What are the differences between bridges and repeaters?

Bridges and repeaters are both important devices in the field of telecommunications and computer networking. In this article we discuss about these two and differences between them. 

- The repeaters are deployed at the physical layer whereas one can find bridges at the MAC layer. 

- Thus, we called repeaters as the physical layer device. 

- Similarly, bridge is known as the MAC layer device. 

- Bridge is responsible for storing as well forwarding the data packets in an Ethernet.

- Firstly, it examines the header of the data frame, selects few of them and then forwards them to the destination address mentioned in the frame. 

- Bridge uses the CSMA/CD for accessing a segment whenever the data frame has to be forwarded to it.

- Another characteristic of a bridge is that its operation is transparent. 

- This means that the hosts in the network do not know that the bridge is also present in the network. 

- Bridges learn themselves; they do not have to be configured again and again. 

- They can be simply plugged in to the network. 

- Installing a bridge causes formation of LAN segments by breaking a LAN. 

- Packets are filtered with the help of bridges. 

- The frames that belong to one LAN segment are not sent to the other segments. 

- This implies separate collision domains are formed. 

The bridge maintains a bridge table consisting of the following entries:

1. LAN address of the node
2. Bridge interface
3. Time stamp
4. Stale table entries

- Bridges themselves learn that which interface can be used for reaching which host. 

- After receiving a frame, it looks for the location of the sending node and records it.

- It keeps the collision domains isolated from one another thus, giving the maximum throughput. 

- It is capable of connecting a number of nodes and offer limitless geographical coverage. 

- Even different types of Ethernet can be connected through it. 

- Even the repeaters are plug and play devices but they do not provide any traffic isolation. 

- Repeaters are used for the purpose of regenerating the incoming signals as they get attenuated with time and distance. 

- If physical media such as the wifi, Ethernet etc. is being used, the signals can travel only for a limited distance and after that their quality starts degrading. 

- The work of the repeaters is to increase the extent of the distance over which the signals can travel till they reach their destination. 

- Repeaters also provide strength to the signals so that their integrity can be maintained. 

- Active hubs are an example of the repeaters and they are often known as the multi-port repeaters. 

- Passive hubs do not serve as repeaters. 

- Another example of the repeaters are the access points in a wifi network. 

- But it is only in repeater mode that they function as repeaters. 

- Regenerating signals using repeaters is a way of overcoming the attenuation which occurs because of the cable loss or the electromagnetic field divergence. 

- For long distances, a series of repeaters is often used. 

- Also, the unwanted noise that gets added up with the signal is removed by the repeaters. 

- The repeaters can only perceive and restore the digital signals.

- This is not possible with the analog signals. 

- Signal can be amplified with the help of amplifiers but they have a disadvantage which is that on using the amplifiers, the noise is amplified as well. 

- Digital signals are more prone to dissipation when compared to analog signals since they are completely dependent up on the presence of the voltages. 

- This is why they have to be repeated again and again using repeaters. 

What is reverse path forwarding?

- RPF or reverse path forwarding is a common technique used for ensuring that the multicast packets are forwarded without any loops in the modern routers in multicast routing. 

- This technique is also used for the prevention of the IP address spoofing during the unicast routing.

- Multicast RPF or just RPF is not used alone. 

- Rather, it is used along with some multicast routing protocol. 

- There are various multicast routing protocols such as the PIM – SM, PIM – DM, MSDP and so on. 

- This is for ensuring that no loops are formed in forwarding the multicast packets. 

- Source address is used for deciding whether the traffic has to be forwarded or not in multicast routing. 

- On the other hand in unicast routing, this depends up on the destination address instead of source address. 

- This it achieves either through utilization of either the unicast routing table of the router or a multicast routing table that has been dedicated to the purpose. 

- As and when a packet comes to the interface of the router, it searches in the networks list for the networks that can be reached through this interface. 

- This is nothing but the reverse path checking of the multicast packet.  

- If the appropriate routing entry is found for the multicast packet’s source IP address, it is said to pass the RPF check. 

- After this the packet is sent to all the participating interfaces in that particular multicast group.  

- If the packet fails at this RPF check, the packet is simply dropped. 

