## Topics Contributions

As the distributed computing system becomes widely used today, knowing the internal structure of a network helps people to understand and maintain the network efficiently. Network topology, which extends the topology from mathematics, is used to illustrate the layout of a network, both physically and logically. The devices in network are regarded as nodes, links between them as lines, and they are connected together geometrically to indicate the configuration of the network. The way that they are connected is the topology of that network.

Network topology can be further divided into two types: physical and logical. Physical topology focuses on the physical connection between the devices in a network, while logical topology focuses on the internal data flow regardless of the location of the devices or distances between them1.

### Basic Types of Network Topology

#### Point-to-point

A point-to-point topology is simply connecting two nodes directly.

#### Bus

All the nodes are connected to one central line (the bus) in a bus topology. Data is transmitted in both ways along the central line until the node address matches the the destination address contained in the data. If they don’t match, the data is passed to the next nodes without manipulation to ensure that it is received by the right node.

#### Star

There exists a central hub which all the other nodes are directly connected to and receive datas from. This simple structure also leads to an obvious weakness, once the central hub performs poorly or even breaks down, the rest of the network will be affected.

#### Ring

A ring topology can be regarded as an end-to-end chain, where every nodes is either directly or indirectly connected to the others. Data is transmitted in one direction along the ring, looking for the right destination address.

#### Mesh

There is a direct path between every pair of nodes in a fully connected mesh topology, which makes the data transmission faster by choosing the shortest path.

#### Tree

A tree network combines star topology and bus topology together. It has a central cable (the bus) as the backbone, with several branches coming out of both sides, each of the branches is connected to the central hub of a star network2.

#### Hybrid

There is a combination of at least two different physical topology structures in a hybrid topology. It is commonly used in the network with relatively large scale because of its high cost, on the other side, it combines the advantages of the topologies that are being used to make the whole network run more efficiently and stably3.

### References

1. Santanu Santra , Pinaki Pratim Acharjya. A Study And Analysis on Computer Network Topology For Data Communication. International Research Journal of Engineering and Technology. 2015;01 88-90. Available from: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.413.7099&rep=rep1&type=pdf [Accessed 28th February 2016]
2. Margaret Rouse. Tree Networking. Available from: http://searchnetworking.techtarget.com/definition/tree-network [Accessed 9th March 2016]
3. ELearnSite. Topology – Types of Topology. Available from: http://networkingelearnsite.blogspot.com/2012/07/topology-types-of-topology-define.html [Accessed 10th March 2016]

One issue that should be considered before building a CDN is how densely the servers should be distributed. The choice of CDN topology – the distribution of CDN servers – will lead to differences in performance. The two main approaches nowadays are the co-location approach and the network-core approach1. The servers of a CDN following the former approach are widely scattered geographically, while the latter are consolidated.

### Scattered CDN

A scattered CDN usually has a large number of surrogate servers distributed pervasively around the target area, with each of them having a relatively low capacity, the proximity to the end-users is therefore maximised2. One of the biggest CDN providers, Akamai, has a highly scattered CDN network. It claims to cover 110 countries with 200,000 servers within over 1,400 networks3.

### Consolidated CDN

A consolidated CDN, on the contrary, has centralized and high-capacity data centres comprising thousands of servers, located in fewer places4. Companies like Limelight and AT&T use this approach and they believe this will lead to a better performance overall1.

### References

1. Sipat Triukose, Zhihua Wen, Michael Rabinovich. Content Delivery Networks: How Big is Big Enough?, Case Western Reserve University. 2009. Available from: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.413.7099&rep=rep1&type=pdf [Accessed 28th February 2016]
2. Incapsula. The Essential CDN Guide | CDN Infrastructure. Available from: https://www.incapsula.com/cdn-guide/cdn-architecture.html [Accessed 2nd March 2016]
3. Akamai. About Akamai | Facts and Figures. Available from: https://www.akamai.com/us/en/about/facts-figures.jsp [Accessed 2nd March 2016]
4. Sipat Triukose, Zhihua Wen, Michael Rabinovich. Measuring a Commercial Content Delivery Network, Case Western Reserve University. 2011. Available from: http://wwwconference.org/proceedings/www2011/proceedings/p467.pdf [Accessed 28th February 2016]