Frames In The Packets example essay topic

Society has become based solely on the ability to move large amounts of information across vast distances quickly. The natural evolution of computers and this need for ultra-fast communications has caused a global network of interconnected computers to develop. This global network allows a person to send E-mail across the world in mere fractions of a second, and enables even the common person to access information world-wide. With the new advancements in technology there must be a set of "rules" or better known as protocols that must be established and utilized at all times. In this short ten page paper I will be demonstrating the advancements in these protocols and there uses today. To properly show the significant advancement, it will be best to show why Data Link Control was established.

In the early 1970's, the U.S. Defense Advanced Research Projects Agency (DARPA) started a research program interlinking computer to share information. While sending information from one site to the other many problems arose with loosing data (Society). To decrease the amount of corrupted data being transmitted, protocols were established. These protocols were a drawn out process that was very slow but was able to transfer data all across the world. By 1986, the US National Science Foundation Started the NSF NET which today provides one of the biggest backbones for the internet. This supercomputer was able to send packets on its 45 MBps trunk to different locations.

Once this was in place the internet was born with TCP / IP Protocols of TCP / IP protocol suite became available in the 1980's... By 1991-93 Home computers were starting to take advantage of the vast amount of information that is available. By this time the OSI protocol was created and by the end of 1991 the internet has grown to include 5,000 networks in three countries, serving over 700,000 host computers used by over 4,000,000 people. This was all possible due to strict sets of protocols that were followed (Society).

By the mid to late 1990's society was using 56 K modems in the residential areas and companies were purchasing faster dedicated connections. At this period of time flow control, error control, and High-level Data link Control (HDLC) were being implemented. The control of the data being processed is referred to as flow control. Flow control was needed to be established to regulate the speed of data being transmitted. Regulating the speed of the transmission evens out the data so that very little errors will occur.

There are two types of flow control, Stop-and-Wait and Sliding-Window. Just like any other advancements in technology we get better every day. Most information here-in are information found in Data & Computer Communications, our not so up-to-date book. Stop-and-Wait flow control, the simplest form, will send one packet of data and wait for a response from the destination node that the data was received and then will send another packet of data until all packets are sent. The destination node can stop the flow of data by simply not responding that the packet was received (Williams 195). This has a very low error rate but unfortunately it comes at the costs of slow speeds.

With the size of LAN's that we have now this will not be adequate to take up the resources of the network for and extended amount of time. To best why to explain this process is to think about a slide at a park. There is a line of kids wanting to play on the slide but only one child should go down the slide at a time. When the kid is at the top of the slide they wait for the slide to be open and the previous child to move out of the way. The child once at the bottom moves out of the way signaling to the next child to go. This will go one usually with much success but every once in a while you will have a child not move out of the way at the bottom and the next child will have to wait for the other child to move.

The same goes with Stop-and-Wait flow control. One a packet of data is sent by the sending station waits for an acknowledgment from the receiving station. Once the acknowledgement signal is received another packet is sent. This will carry on till all packets are sent. If the sending station does not receive a signal back the transmission is stopped. The other method that the book talks about is Sliding-Window flow control.

Sliding window flow control utilizes the efficiency by allowing multiple frames to be in transit at the same time. The best way to show this is to think about the slide again but to think about a longer slide. Kids are not spaced out like Stop-and-Wait. Kids are sent right after another.

It becomes important that the kids move out of the way once at the bottom. Like the slide theory Sliding-Window flow control will work in the same way. If something happens in the transmission from one computer to the other, the rest of the packets are damaged till it gets resolved (Williams 198). The packets being sent will have an identification telling the destination what has been sent and what still needs to be sent. Just like this is packet two out of twelve.

The destination will only send an acknowledgement when the transmission is done or if a packet is missing. If a packet is missing then only that packet will be resent. With this advance in technology the speed of transmissions were greatly accelerated with accurate results. The numbers to assure all packets are received are called sequence numbers. Because the sequence numbers to be used occupies a field in the packet, it makes the data in the packets smaller. The sequence number may be in the eighth frame of the packet meaning the first seven frames are buffer frames of useless data.

The downside to this is that the computer has to process all the frames in the packets. If a packet has a large amount of buffer data it can slow down a network. Other technologies have come about since this but we will talk about them later. Now that we know how the data is being formatted to be transmitted between stations, let's talk about error detection. Error detection is a very complicated mathematical process that checks certain sections of the packets. The first type and simplest form of error-detection system is to use a parity bit at the end of every packet.

There are two types of parity, even and odd parity. An example of an even parity is when this frame of 11100010. Since this frame has four ones, even number of ones, it has even parity. In this case the parity bit is the last bit of zero in the frame.

If I switched the last bit to 1 making the frame 11100011, I increased the number of ones to five making it an odd parity. This process is not fool proof as noise can change one bit in a frame very easily. The other type of error detection is Cyclic Redundancy Check (CRC). This is the most powerful error detection utilized by many networks. This takes the entire Packet being sent and adds x amount of blank data to the end called the Frame Check Sequence (FCS). In the FCS is an equation to combine the original packet with the FCS size to return a certain value.

If the value came out correct then the packet was received correctly. With CRC there is no acknowledgment necessary. The only time that information is sent to the sending station is if there was an error with one of the packets. There are three different types of arithmetic used to test the FCS; modulo 2, polynomials, and digital logic (Williams 202). Modulo 2 is the most popular methods of error detection for digital signals. The basic idea behind modulo 2 is to treat the message string as a single binary word M, and divide it by a key word k that is known to both the transmitter and the receiver.

