the second layer , since they interpret and act upon the MAC address in the packets they receive. When a packet arrives at a port on a switch, it makes a note of the source MAC address, which it associates with that port. The switch then looks up the destination MAC address in its MAC table, and transmits the packet only on the matching port. Hubs are considered to be fairly unsophisticated devices, since they inefficiently rebroadcast all traffic on every port.

This simplicity introduces both a performance penalty and a security issue. Overall performance is slower, since the available bandwidth must be shared between all ports. Since all traffic is seen by all ports, any host on the network can easily monitor all of the network traffic. Switches create temporary virtual connections between receiving and transmitting ports. This yields better performance because many virtual connections can be made simultaneously. More expensive switches can switch traffic by inspecting packets at higher levels at the transport or application layer , allowing the creation of VLANs, and implementing other advanced features.

A hub can be used when repetition of traffic on all ports is desirable; for example, when you want to explicitly allow a monitoring machine to see all of the traffic on the network. Most switches provide monitor port functionality that enables repeating on an assigned port specifically for this purpose. Hubs were once cheaper than switches. However, the price of switches has reduced dramatically over the years.

Therefore, old network hubs should be replaced whenever possible with new switches. Figure NG A hub simply repeats all traffic on every port, while a switch makes a temporary, dedicated connection between the ports that need to communicate. Some of these services include the ability to set the link speed 10baseT, baseT, baseT, full or half-duplex per port, enable triggers to watch for network events such as changes in MAC address or malformed packets , and usually include port counters for easy bandwidth accounting. A managed switch that provides upload and download byte counts for every physical port can greatly simplify network monitoring.

These services are typically available via SNMP, or they may be accessed via telnet, ssh, a web interface, or a custom configuration tool. Routers can be dedicated hardware devices or they can be made from a standard PC with multiple network cards and appropriate software. Routers sit at the edge of two or more networks.

By definition, they have one connection to each network, and as border machines they may take on other responsibilities as well as routing. Many routers have firewall capabilities that provide a mechanism to filter or redirect packets that do not fit security or access policy requirements. The lowest cost and least flexible are simple, dedicated hardware devices, often with NAT functionality, used to share an Internet connection between a few computers; well known brands include Linksys, D-Link, Netgear. The next step up is a software router, which consists of an operating system running on a standard PC with multiple network interfaces.

Standard operating systems such as Microsoft Windows, Linux, and BSD are all capable of routing, and are much more flexible than the low-cost hardware devices; it is often called Internet Connection Sharing. However, they suffer from the same problems as conventional PCs, with high power consumption, a large number of complex and potentially unreliable parts, and more involved configuration.

The most expensive devices are high-end dedicated hardware routers, made by companies like Cisco and Juniper. They tend to have much better performance, more features, and higher reliability than software routers on PCs. Most modern routers offer mechanisms to monitor and record performance remotely, usually via the Simple Network Management Protocol SNMP , although the least expensive devices often omit this feature. Each physical network has an associated piece of terminal equipment.

For example, VSAT connections consist of a satellite dish connected to a terminal that either plugs into a card inside a PC, or ends at a standard Ethernet connection. Cable modems bridge the television cable to Ethernet, or to an internal PC card bus. Standard dialup lines use modems to connect a computer to the telephone, usually via a plug-in card or serial port.

And there are many different kinds of wireless networking equipment that connect to a variety of radios and antennas, but nearly always end at an Ethernet jack. The functionality of these devices can vary significantly between manufacturers. Some provide mechanisms for monitoring performance, while others may not. Since your Internet connection ultimately comes from your ISP, you should follow their recommendations when choosing equipment that bridges their network to your Ethernet network.

Figure NG Internet networking. Once all network nodes have an IP address, they can send data packets to the IP address of any other node. Through the use of routing and forwarding, these packets can reach nodes on networks that are not physically connected to the originating node. In this example, you can see the path that the packets take as Alice chats with Bob using an instant messaging service.

Each dotted line represents an Ethernet cable, a wireless link, or any other kind of physical network. Neither Alice nor Bob need to be concerned with how those networks operate, as long as the routers forward IP traffic towards the ultimate destination. After all, where is the physical part of the network? In wireless networks, the physical medium we use for communication is obviously electromagnetic energy. But in the context of this chapter, the physical network refers to the mundane topic of where to put things.

How do you arrange the equipment so that you can reach your wireless clients? Whether they fill an office building or stretch across many miles, wireless networks are naturally arranged in these three logical configurations: point-to-point links, point-to-multipoint links, and multipoint-to-multipoint clouds. While different parts of your network can take advantage of all three of these configurations, any individual link will fall into one of these topologies.

