CN - Hybrid Topologies, Backbone networks, Switching

Hybrid topology

A hybrid topology is a type of network topology that uses two or more other network topologies, including bus topology, mesh topology, ring topology, star topology, and tree topology.

For example, if there exists a ring topology in one office department while a bus topology in another department, connecting these two will result in Hybrid topology. Remember connecting two similar topologies cannot be termed as Hybrid topology. Star-Ring and Star-Bus networks are most common examples of hybrid network. 

Advantages of Hybrid Network Topology:

Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of network and required corrective measures can be taken, WITHOUT affecting the functioning of rest of the network. 

Scalable: Its easy to increase the size of network by adding new components, without disturbing existing architecture. 

Flexible: Hybrid Network can be designed according to the requirements of the organization and by optimizing the available resources. Special care can be given to nodes where traffic is high as well as where chances of fault are high. 

Effective: Hybrid topology is the combination of two or more topologies, so we can design it in such a way that strengths of constituent topologies are maximized while there weaknesses are neutralized. For example we saw Ring Topology has good data reliability (achieved by use of tokens) and Star topology has high tolerance capability (as each node is not directly connected to other but through central device), so these two can be used effectively in hybrid star-ring topology. 

Disadvantages of Hybrid Topology

Complexity of Design: One of the biggest drawback of hybrid topology is its design. Its not easy to design this type of architecture and its a tough job for designers. Configuration and installation process needs to be very efficient.

Costly Hub: The hubs used to connect two distinct networks, are very expensive. These hubs are different from usual hubs as they need to be intelligent enough to work with different architectures and should be function even if a part of network is down.

Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a lot of cables, cooling systems, sophisticate network devices, etc.

 

Hybrid Network Topology

Backbone networks

A backbone network is a larger transmission line that carries data gathered from smaller lines that interconnect with it.

Backbone part of the network is a core of the network with its high throughput capability and significant bandwidth. It is made for the ability of network to communicate with external networks (like Internet). It is a root of the network tree, that has rest of the network growing from it.

     At the local level, a backbone is a line or set of lines that local area networks connect to for a wide area network connection or within a local area network to span distances efficiently (for example, between buildings).  

    

    On the Internet or other wide area network, a backbone is a set of paths that local or regional networks connect to for long-distance interconnection. The connection points are known as network nodes or telecommunication data switching exchanges (DSEs).

Serial Backbone

Serial backbone is formed of two or more devices that are connected in a daisy chain (linked series). It is a simplest kind of backbone. As the one can see from Figure 1, serial backbone can be made not only from switches, but also from gateways and routers.

While designing the backbone, the one should consider the limit of the devices that can be connected to the backbone in the repeating fashion. Exceeding the limit would result in the unexpected errors and data loss in the network. Serial backbone networks are not very fault tolerant and not very scalable, that make them less commonly used that the distributed backbone.

Figure 1. Serial backbone.

Distributed Backbone

Distributed backbone uses hierarchical design of the network, where number of intermediate devices are connected to single or multiple connectivity devices. These central connectivity devices could be switches or routers and shown with purple color in Figure 2.

Figure 2. Simple distributed backbone.

This type of backbone is easily scalable since new layers of devices can be added with no troubles. Distributed backbone allows simple administration and management of the network due to its segregation. This type of network can have daisy chain linked devices for the backbone, however, designer should consider the same limitations as for the serial backbone.

Figure 3 provides a view of more complicated distributed backbone that connects multiple LANs together. LANs are interconnected with routers that form the backbone.

Figure 3. Distributed backbone that connects multiple LANs.

Overall, it is cheap, easy, and quick to implement the distributed backbone network.

Collapsed backbone

This type of backbone uses single, powerful router as the central connection point for multiple subnetworks. As Figure 4 shows, the central device is the highest level of the backbone. It should have powerful computational power in order to manage big traffic coming in. This is highly risky, since if the central device fails, the whole network would be down. However, this type of backbone is useful for the one who wants to interconnect two types of subnetworks, with ability to manage and troubleshoot them. 

