What is a choke packet?

- The networks often experience problems with congestion and flow of the traffic. 

- While implementing flow control a special type of packet is used throughout the network. 

- This packet is known as the choke packet. 

- The congestion in the network is detected by the router when it measures the percentage of the buffers that are actually being used. 

- It also measures the utilization of the lines and average length of the queues. 

- When the congestion is detected, the router transmits choke packets throughout the network. 

- These choke packets are meant for the data sources that are spread across the network and which have an association with the problem of congestion. 

- These data sources in turn respond by cutting down on the amount of the data that they are transmitting. 

- A choke packet has been found to be very useful in the maintenance tasks of the network. 

- It also helps in maintaining the quality to some extent. 

- In both of these tasks, it is used for informing the specific transmitters or the nodes that the traffic they are sending is resulting in congestion in the network. 

- Thus, the transmitters or the nodes are forced to decrease the rate at which they are generating traffic. 

- The main purpose of the choke packets is controlling the congestion and maintaining flow control throughout the network. 

- The router directly addresses the source node, thus causing it to cut down its data transmission rate. 

- This is acknowledged by the source node by making reductions by some percentage in the transmission rates. 

- An example of the choke packet commonly used by the most of the routers is the source quench packet by ICMP (internet control message protocol).  

- The technique of using the choke packets for congestion control and recovery of the network involves the use of the routers. 

- The whole network is continuously monitored over by the routers for any abnormal activity.

- Factors such as the space in the buffers, queue lengths and the line utilization are checked by the routers. 

- In case the congestion occurs in the network, the choke packets are sent by the routers to the corresponding parts of the network instructing them to reduce the throughput. 

- The node that is the source of the congestion has to reduce its throughput rate by a certain percentage that depends on the size of the buffer, bandwidth that is available and the extent of the congestion. 

- Sending the choke packets is the way of routers telling the nodes to slow down so that the traffic can be fairly distributed over the nodes. 

- The advantage of using this technique is that it is dynamic in nature. 

- The source node might send as much data as required while the network might inform that it is sending large amounts of traffic.

- The disadvantage is that it is difficult to know by what factor the node should reduce its throughput.

- The amount of the congestion being caused by this node and the capacity of the region in which congestion has occurred is responsible for deciding this. 

- In practical, this information is not instantly available. 

- Another disadvantage is that after the node has received the choke packet, it should be capable of rejecting the other choke packets for some time. 

- This is so because many additional choke packets might be generated during the transmission of the other packets. 

The question is for how long the node is supposed to ignore these packets? 

- This depends up on some dynamic factors such as the delay time. 

- Not all congestion problems are same, they vary over the network depending up on its topology and number of nodes it has. 

What are applications of flooding algorithm?

- Flooding algorithm and its many other variants are used as material distributing algorithm.

- This algorithm distributes the messages to all the hosts in the entire graph. 

- This algorithm since it acts like a flood and therefore has been named so. 

- In this simple yet useful distribution or routing algorithm, every packet that a node receives is transmitted to every other outgoing link. 

This algorithm is available in many variants but in every variant the following two things are common:

1. Each node acting as receiver and transmitter.

2. Each node responsible for forwarding received message to all its neighboring nodes except the one from which the message came. 

- Thus, the messages are eventually delivered to the hosts spread across the network. 

- Flooding algorithm might have been more useful if it would have been more complex. 

- Also, then it would have been possible to avoid the duplicate messages and infinite loops that occur because of them. 

Applications of Flooding Algorithm

In this article we list some of the application of the flooding algorithm.

1. Used in computer networking

2. Used in graphics

3. These algorithms are quite useful for solving numerous mathematical problems such as the maze problems.

4. Used for solving problems in the graph theory. 

5. Used in systems which make use of bridging. 

6. Used in systems like usenet.

7. Implemented in peer – to – peer file sharing

8. Flooding algorithms are often implemented as a part of some of the routing protocols as in OSPF, DVMRP and so on. 

9. It is also used in the protocols used in the ad hoc wireless networks.

10.There is a variant of flooding algorithm called the selective flooding which is capable on addressing various issues of flooding algorithm partially by allowing the packets to be sent only in the appropriate right direction. The packets are not sent on each and every line. 

11. Another variant of the flooding algorithm called the similarity algorithm is used graph matching algorithm. 

- This variant of the flooding algorithm is quite versatile and has got an application in the schema matching. 

- Matching the contents of the two data schemas has got an important role to play in many biochemical applications, e – business and other data warehousing applications etc. 

- The similarity flooding algorithm is based on a computation that is fixed and can be used across a number of scenarios. 

- Two graphs are passed to the algorithm as the input parameters. 

- These graphs might be of catalogs, schemas or even data structures etc. 

- The algorithm then produces a mapping between the corresponding nodes of the two graphs as output. 

- It depends on the goal of the matching what subset of the mapping has to be chosen using filters. 

- After the algorithm has been executed, a human tester is expected is expected to check and verify the results. 