- Because of this, the packet forwarding has to be decided depending up on its reverse path. 

- Otherwise, the forward path can be used as usual. 

- Only those packets are forwarded by the RPF routers which pass this RPF check. 

- Passing this RPF check means breaking any loop that might otherwise exist. 

- This is of critical importance in the multicast topologies that are redundant. 

- This is so because it is possible for the same packet to come again and again to the same router through a number of multiple interfaces. 

- The RPF check is an integral part of the decision concerning forwarding of the packets. 

- Consider a router forwarding a packet from first interface to the second interface and also from second interface to the first one. 

- Thus, the same packet is received by the two packets, thus creating a common routing loop. 

- This loop will keep on forwarding the packets until the expiry of their TTLs. 

- Even if the TTL expiry is considered, the best thing to do is to avoid the routing loops because they are a main cause of the temporary network degradation.

RPF check has the following underlying assumptions:

1. The given unicast routing table is converged as well as correct.
2. There is symmetry between the path that goes from sender to router and the path that comes back from the router to the sender.

- RPF check uses the unicast routing table as the fallback. 

- Therefore, if the first assumption is not satisfied, the check will fail. 

- But in case the second assumption is false, the multicast traffic is rejected by the RPF check save the traffic on the shortest path that exists between the sender and the router. 

- This results in a multicast tree that is non–optimal.

- The reverse path forwarding will not work if there are uni-directional links present in the network.

Unicast RPF: 

- This type of the reverse path forwarding is based up on the concept that the interface which does not originate traffic must not accept it. 

- It is good for the organizations to not allow private address propagation on their network until and unless they are continuously using it. 

How can we reduce the risk by using a test plan?

A test plan apart from just serving the purpose of a guide for the software testing life cycle also plays a great role in the assessment of the risks associated with the development of that particular software system or application and also provides the alternatives or builds for overcoming those factors which are the root cause of the risk.

The preparation of the test plan must be based on all the risks identified during the process of risk assessment. This article is focussed up on the relation that exists between the risk and the test plan and how a risk based test plan can help reducing the overall risk.

How a risk based test plan reduce the overall risk?

- Such a test plan is effective only when a test oriented risk analysis is carried out during the software system or application development life cycle.

- With the advancing and sophisticated technology the complexity of the software systems and applications is also increasing and so does it lengthens the testing process which is quite exhaustive.

- The software testing methodologies therefore should be quite selective and should also be chosen keeping in mid the time limit and the budget of the project.

- It is often stressed by many testers that the testing should be based on the risk which is not possible until and unless the testers are well equipped with the knowledge of the risk.

- It is a wise decision of concentrating the testing more on the area which is at higher risk compared to other parts of the software system or application.

- Many researchers have made a rigorous research on the subject of risk based testing and have stated that a software system or application requires apart from the understanding of software testing, the knowledge of risk and its analysis also.

Types of Risks
There are two types of risks namely:

1. Forward Risks
These are the risks associated with the operation of the system and one of the major cause of the software failure.

2. Backward Risks
These are the risks associated with the development issues like those mentioned below:
(a) Inappropriate design
(b) Casual programming
(c) Incorrect specifications
(d) Inadequate management

Risk is thought of as a function of two components as described below:

1. The probability of occurrence of an undesirable event that is defined and
2. The degree of the severity of the consequences if the event of the system failure does occur.

In most of the cases, the consequences following the failure of the software system are related to the purpose of the software and therefore reducing the risk is not always an option.

Thus, this leads us to the conclusion that the risk must be reduced by reducing the probability of the failure of the software system which in turn can be achieved only with an efficient test plan.

Now you must be wondering can this actually happen? Yes of course.

- The software testing reduces the risks by digging out many of the bugs.

- But, the actual risk reduction is based up on the implementation of the corrected code and functions.

- The failure of software system is not systematic and hence cannot be predicted by just checking out the history.

- So for the cases like these, only the estimation of the potential consequences of the failure works via the risk based testing that acts as a single factor analysis in
such cases.

- For some it may seem like a trivial process but, it is not so and calls for a thorough examination of the system failure.

- The single factor analysis though does not produce a very correct estimate; it does help the testers in focussing their testing on the code that is buggier than the other units.