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. 

Explain the technique of admission control to control congestion in virtual circuit subnets?

- Virtual circuits can be thought of as a virtual channel in the telecommunication networks as well as computer networks. 

- Virtual circuit sub-nets represent a communication service that is connection oriented. 

- This service is used through the packet mode communication.

- A stream of data bytes can be exchanged between the two nodes only if a virtual circuit has been established between them. 

- Without the presence of the higher level protocols, data division has to be dealt with unnecessarily. 

- Therefore, the virtual circuits always allow the high level protocols. 

- There is a resemblance between the circuit switching mode and the virtual circuits because of the fact that both of them are based up on connection. 

- The packets that are transmitted through a virtual circuit sub-net consist of a circuit number and not the destination address. 

- This is why the memory requirement of the packets is less when using virtual circuits and more in the others. 

- This also makes the virtual circuit sub nets less expensive when compared to other sub nets. 

In this article we discuss about the technique for congestion control in virtual circuit sub nets. 

- One most popular technique is of the admission control. 

- Most of the congestion control methods are based up on an open loop i.e., the congestion is prevented rather than managing it after it has occurred. 

- Admission control is a dynamic method for controlling the congestion problems in the virtual circuit sub nets. 

- Admission control technique has been widely accepted for preventing the congestion problem from getting worse over the time. 

- The technique is based up on a very simple idea which is that no virtual circuit is set up until and unless the congestion problem that was detected has been resolved. 

- Therefore, any attempt that is made for establishing a new virtual connection with the transport layer is failed. 

- The things get even worse if the access is granted to more and more people. 

- Simplicity of the technique is one of its characteristic that makes it easy to be implemented. 

- The technique can be implemented in a straightforward manner. 

- The admission control technique is also used by the telephone systems for combating with the congestion problems. 

- The admission control technique is implemented whenever a switch in the network gets overloaded. 

- At this time no dial tone is heard. 

- Establishing new virtual connections represents another way of coping with this problem. 

- Here, the new connections have to be routed carefully such that there are no problems. 

Another method for eliminating the problem of congestion is to strike an agreement between the virtual circuit subnet and the host. 

- By this we mean establishing a new virtual circuit. 

- But this arrangement requires specifying how the traffic has to be shaped and what would be its volume, QoS (quality of service), other parameters and so on. 

- The virtual circuit sub net has to reserve the resources on its part of the agreement established. 

- These resources lie on the route where the virtual circuit has been established. 

- The resources might include the following:

Ø  Space in the router’s buffer.

Ø  Tables

Ø  Bandwidth of the lines and so on.

- The newly virtual circuits are less likely to experience the congestion problems. 
- This is because to them the availability of the resources has been guaranteed.
- Resources can be reserved in this way only if the existing sub nets are experiencing congestion problem or when the standard operating procedure is being followed. 
- One disadvantage of the admission control technique is that it leads to the wastage of the resources. 
- Also, sometime the bandwidth is left unused. 

What is a virtual circuit? What are the advantages of virtual circuit?

The term VC or virtual circuit is synonymous with the term virtual channel or connection in the field of computer networks as well as in the telecommunications. 

- These are the connection oriented communication service.  

- The packet mode communication is the means through which this service is delivered. 

- After the establishment of a virtual circuit or connection between the two application processes or nodes, the stream of bytes or bit stream can be delivered between the two. 

- The higher level protocols are allowed by the virtual circuits in order to avoid the task of unnecessary dealing with the data division.

- This task may involve dividing data in to frames, packets or segments. 

- The virtual circuits bear a resemblance to the circuit switching mode under the fact that both of them are connection oriented.

- This means that both of them require correct order of delivery of the data. 

- Also, they both require signaling overhead during the establishment phase of the connections.

- The difference between the two is in the terms of the latency and bit rate. 

- It is constant in circuit switching and may vary in virtual circuits. 

This happens because of the following 3 major causes:

1. Varying length of the packet queues in the nodes.

2. Varying bit rate as generated by the application.

3. Varying load generated by the users who share the same resources on the network through statistical multiplexing. 

- A number of virtual circuit protocols are known for providing reliable services for communication but not all. 