- The results might be adjusted by the tester if required. 

- In this method, the accuracy of the algorithm is evaluated by number of the adjustments that are necessary.

- In some cases, an accuracy metric might be used for the estimation of the labor savings that could be obtained by the users by means of this similarity flooding algorithm for obtaining a first matching pair.

- Finally, this algorithm can be deployed as an operator of very high level in a test bed that has been implemented for the management of the output mappings and the information models. 

- There are different types of matching problems and thus each type requires following a different approach. 

- For example, the relational schemas can only be matched using SQL data types.

Sliding Window Protocols? – Part 1

- There are many types of data transmission protocols of which one type is the packet based data transmission protocols. 

- These protocols have a feature called the sliding window protocol.

- The sliding window protocols are a great help wherever the in-order delivery of the data packets demand reliability. 

- For example, the Data link layer of the TCP (transmission control protocol) model and OSI model demand such reliability and thus use window sliding protocol. 

- According to the concept of the sliding window protocols, a consecutive number which is unique is assigned to each and every portion of the transmission i.e., the packets.

- These numbers are used by the receiver for placing the packets it will receive in their correct order. 

- Also, with the help of these numbers, the missing packets can be identified and the duplicate packets can be removed. 

- One problem regarding the sliding window protocols is that it has kept no limits for the size of these numbers that are required. 

- An unlimited number of data packets can be allowed to be communicated at any instant of time if limits are placed on the number of packets involved in transmission or reception. 

- By this, we mean using the sequence numbers of fixed size. 

- By term window we refer to the transmission side. 

- It actually represents the logical boundary or limit of the number of packets that the receiver has to acknowledge. 

- The transmitter has to be informed by the receiver for each ACK (acknowledgement) packet regarding the maximum size or the window boundary of the current receiver buffer. 

- For reporting the window size of the received buffer, a 16 bit field is used in the TCP header. 

- The maximum limit or boundary of the window that we can have is 2¹⁶ i.e., 64 KB. 

- When operating in the slow start mode, the counting of the transmitter begins with a low packet count.

- Gradually, the number of packets involved increases in every transmission after the ACK packet has been received. 

- Whenever it receives an ACK packet, the window slides logically by one packet for the transmission of a new packet. 

- On reaching the window threshold, one packet is sent by the transmitter for every one packet of ACK received. 

- Suppose the limit of the window is 10 packets and the transmitter is in slow start mode. 

- Then, first one packet will be transmitted followed by another two. 

- Between these two transmissions, it will send an ACK packet also. 

- This process will continue until the limit of 10 has reached. 

- After crossing the limit, the transmission is restricted to one i.e., for every ACK packet received only one data packet is transmitted. 

- When viewed during simulation, it seems as if the window is shifting by distance of one packet whenever an ACK packet is received. 

- For avoiding the traffic congestion, the sliding window protocol works up a great deal.

- In this way the application layer would not have to worry about transmission the next set of data packets. 

- It can continue to do so since the sliding windows of the packet buffer will be implemented on both the sides i.e., the receiver’s and the sender’s side by the TCP. 

-However, the network traffic influences the window size dynamically to a great extent. 

- In order to achieve the highest possible throughput, care should be taken for not forcing the transmitter to stop the transmission before one RTT or round trip delay time by the sliding window protocol. 

- The bandwidth delay product of the links in the communication should be less than the limit of the data amount that can be sent before sending ACK packet. - If this condition is not met, the links’ effective bandwidth will be limited by the protocol. 

Explain CSMA with collision detection?

- CSMA with collision detection is abbreviated as CSMA/CD. 

- CSMA in itself makes use of the LBT technology i.e., listen or sense before talk. 

- But when incorporated with the ability of collision detection, it gets much better. 

- If the channel is sensed to be idle the data packets or frames are transmitted immediately but if not, the transmitter is bound to wait for some time before it can re-transmit. 

- Sensing the channels prior to transmission is absolutely necessary if the collisions are to be avoided. 

- Sensing the channel is the most effective way of avoiding the collisions. 

- There are two types of CSMA protocols namely persistent and the non-persistent CSMA.

- In CSMA/CD protocol all the hosts have freedom for transmitting and receiving the data frames on one and the same channel. 

- Also, the size of the packets is variable.

CSMA/CD comprises of two processes:

Carrier Sense: In this process the transmitter or the host checks if the channel or the line is not occupied before starting the transmission.

Collision Detection: CSMA/CD tries to detect the collisions in the shortest possible time. If it happens to detect a collision, it stops the transmission then and there and waits for a random amount of time which is equal to the binary exponential back-off. It then again senses the channel.

- For ensuring there occurs no collision during the transmission of a packet, a host must have the capability of detecting the collision before the transmission process is complete. 

- What happens is that the host A sensing the line to be idle starts transmitting a frame. 

- Just before the first unit of this frame reaches host B, it also senses the line to be idle and starts its transmission. 