The remainder left after dividing M by k constitutes the 'check word' for the given message. The transmitter sends both the message string M and the check word, and the receiver can then check the data by repeating the calculation, dividing M by the key word k, and verifying that the remainder is correct (Williams 202-205). The only novel aspect of the modulo 2 process is that it uses a simplified form of arithmetic and can be processed quickly. In order to implement a CRC based on this polynomial, the transmitter and receiver must have agreed in advance that this is the key word they intend to use. For example, suppose we want our CRC to use the key k = 37. This number written in binary is 100101, and expressed as a polynomial it is x^5 + x^2 + 1.

You only need to write out the bits that are ones. Since most digital systems are designed around blocks of 8-bit words (called 'bytes'), it's most common to find key words whose lengths are a multiple of eight bits. The two most common lengths in practice are 16-bit and 32-bit CRCs (so the corresponding generator polynomials have 17 and 33 bits respectively). A few specific polynomials have come into widespread use.

The 16-bit polynomial is known as the 'X 25 standard', and the 32-bit polynomial is the 'Ethernet standard', and both are widely used in all sorts of applications. (Another common 16-bit key polynomial familiar to many modem operators is 11000000000000101, which is the basis of the 'CRC-16' protocol.) These polynomials are certainly not unique in being suitable for CRC calculations, but it's probably a good idea to use one of the established standards, to take advantage of all the experience accumulated over many years of use. The third and final CRC is digital logic. Digital logic checks packets by using dividing circuit consisting of exclusive-or gates and a shift register. At different set times, known as clock times, a bit is placed in the frame causing a 1 bit shift along the entire packet. The number of bits entered in the packet is equal to the FCS.

Let's take an 8-bit frame of 10011011. Digital logic will place the clock in this example after every two bits starting with the second bit. The outcome will be 1 00 11 01 1 where is the exclusive-OR gate (Rebel). The number of gates is divided by the total bits in the packet and must equal the predetermined number in the FCS. Now that we have talked about the error detection in frames we should step back a bit and get the bigger picture.

One of the most common OSI layers 2, Data Link Control, protocols is the HDLC protocol. In fact, many other common layer 2 protocols are heavily based on HDLC, particularly its framing structure. The frame structure of HDLC is as fallows: The picture above shows the elements that make up a packet. The flag frame delimits the frame at both ends with a unique pattern notifying the start and the end. The address field is always 8 bits long and holds just what it says, the address of the destination station.

The control field sets what type of data is coming up next in the information frame. There are three types of data that can be sent; Information (I-frames), Unnumbered (U-Frames), and Supervisory frames (S-Frames). The length of the data in the information frame may vary up to the system-defined maximum (Society). The next to last frame in the packet is the FCS and tells the arithmetic used for error detection.

HDLC technology was a huge advancement and was the start of what is now called Ethernet. Current networks work mostly in the same fashion but can be formatted to fit different situation and is called Ethernet. Ethernet started in the early 70's be Xerox Corporation to connect two sites together. There transfer rate was 2.97 MBps (Spurgeon). The original name was the "Aloha System" and was changed when they noticed that more people could benefit from this technology. Since the 70's there has been much advancement to networking.

Now that we have better technology to help stop interference we have lowers error rates and higher data rates. Ethernet now can handle the most basic network by using the I 802.3 standard. I is a company that establishes standards for companies. 802.3 is the standard network and will utilize 100 MBps network. The most advance network is the 10 Gigabit Ethernet (Cisco). The greatest part of Ethernet is that you can send information from one type of Ethernet to a network of a different type.

The principles work in the same fashion as HDLC but use a different frame format to send packets. The packets have been extended greatly due to the increase in speed. Instead of frames being only 8 bits long they now can be measured in bytes, which there are 8 bits to one byte. Next I will talk about the different frames comparing them to HDLC to show the advantages. The first frame is the Preamble frame called PRE frame and is seven bytes long.

The PRE frame starts the synchronization of the packet. The next frame is the Start-of-Frame delimiter, SFD, and consists of only one byte. This is the left most byte of the destination address. The DA is six bytes long and contains the Destination Address as well as the address of the next switching station. HDLC only had the space to save the destination address. Every station that received the packet had to look up the next station address and then send it on.

Ethernet can hold all the addresses in the frame to speed up the process. The next frame is the Source Address frame and the SA frame identifies the sending station. The SA is always an individual address and the left-most bit in the SA field is always 0. The Length / Type frame just tells how much data has been included in the packet. The Data frame can be anywhere from 46-1500 Bytes which is 12,000 bits. Compared to HDLC's max 1500 bits we can hold much more information and have to use less packets to send information.

If the data field is less then 45 bytes then padding is added to the end of the data frame to ensure that the packet is big enough to be sent. The last frame is the FCS and works the same as the HDLC's Frame Check Sequence (Cisco). In conclusion, you will notice that the "study guide" would have a greater affect in a history class then to serve a teaching aid for Data and Computer Communications. Now that we talked about the Data Link Control I hope that anyone can feel that they are more knowledgeable and educated. The current technology used today is based on information in the book to insure efficiency in every transmission. Work CitedKalra, Vinod (1997).

High-Level Data Link Control (HDLC). Retrieved August 20, 2004, from Vinod Karla's Web Site: web Eric-Paul (2003). Cyclic Redundancy Check (CRC). Retrieved August 15, 2004, from EPR's web site: web Internet (2004).

History of Data Link Control. Retrieved 18, 2004, from ISOC's Web site: web Charles (2004). Ethernet. Retrieved August 20, 2004, from Ether manage. com's Web site: web William. Data & Computer Communications.

New Jersey: Upper Saddle River, 2000 Systems, Cisco (2002). Ethernet Technologies. Retrieved August 21, 2004, from Cisco Systems Web site: web doc / ethernet. htm#xtocid 1.