- These services are provided by the means of data re-transmission because of the ARQ (automatic repeat request) and error detection. 

- Data-gram represents an alternate configuration for the virtual circuit. 

There are two types of virtual circuits namely:

Layer 4 virtual circuits: Data-link protocols such as TCP which are connection oriented and include segment numbering and thus reordering on the receiver’s side use this kind of virtual circuits. Thus out of order delivery is prevented. 

Layer 2/3 virtual circuits: The virtual circuit protocols of the data link  layer and the network layer are based up on the packet switching that is connection oriented. this implies that the delivery path of the data is always the same. 

Advantages of Virtual Circuits

There are several advantages of this kind of virtual connections:

1. They support the bandwidth reservation while the connection is being established.  This in turns increases the possibilities of QoS (quality of service). 

2. They produce less overhead. This is because of the fact that there is no individual packet routing and exclusion of the complete addressing info from the packet header. Each packet contains only a small VI or virtual channel identifier. The remaining routing info is provided to the network nodes during the establishment of the connection. 

3. Theoretically speaking, the nodes here have high capacity and are faster because their only task is to carry out routing. On the other hand, the network nodes in a connection-less network carry out routing for every packet individually. In switching, it requires only to look up the VCI in the table instead of analyzing the full address. Implementation of the switches is quite easy in the ASIC hardware but the complexity of the routing increases and demands software implementation. But as we know there is a huge market of the IP routers and layer 3 switching is supported by the advanced IP routers. 

Below mentioned are some protocols that provide VC facility:

- TCP or transmission control protocol

- SCTP or stream control transmission protocol

- X.25

- Frame relay

- ATM or asynchronous transfer mode

- MPLS or multi-protocol label switching

What is the difference between a passive star and an active repeater in fiber optic network?

There are two important components of a fiber optic network namely passive star coupler and active repeaters. 

Passive Star in Fiber Optic Network

- Passive star couplers are single mode fiber optic couplers with reflective properties.  

- These couplers are used for optical local area networking at very high speeds. 

- These couplers are made from very simple components such as mirrors and 3 db couplers. 

- Besides this, these star couplers save a lot of optical fiber when compared to its trans-missive counterpart. 

- They are free of any multi-paths so as to avoid any interference. 

- A fiber optic network may consist of any number of passive star couplers and each of them is capable of connecting a number of users. 

- The input and output from every passive star coupler is given to the output and input of an active coupler. 

- The round trip transmission tile is stored by the active star coupler. 

- When it receives a signal from a passive star coupler, it stops the output to that coupler for the duration of the signal.

- It also inhibits the incoming data from all the other passive star couplers for the round trip transmission delay plus signal duration. 

- The purpose of a star coupler is to take one input signal and then splitting it in to a number of output signals. 

- In telecommunications industry and fiber optics communication, this coupler is used in network applications being a passive optical device. 

- If an input signal is introduced to one of the input ports, it is distributed to all of the output ports of the coupler. 

- As per the construction of the passive star coupler, the number of ports it will have is given by the power of 2. 

- For example, in a two port coupler or in a directional coupler or splitter, there are 2 input ports and 2 output ports.

- In a four port coupler, there are 4 i/p ports and 4 o/p ports and so on. 

- The digital equipment corporation also sold a device by the name of star coupler which was used for interconnecting the links and computers through coaxial cable instead of using optical fibers. 

Active Repeater in Fiber Optic Network 

- Active repeater is an important telecommunications device used for re transmitting the signal it receives to a higher level and with higher basically to the other side of an obstacle so that long distances can be covered. 

- Repeater is an electro-mechanical device that helps in regenerating the telegraphy signals. 

- It may be defined as an analog device for amplifying the input signal, reshaping it, re-timing it for re-transmission. 

- A re-generator is a repeater that can perform the re-timing operation. 

- Repeaters just tend to amplify the physical signal without interpreting the data transmitted by the signal. 

- The 1^st layer i.e., the physical layer is where the repeaters operate. 

- Repeaters are employed for boosting the signals in optical fiber lines as well as in twisted pair and coaxial cables. 