- Now the host B receives data while its transmission is still in progress and so it detects that a collision is about to occur. 

- A collision occurs close to the host B. the host A also receives data in midst of its transmission and therefore detects the collision. 

- For making the hosts detect collision before transmission, a minimum length has to be decided for the packets that are transmitted via CSMA/CD networks. 

There are 3 states for a CSMA/ CD channel namely:

1. Contention
2. Transmission
3. Idle

- Ethernet is the most popular example of the CSMA/CD networks. 

- A minimum slot time is required for collision detection between the stations.

- This slot time must equal twice the maximum value of the propagation delay. - The host acquires the channel on the basis of the 1 – persistence. 

- Also, a jam signal is transmitted if a case of collision detection occurs. 

- CSMA/CD make use of the binary exponential back-off algorithm. 

- It is obvious that the idle time of the channel will be small if the load is heavy. 

- It normalizes all the packets with respect to the time of the packet transmission.

- CSMA/CD represents a very effective method for media access control. 

- There are different methods available for detecting the collisions. 

- Which method is to be followed depends largely on the transmission medium that exists between the two stations. 

- For example, if the two stations are connected via electrical buses, the collision can be detected by making comparison between the transmitted and the received data. 

- Some other way involves recognition of a signal of higher amplitude than the normal one. 

- The jam signal used in the CSMA/CD networks is constituted of 32 bit binary pattern.

RS232 Standard

RS232 is a asynchronous serial communication protocol widely used in computers and digital systems. It is called asynchronous because there is no separate synchronizing clock signal as there are in other serial protocols like SPI and I2C.
In RS232 there are two data lines RX and TX. TX is the wire in which data is sent out to other device. RX is the line in which other device put the data it need to sent to the device.
Voltage levels in RS232 are HIGH=-12V and LOW=+12V.

RS-232 Specifications :
- Cabling : Single-ended
- Number of Devices : 1 transmit, 1 receive
- Communication Mode : Full duplex
- Distance(max) : 50 feet at 19.2kbps
- Data Rate(max) : 1Mbps
- Signaling : Unbalanced
- Mark(data 1) : -5V (min) -15V (max)
- Space(data 0) : 5V (min) 15V (max)
- Input Level(min) : ±3V
- Output Current : 500mA
- Impedance : 5kW (Internal)
- Bus Architecture : Point-to-Point

RS232 Data Transmission :
Transmission
1. When there is no transmission the TX line sits HIGH (STOP CONDITION).
2. When the device needs to send data it pulls the TX line low for 104uS (This is the start bit which is always 0).
3. Then it sends each bit with duration of = 104uS.
4. Finally it sets TX lines to HIGH for at least 104uS (This is stop bit and is always 1).
Reception :
1. The receiving device is waiting for the start bit i.e. the RX line to go LOW.
2. When it gets start bit it waits for half bit time i.e. 104/2 = 51uS, i.e.it is in middle of start bit, it reads it again to make sure it is a valid start bit and not a spike.
3. Then it waits for 104uS and now it is in middle of first bit. It then reads the value of RX line.
4. In the same way it reads all the 8 bits.
5. Now the receiver has the data.

Limitations of RS232 Standard :
* The large voltage swings and requirement for positive and negative supplies increases power consumption of the interface and complicates power supply design. The voltage swing requirement also limits the upper speed of a compatible interface.
* Single-ended signaling referred to a common signal ground limits the noise immunity and transmission distance.
* Multi-drop connection among more than two devices is not defined. While multi-drop "work-arounds" have been devised, they have limitations in speed and compatibility.
* Asymmetrical definitions of the two ends of the link make the assignment of the role of a newly developed device problematic; the designer must decide on either a DTE-like or DCE-like interface and which connector pin assignments to use.
* The handshaking and control lines of the interface are intended for the setup and takedown of a dial-up communication circuit; in particular, the use of handshake lines for flow control is not reliably implemented in many devices.
* No method is specified for sending power to a device. While a small amount of current can be extracted from the DTR and RTS lines, this is only suitable for low power devices such as mice.
* The 25-way connector recommended in the standard is large compared to current practice.

Handshaking Mechanism

Handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transfer.
It is usually a process that takes place when a computer is about to communicate with a foreign device to establish rules for communication. When a computer communicates with another device like a modem, printer, or network server, it needs to handshake with it to establish a connection.
With handshaking signals, a transmitter can indicate when it has data to send, and a receiver can indicate when it is ready to receive data. The exact protocols that signals follow may vary, though many RS-232 and RS-485 links follow standard or conventional protocols.

In hardware handshaking, the receiver brings a line high when it is ready to receive data, and the transmitter waits for this signal before sending data. The receiver may bring the line low any time and the transmitter must detect this, stop sending, and wait for the line to return high before finishing the transmission.
Other links accomplish the same thing with software handshaking, by having the receiver send one code to indicate that it is ready to receive, and another to signal the transmitter to stop sending.