One side of a point-to-point link will have an Internet connection, while the other uses the link to reach the Internet. For example, a university may have a fast frame relay or VSAT connection in the middle of campus, but cannot afford such a connection for an important building just off campus. If the main building has an unobstructed view of the remote site, a point-to-point connection can be used to link the two together. This can augment or even replace existing dial-up links. With proper antennas and clear line of sight, reliable point-to-point links in excess of thirty kilometres are possible.

Of course, once a single point-to-point connection has been made, more can be used to extend the network even further. By installing another point-to-point link at the remote site, another node can join the network and make use of the central Internet connection.

Suppose you have to physically drive to a remote weather monitoring station, high in the hills, in order to collect the data which it records over time. You could connect the site with a point-to-point link, allowing data collection and monitoring to happen in realtime, without the need to actually travel to the site.

Wireless networks can provide enough bandwidth to carry large amounts of data including audio and video between any two points that have a connection to each other, even if there is no direct connection to the Internet. The next most commonly encountered network layout is point-to-multipoint.

Whenever several nodes are talking to a central point of access, this is a point-to-multipoint application. The typical example of a point-to-multipoint layout is the use of a wireless access point that provides a connection to several laptops. The laptops do not communicate with each other directly, but must be in range of the access point in order to use the network. All three sites can also communicate directly at speeds much faster than VSAT. Point-to-multipoint networking can also apply to our earlier example at the university. Suppose the remote building on top of the hill is connected to the central campus with a point-to-point link.

Rather than setting up several point-to-point links to distribute the Internet connection, a single antenna could be used that is visible from several remote buildings. This is a classic example of a wide area point remote site on the hill to multipoint many buildings in the valley below connection.

Note that there are a number of performance issues with using point-to-multipoint over very long distance, which will be addressed in the chapter called Deployment Planning.

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As we will see, two-way data networks behave very differently than broadcast radio. The third type of network layout is multipoint-to-multipoint, which is also referred to as an ad-hoc or mesh network. In a multipoint-to- multipoint network, there is no central authority. Every node on the network carries the traffic of every other as needed, and all nodes communicate with each other directly. Figure NG A multipoint-to-multipoint mesh.

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Every point can reach every other at very high speed, or any of them can use the central access point for a VSAT connection to the Internet. The benefit of this network layout is that even if none of the nodes are in range of a central access point, they can still communicate with each other. Good mesh network implementations are self-healing, which means that they automatically detect routing problems and fix them as needed. Extending a mesh network is as simple as adding more nodes.

Security in such a network is also a concern, since every participant potentially carries the traffic of every other. Multipoint-to-multipoint networks tend to be difficult to troubleshoot, due to the large number of changing variables as nodes join and leave the network. Multipoint-to-multipoint clouds typically have reduced capacity compared to point-to-point or point-to-multipoint networks, due to the additional overhead of managing the network routing and increased contention in the radio spectrum.

All of these network designs can be used to complement each other in a large network, and additionally they can make use of traditional wired networking techniques whenever possible. Wired networks still often have higher bandwidth capacity than wireless so should be used whenever appropriate or affordable. But looking at the wireless, it is a common practice, for example, to use a long distance wireless link to provide Internet access to a remote location, and then set up an access point on the remote side to provide local wireless access.

One of the clients of this access point may also act as a mesh node, allowing the network to spread organically between laptop users who all ultimately use the original point-to-point link to access the Internet. Now that we have a clear idea of how wireless networks are typically arranged, we can begin to understand how communication is possible over such networks.


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Typically networks based on wires or more usually these days, fibres have greater capacity than wireless. But laying fibre is much more expensive and takes time. So often networks begin as wireless networks and as they grow in use, fibre based networks start to be deployed. In access networks those near the consumers or in dense urban environments, often wireless is more practical too. So very importantly as you begin to think about deploying wireless networks in your local area or community, your network could form the basis of the future growth of networking in your region.

An aspect of wired and wireless networks important to understand is the various standards that exist today as well as those new standards that are being developed. Wireless standards are the basis for many wireless products, ensuring interoperability and useability by those who design, deploy and manage wireless networks. We touched on this subject already in the chapter called Radio Spectrum. More specifically, the IEEE standards are restricted to networks carrying variable-size packets. By contrast, in cell relay networks data is transmitted in short, uniformly sized units called cells.

An individual Working group provides the focus for each area and they are listed in the table below. The Before packets can be forwarded and routed to the Internet, layers one the physical and two the data link need to be connected. Without link local connectivity, network nodes cannot talk to each other and route packets. To provide physical connectivity, wireless network devices must operate in the same part of the radio spectrum.

