Understanding and Using Cyclic. Redundancy Checks 01

Lecture 9Cyclic Redundancy Checking (CRC), Idle RQ, Continuous RQ, Selective Retransmission and Go-Back-N error recovery(Recap last lecture) Idle RQ and Continuous RQ are two forms of ARQ – where data packets are automatically re-sent if the receiving station does not acknowledge correct reception of the packet. The CRC is what the receiver uses to determine whether or not it has received the data correctly.Cyclic Redundancy Checking (CRC):A powerful method for detecting errors in the received data is by grouping the bytes of data into a block and calculating a Cyclic Redundancy Check (CRC). This is usually done by the data link protocol and the calculated CRC is appended to the end of the data link layer frame. The CRC is the only field which is by convention sent most significant bit first. (This is contrary to all header and payload bytes which are sent LSB first.) Thus the first bit of a CRC-16 to be sent is the bit corresponding to X16 and the last, the bit corresponding to X1.The CRC is calculated by performing a modulo 2 division of the data by a generator polynomial and recording the remainder after divisionFor a generator of R bits, the resulting (R - 1) -bit checksum will detect: •all single bit errors•most double bit errors•most odd number-of-bit errors•all error bursts less than R in length•most error bursts greater than or equal to R in lengthThree polynomials are in common use they are:•CRC-16 = x16 + x15 + x2+ 1•CRC-CCITT = x16 + x12 + x5 + 1•CRC-32 = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1 (used in Ethernet)Although this division may be performed in software, it usually performed using a shift register and X-OR gates. The hardware solution for implementing a CRC is much simpler than a software approach. One example for a CRC-16 is:On reception, the process is reversed. The CRC register is first set to zero (or the initial value on transmission, if non-zero). The bits (this time including the CRC) are fed into the register on each clock cycle. If the CRC contains the value zero (assuming initialisation was zero), the CRC is valid, if not it has detected an error. The CRC-16 is able to detect all single errors, all double errors, all odd numbers of errors and all errors with burst less than 16 bits in length. In addition 99.9984 % of other error patterns will be detected.Selective retransmission is simply another name for ARQ: only corrupted or lost packets are retransmitted.Go-back-N error recovery:The recovery of a corrupted frame proceeds in three stages:•First, the corrupted frame is discarded at the remote node's receiver. The loss of the frame is revealed when a correct (but out-of-sequence) frame issubsequently received.•Second, the remote node requests retransmission of the missing frame (s).•The final stage consists of retransmission of the lost frame(s).A remote node may request retransmission of corrupted frames by initiating Go-Back-N error recovery by sending a REJ (Reject) frame. The remote node sends a REJ frame to instruct the sending station to begin retransmission of frames at the frame number indicated (in the N(R) value of the REJ frame). The receivercontinues to discard received frames until one is received with the expected sequence number.Upon receipt of a REJ (by the local node), the transmitter winds-back the sequence of frames pending transmission to the indicated frame. The transmitter then retransmits the requested frame followed by all successive I-frames. This is sometimes known as "wind back" of the transmitter.Factors affecting the efficiency of data transmission on satellite links.First a bit about Propagation Time ( t p )Light & radio waves travel through space at a velocity of 3x108 ms-1 .This is a propagation time of 3.3 ns per metre.Signals propagate down cables at about 0.67 times this velocity: ie about 2x108 ms-1. In practical terms this is a propagation time of 5ns per metre.We saw in Question 4 that a minimum frame size is 64 Bytes = 512bits. So a minimal frame occupies512 / 10,000,000 = 0.0152ms.At a speed of 5ns per metre, 512 bits will therefore occupy 10.24km of cable. Similary, a maximum frame of about 1.5kBytes (12k bits) will take 1.2msThe only time that an Ethernet controller can detect collisions on the wire is when it is in the transmit mode. When an Ethernet NIC has finished transmitting and switches to receive mode, the only thing it listens for is the 64 bit preamble that signals the start of a data frame. The minimum frame size in Ethernet is specified such that, based on the speed of propagation of electrical signals in copper cables, an Ethernet card is guaranteed to remain in transmit mode and therefore detecting collisions long enough for a collision to propagate back to it from the farthest point on the wire from it.Take, for example, a length of 10base5 thick Ethernet cabling exactly 500 meters long (the maximum that the spec allows) with two stations, Station A and Station B, attached to the farthest ends of it. If Station A begins to transmit, it will have transmitted 25 bits by the time the signal reaches Station B, 500 meters away. If Station B begins to transmit at the last possible instant before Station A's signal reaches it, the collision will reach Station A 25 bit-times later (the time it takes for the signal on the wire to travel one bit-length -- 20 metres in cable). Station A will have transmitted only 50 bits when the collision reaches it – which is well within the 64bit preamble period. The collision will, therefore be correctly detected.Meanwhile, back at