Figure 4. Collapsed backbone.

Parallel backbone

Parallel backbone is a variation of the collapsed backbone, where devices are having more than one connection between them. As Figure 5 shows, there are multiple connections between the high level routers and the network segments. Duplicate connections ensure networks availability at anytime, higher speeds, and high fault tolerance. Logical drawback of this solutions is the increased price, since amount of required cabling is highly increased. It is not obligatory to have duplicate connections between all the devices, selective implementation of parallel structure would significantly lower the overall price and make additional ports of the devices available.

Figure 5. Parallel backbone

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Switching

a switch determines from the physical device (Media Access Control or MAC) address in each incoming message frame which output port to forward it to and out of. In a wide area packet-switched network such as the Internet, a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination.

Circuit Switching

In this networking method, a connection called a circuit is set up between two devices, which is used for the whole communication. Information about the nature of the circuit is maintained by the network. The circuit may either be a fixed one that is always present, or it may be a circuit that is created on an as-needed basis. Even if many potential paths through intermediate devices may exist between the two devices communicating, only one will be used for any given dialog. This is illustrated in  Figure 1.

Figure 1: Circuit Switching

In a circuit-switched network, before communication can occur between two devices, acircuit is established between them. This is shown as a thick blue line for the conduit of data from Device A to Device B, and a matching purple line from B back to A. Once set up, all communication between these devices takes place over this circuit, even though there are other possible ways that data could conceivably be passed over the network of devices between them. Contrast this diagram to Figure 2.

Packet Switching

In this network type, no specific path is used for data transfer. Instead, the data is chopped up into small pieces called packets and sent over the network. The packets can be routed, combined or fragmented, as required to get them to their eventual destination. On the receiving end, the process is reversed—the data is read from the packets and re-assembled into the form of the original data. A packet-switched network is more analogous to the postal system than it is to the telephone system (though the comparison isn't perfect.) An example is shown in Figure 2.

Figure 2: Packet Switching

In a packet-switched network, no circuit is set up prior to sending data between devices. Blocks of data, even from the same file or communication, may take any number of paths as it journeys from one device to another.

Comparing Circuit Switching and Packet Switching

A common temptation when considering alternatives such as these is to ask which is “better”—and as usually is the case, the answer is “neither”. There are places where one is more suited than the other, but if one were clearly superior, both methods wouldn't be used.

One important issue in selecting a switching method is whether the network medium is shared or dedicated. Your phone line can be used for establishing a circuit because you are the only one who can use it—assuming you can keep that pesky wife/husband/child/sister/brother/father/mother off the phone.

However, this doesn't work well in LANs, which typically use a single shared medium and baseband signaling. If two devices were to establish a connection, they would “lock out” all the other devices for a long period of time. It makes more sense to chop the data into small pieces and send them one at a time. Then, if two other devices want to communicate, their packets can be interspersed and everyone can share the network.

The ability to have many devices communicate simultaneously without dedicated data paths is one reason why packet switching is becoming predominant today.

However, there are some disadvantages of packet switching compared to circuit switching. One is that since all data does not take the same, predictable path between devices, it is possible that some pieces of data may get lost in transit, or show up in the incorrect order. In some situations this does not matter, while in others it is very important indeed.

While the theoretical difference between circuit and packet switching is pretty clear-cut, understanding how they are used is a bit more complicated. One of the major issues is that in modern networks, they are often combined. For example, suppose you connect to the Internet using a dial-up modem. You will be using IP datagrams (packets) to carry higher-layer data, but it will be over the circuit-switched telephone network. Yet the data may be sent over the telephone system in digital packetized form. So in some ways, both circuit switching and packet switching are being used concurrently.

Another issue is the relationship between circuit and packet switching, and whether a technology is connection-oriented or connectionless. The two concepts are related but not the same; the next topic discusses this in much more detail.