- When a signal travels through a channel, it gets attenuated with the distance and time because of the energy loss (dielectric losses, conductor resistance etc.). 

- When light travels in optical fibers, it scattered and absorbed and hence is attenuated. 

- Therefore, in long fiber lines, repeaters are installed at proper intervals for regenerating and strengthening the signal. 

Repeater in optical communication performs the following functions:

Ø  Takes the input signal

Ø  Converts it in to electrical signal

Ø  Regenerates it.

Ø  Converts it in to optical signal

Ø  Re-transmits it

- These repeaters are usually employed in submarine as well as transcontinental communication cables as the loss is unacceptable in these cases.  

RS422 Standard

RS-422 is a telecommunications standard for binary serial communications between devices. It is the protocol or specifications that must be followed to allow two devices that implement this standard to speak to each other. RS-422 is an updated version of the original serial protocol known as RS-232.

This standard was introduced in 1975 to offer improvements over the older RS-232 standard. It provides a balanced line with optional termination. The standard uses a voltage differential of 2v min to 5v max to represent the binary 0's and 1's. The specification allows data rates up to 10M baud at 40 feet maximum cable length.
The maximum cable length that can be driven will depend on the baud rate, the driver/receiver IC's, the cable type, and the amount of electrical noise in the surrounding environment. RS-422 can be used for point-to-point communication or for multi-drop one-master/many-slave systems.

The RS-422 standard only defines the characteristic requirements for the balanced line drivers and receivers. It does not specify one specific connector, signal names or operations. RS-422 interfaces are typically used when the data rate or distance criteria cannot be met with RS-232. The RS-422 standard allows for operation of up to 10 receivers from a single transmitter. The standard does not define operations of multiple tri-stated transmitters on a link.

RS-422 is a balanced four wire system. The signal sent from the DTE device is transmitted to the DCE device through two wires and the signal sent from the DEC device to the DTE device is transmitted through the other two wires. The signals on each pair of wires are the mirror opposite of each other, i.e., a "1" datum is transmitted as a plus 2 volt reference on one wire and a minus 2 volt reference on the other wire. To send a "0" datum, a minus 2 volt reference is transmitted through one wire and a plus 2 volt reference on the other wire. That is the opposite of what was done to transmit a \'1\' datum.

The RS-422-A interfaces between the Data Terminal Equipment (DTE) and Data Communication Equipment (DCE) or in any point-to-point interconnection of signals between digital equipment. It employs the electrical characteristics of balanced-voltage digital interface circuits.

RS485 Standard

RS-485 is a telecommunications standard for binary serial communications between devices. It is the protocol or specifications that need to be followed to allow devices that implement this standard to speak to each other. A RS-485 compliant network is a multi-point communications network. The RS-485 standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. RS-485 drivers are now even able to withstand bus contention problems and bus fault conditions. A RS-485 network can be constructed as either a balanced 2 wire system or a 4 wire system. If a RS-485 network is constructed as a 2 wire system, then all of the nodes will have equal ranking. A RS-485 network constructed as a 4 wire system, has one node designated as the master and the remaining nodes are designated as slaves. The maximum cable length can be as much as 4000 feet because of the differential voltage transmission system used. The typical use for RS485 is a single PC connected to several addressable devices that share the same cable.

RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions.

SPECIFICATIONS RS485
- Mode of Operation DIFFERENTIAL
- Total Number of Drivers and Receivers on One Line 1 DRIVER & 32 RECEIVER
- Maximum Cable Length 4000 FT.
- Maximum Data Rate 10Mb/s
- Maximum Driver Output Voltage -7V to +12V
- Driver Output Signal Level (Loaded Min.) Loaded +/-1.5V
- Driver Output Signal Level (Unloaded Max) Unloaded +/-6V
- Driver Load Impedance (Ohms) 54
- Max. Driver Current in High Z State Power On +/-100uA
- Max. Driver Current in High Z State Power Off +/-100uA
- Slew Rate (Max.) N/A
- Receiver Input Voltage Range -7V to +12V
- Receiver Input Sensitivity +/-200mV
- Receiver Input Resistance (Ohms) >=12k