efficiency …A data link (whether cable or a satellite link) is most efficiently used when it is always occupied by data but avoiding collisions. A satellite link that is not actively relaying data is costing money. Satellites were chosen for this example because they are 35,800 km above the Earth’s surface and thus the propagation time is 0.12s or 0.24s for the round-trip of earth back to earth. Assuming that an =ARQ protocol is used it means that one packet of data cannot be sent until the first is acknowledged – a delay of at least two round- trips or 0.48s (480ms) . If a packet is the minimal length of 512 bits (0.0152ms) then the link is only actually transmitting data for 0.0152ms every packet exchange.In other words the time for a packet is 0.0152ms and the time before another can be sent is 480 + 0.0152, so the occupancy is 0.0152 / (480 + 0.0152) = 3.17 x 10-5 or 0.00317% !!General efficiency equation:U = t F / (t F + 2t P) x 100%Where U is the efficiencyt F is the frame time lengtht P is the A-to-B propagation timeLet’s do an efficiency calculation again, this time for 500 metres of cable and a 0.0152ms data packet where we are using ARQ.t F = 0.015ms, t P = 500m / 2x108 ms-1 = 0.0025msSo U =0.0015 / (0.0015 + 2x0.0025)= 0.0015 / 0.0065= 0.230723.07%Conclusions:It seems that the longer the path length, the lower the efficiency when using an ARQ system. With satellite links, it seems that ARQ is not the system to use!What are some likely internet connection routes?ISP = Internet Service Provider (BT, AOL, NTL etc)In the earlier days of internet availability to home users, it is true that your ISP would have had thousands of phone lines, from the Public Switched Telephone Network, PSTN, coming into their premises, each of which would be connected to an individual modem. The RS232 output from the modems would then be connected to a computer system and thence to servers for e-mail, internet and other online content.Nowadays, third-party network providers, like BT, provide local “Points of Presence” – POPs – which are the large rooms containing thousands of modems. Whatever your ISP company, you are likely to use the same POP as other company’s customers, but with a different dial-up number. POPs are run bycompanies called NSPs – Network Service Providers. The modem outputs at the POP are fed to a router which decides which ISP network you belong to and then forwards your network connection to a Network Access Point (NAP) via a high-capacity data link. This then connects you to your ISP’s network, which contains their online content and e-mail servers as well as a Gateway through to the Internet. Many ISPs are connected at more than one point to one or more NSPs. The additional connections improve resilience to failuresNote: A T3 link is a 45 Mbps backbone.Backbones are typically fibre optic trunk lines. The trunk line has multiple fibre optic cables combined together to increase the capacity. Fibre optic cables are designated OC for optical carrier, such as OC-3, OC-12 or OC-48. An OC-3 line is capable of transmitting 155 Mbps while an OC-48 can transmit 2,488 Mbps (2.488 Gbps). Today there are many companies that operate their own high-capacity backbones, and all of them interconnect at various NAPs around the world. In this way, everyone on the Internet, no matter where they are and what company they use, is able to talk to everyone else on the planet. The entire Internet is a gigantic, sprawling agreement between companies to intercommunicate freely.Protocol of the InternetEvery machine on the Internet has a unique identifying number, called an IP Address. The IP stands for Internet Protocol, and a typical IP address looks like this: 216.27.61.137The four numbers in an IP address are called octets, because they each have eight positions when viewed in binary form. If you add all the positions together, you get 32, which is why IP addresses are considered 32-bit numbers. The total number of possible combinations per octet is 28 or 256. Combine the four octets and you get 232 or a possible 4,294,967,296 unique values!Out of the almost 4.3 billion possible combinations, certain values are restricted from use as typical IP addresses. For example, the IP address 0.0.0.0 is reserved for the default network and the address 255.255.255.255 is used for broadcasts. The octets serve a purpose other than simply separating the numbers. They are used to create classes of IP addresses that can be assigned to a particular business, government or other entity based on size and need. The octets are split into two sections: Net and Host. The Net section always contains the first octet. It is used to identify the network that a computer belongs to. Host (sometimes referred to as Node) identifies the actual computer on the network. The Host section always contains the last octet. There are five IP classes plus certain special addresses.In 1983, the University of Wisconsin created the Domain Name System (DNS), which maps text names to IP addresses automatically. This way you only need to remember for example, instead of the IP address.Servers and those with permanent connections to the internet have fixed – or static IP addresses – but when dial-up connections are made, the IP address is assigned dynamically – so any one user may never have the same IP address twice. This helps to conserve the limited number of IP addresses – with the limitation of 4.3 billion.There are already an estimated 600-800 million Internet users worldwide. If this doubles by the year 2005 it will still only account for 16% of the world’s population.。