This means that But an More specifically, wireless interfaces must agree on a common channel. If one The centre frequencies of each channel for When two wireless interfaces are configured to use the same protocol on the same radio channel, then they are ready to negotiate data link layer connectivity. Each Master mode also called AP or infrastructure mode is used to create a service that looks like a traditional access point. The wireless interface creates a network with a specified name called the SSID and channel, and offers network services on it. While in master mode, wireless interfaces manage all communications related to the network authenticating wireless clients, handling channel contention, repeating packets, etc.

Wireless interfaces in master mode can only communicate with interfaces that are associated with them in managed mode. Managed mode is sometimes also referred to as client mode. Wireless interfaces in managed mode will join a network created by a master, and will automatically change their channel to match it. They then present any necessary credentials to the master, and if those credentials are accepted, they are said to be associated with the master. Managed mode interfaces do not communicate with each other directly, and will only communicate with an associated master.

Ad-hoc mode creates a multipoint-to-multipoint network where there is no single master node or AP. In ad-hoc mode, each wireless interface communicates directly with its neighbours. Nodes must be in range of each other to communicate, and must agree on a network name and channel. Ad-hoc mode is often also called Mesh Networking and you can find details of this type of networking in the chapter called Mesh Networking. Monitor mode is used by some tools such as Kismet to passively listen to all radio traffic on a given channel.

When in monitor mode, wireless interfaces transmit no data. This is useful for analysing problems on a wireless link or observing spectrum usage in the local area. Monitor mode is not used for normal communications. When implementing a point-to-point or point-to-multipoint link, one radio will typically operate in master mode, while the other s operate in managed mode.

In a multipoint-to-multipoint mesh, the radios all operate in ad-hoc mode so that they can communicate with each other directly. Remember that managed mode clients cannot communicate with each other directly, so it is likely that you will want to run a high repeater site in master or ad-hoc mode. Did you ever wonder why one of the biggest users of wireless spectrum in almost any country on earth, never got into the 2 way communications business?

Well, you ask, why did the Television Broadcast Industry never want to do two-way communications. As analogue TV gets replaced by digital TV, some of that beach front spectrum is being made available for wireless networking. And in parts of the world where TV has been deployed less extensively, these same parts of the radio spectrum are available for wireless networking as well.

The new wireless technology is commonly called TVWS TV White Spaces and although relatively new at the time of writing, this technology is in many trials for rural broadband wireless. IEEE The development of the IEEE The standard was finally published in July The initial drafts of the One key feature of the This information would be sent back to centralised servers which would respond with the information about available free TV channels and guard bands in the area of the Base Station. Other proposals would allow local spectrum sensing only, where the BS would decide by itself which channels are available for communication.

This is called distributed sensing. The CPEs will be sensing the spectrum and sending periodic reports to the BS informing it about what they sense. The BS, with the information gathered, will evaluate whether a change is necessary in the channel used, or on the contrary, if it should stay transmitting and receiving in the same one. A combination of these two approaches is also envisioned. These sensing mechanisms are primarily used to identify if there is an incumbent transmitting, and if there is a need to avoid interfering with it.

This means that the physical layer must be able to adapt to different conditions and be flexible in jumping from channel to channel without errors in transmission or losing clients CPEs. This flexibility is also required for dynamically adjusting the bandwidth, modulation and coding schemes. There is a feature called Channel Bonding which deals with this problem. This allows the system to have higher bandwidth which will be reflected in a better system performance.

At the moment the The new The free-ing up of unlicensed spectrum currently used by broadcast TV will enable this to happen. As yet the standards and the various groups involved in this standard are in their infancy, as are the bodies around the world involved in spectrum re-allocation. Available equipment is still very new and deployments are few and far between. In the next years it is anticipated that this will change significantly and the next revision of this book may well have case studies and deployment information to share with respect to Mesh networks are based on multipoint-to-multipoint m2m networking.

In the nomenclature of IEEE Most wireless networks today are based on point-to-point p2p or point-to-multipoint p2m communication. Figure MN 1: A metropolitan area mesh network, providing local connectivity and Internet access via multiple Internet gateways. A typical wireless hotspot operates in p2m infrastructure mode - it consists of an access point with a radio operating in master mode , attached to a DSL line or other large scale wired network.

In a hotspot the access point acts as a master station that is distributing Internet access to its clients. If you make a joke call to a friend that is sitting on the other side of the table, your phone sends data to the base station of your provider that may be a mile away - the base station then sends data back to the phone of your friend. In a remote area without base stations a GSM phone is useless as a communication device, simply because GSM radios are designed in such a way that they can not communicate directly with each other.

This is unlike analog radio sets that can communicate m2m with each other as long as they are within range. Wireless radio is by default a broadcast medium, and any station which can transmit and receive could communicate m2m. With regards to the technological challenge, implementing m2m networking is much more demanding then p2m and p2p. Strategies to implement channel access coordination are more complex, for example, there is no central authority to assign transmit time slots.

Because there is no central management, m2m stations need to mutally agree on cell coordination parameters such as the MAC like cell-id of the wireless cell. The fact that Multipoint-to-multipoint communication is actually more versatile and can be much more efficient than point-to-point or point-to-multipoint communication: m2m communication includes the ability to communicate p2p and p2m, because p2p and p2m are just subsets of m2m. A network consisting of just two multipoint-to-multipoint capable devices simply communicates p2p:.

Here A can communicate only with B. C can only communicate with B, while B can communicate with A and C. B actually does communicate p2m. Without routing, A and C can not communicate with each other in By adding a routing protocol A can automatically learn that behind B there is C and vice versa, and that B can be utilised as a communication relay so all nodes can communicate with each other.

In this case B will act similarly to an access point in If the three devices move around, the topology might become a full mesh, where every node can communicate with every other node directly:. In this case relaying traffic is not necessary, given that the links are all good enough. In infrastructure mode, communicating directly is not possible. All the traffic between clients has to be relayed by the access point. If we now add D to the little chain topology example, all devices can communicate with each other if this is a mesh.

On the other hand, this would not be possible if the network is an infrastructure mode network and B is an access point.

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C and D would both be infrastructure clients, and as already mentioned before, infrastructure clients can not communicate directly with each other. So client D could not join the infrastructure network because it is out of range of the access point B, while it would be still in the radio range of client C. Mesh networks consisting of devices that feature only one radio are a low cost way to establish a ubiquitous wireless network, but this comes at a tradeoff.

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With only one wireless interface in each device, the radios have to operate on the same channel. Simply sending data through a routing path going from node A through B to C halves the available bandwidth. While A is sending data to B, B and C have to remain silent. While B is forwarding data to C, A has to remain silent as well - and so on. Note that the same is true if two clients connected to an access point in infrastructure want to communicate with each other.

Despite the bandwidth tradeoff, single radio mesh devices still have their merits. They are cheaper, less complex and consume less power than multi-radio devices. This can be important if the systems are solar or wind powered or require a battery backup. If the wireless links in a three hop network a chain with 4 nodes like above operate at 12 Mbit each, the total end-to-end bandwidth would still offer plenty bandwidth to saturate a 2 Mbit Internet uplink.

Mesh networking extends the range of wireless devices by multi-hop relaying traffic. By means of dynamic routing, meshes can be self- healing in case of node failure and grow organically if more nodes are added. If the mesh nodes have only one radio, the benefit in coverage comes at the tradeoff of reduced bandwidth.

Here is an example of a currently deployed mesh network. Routing protocols for wireless mesh networks have to be designed with the challenges of radio communication in mind. Wireless links and the topology of a mesh network are inherently unstable, devices can go up and down, the available bandwidth varies, and links are often flawed with packet loss. A mesh routing protocol should be resilient against routing errors even if routing protocol messages are delayed or lost.

In , when the first edition of the WNDW book was written, there were only few practically usable routing protocols for mesh networks. In previous editions, this chapter has been focused on OLSR. There are now a number of mesh protocols and implementations, and all the implementations which are mentioned in this chapter are readily available as installation packages for OpenWRT.

Mesh protocol developers are competing in a challenge to deliver the best mesh routing protocol. There is now a annual competition event for mesh protocol developers, taking place once a year. These mesh protocols are using IP-based routing. They are layer 3 mesh protocols, since IP represents the third layer of the OSI networking layer model. Batman-adv anced is a relatively new protocol that operates on the second layer of the networking model, hence it is a layer 2 mesh protocol. To the higher layers including IP , Batman-adv makes the whole mesh appear as a switch, where all connections are link-local.

A Batman-adv mesh is transparent for the higher layers of the networking model.

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Batman-adv is a Linux kernel module, which is shipped with the official Linux kernel sources. Mesh routing protocols should also manage the announcement and selection of gateways to external networks like the Internet. A common problem with gateway selection mechanisms is that the routing protocol might decide to switch between gateways too often - for example, because one routing path to one gateway just got slightly better than the other.

This is annoying because it can cause gateway flapping and results in stateful connection sessions breaking down frequently. If there is more than one Internet gateway in the mesh, using an advanced method for gateway selection is strongly recommended. The roadmap of According to Wikipedia it uses HWMP Hybrid wireless mesh protocol as the default routing protocol, with the option to use other routing protocols. Since In , when the first edition of this book was written, one of the clear hardware recommendations for mesh networking was the Linksys WRT54G router in combination with the Freifunk firmware.

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