Commu Communica nications tions and and Netw Networ orking - - PowerPoint PPT Presentation

Business Da Business Data ta Commu Communica nications tions and and Netw Networ

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Abdullah Alfarrarjeh

Most of the slides in this lecture are either from or adapted from the slides provided by Dr. Hussein Alzoubi

▪ Understand the role of the data link layer ▪ Become familiar with two basic approaches to controlling access to the media ▪ Become familiar with common sources of error and their prevention ▪ Understand three common error detection and correction methods ▪ Become familiar with several commonly used data link protocols

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▪ The data link layer is often divided into two sublayers.

▪ The first sublayer (called the logical link control [LLC] sublayer) ▪ The second sublayer (called the media access control [MAC] sublayer) controls the physical hardware.

▪ A data link protocol performs three functions:

▪ Controls when computers transmit (media access control) ▪ Detects and corrects transmission errors (error control) ▪ Identifies the start and end of a message by using a PDU (message delineation)

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▪ Media access control refers to the need to control when computers transmit. ▪ Media access control becomes important when several computers share the same communication circuit, such as

▪ a point-to-point configuration with a half-duplex configuration, or ▪ a multipoint configuration

▪ There are two fundamental approaches to media access control:

▪ contention ▪ controlled access.

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4.2.1 Contention

▪ With contention, computers wait until the circuit is free (i.e., no other computers are transmitting)

▪ and then transmit whenever they have data to send. ▪ There must be some technique to continue the conversation after a collision occurs.

▪ Contention is commonly used in Ethernet— Local Area Networks (LANs).

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4.2.2 Controlled Access

▪ With controlled access, one device controls the circuit and determines which clients can transmit at what time. ▪ There are two commonly used controlled access techniques:

• 1. Access request technique: client computers, that

want to transmit, send a request to transmit to the device that is controlling the circuit (e.g., the wireless access point).

▪ they use a contention technique to send an access request.

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2. Polling: periodically, the controlling device (e.g., a wireless access point) polls the client to see if it has data to send.

▪ Roll-call polling

▪ first polling client 1, then client 2, and so on, until all are polled. ▪ For example, one could increase the priority of client 1 by using a polling sequence such as 1, 2, 3, 1, 4, 5, 1, 6, 7, 1, 8, 9. ▪ Usually, a timer “times-out” the client after waiting several seconds without getting a response. ▪ If some sort of fail-safe time-out is not used, the circuit poll might lock up indefinitely on an out-of-service client.

▪ Hub polling (often called token passing): one device starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so on, until it reaches the first computer, which restarts the process again.

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4.2.2 Controlled Access (Cont.)

Remember

Media Access Control Sub- layer Contention Controlled Access Access Request Polling Roll-call Polling Hub Polling

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4.2.3 Relative Performance (contention vs. controlled access)

▪ The key consideration is throughput—

▪ which approach will permit the most amount of user data to be transmitted through the network.

▪ In general, contention approaches work better than controlled approaches for small networks that have low usage.

▪ Because usage is low, there is little chance of a collision.

▪ Also, controlled approaches work better than contention approaches for large networks that have high usage. ▪ Collisions are very costly in terms of throughput because they waste circuit capacity during the collision and require both computers to re-transmit later.

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4.2.3 Relative Performance (contention vs. controlled access)

• Most experts believe that the crossover point is often around 20 computers
• (lower for busy computers, higher for less-busy computers).
• For this reason, when we build shared multipoint circuits like those often used in LANs
• r wireless LANs, we try to put no more than 20 computers on any one shared circuit.

▪ Human errors, such as a mistake in typing a number, usually are controlled through the application program. ▪ Network errors, such as those that occur during transmission, are controlled by the network hardware and software.

▪ There are two categories of network errors:

▪ corrupted data (i.e., data has been changed) ▪ lost data

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▪ Inter-Exchange Carriers (IXCs), that provide data transmission circuits, provide statistical measures specifying typical error rates and the pattern of errors that can be expected on the circuits they lease.

▪ For example, the error rate might be stated as 1 in 500,000,

▪ In a burst error, more than 1 data bit is changed by the error-causing condition. In other words, errors are not uniformly distributed in time.

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4.3.1 Sources of Errors

▪ Line noise and distortion

▪ noise is undesirable electrical signals

▪ (for fiber-optic cable, it is undesirable light)

▪ Noise is introduced by equipment or natural disturbances, and it degrades the performance of a communication circuit. ▪ Noise manifests itself as:

▪ extra bits, ▪ missing bits, ▪ or bits that have been “flipped”

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4.3.1 Sources of Errors

4.3.1 Sources of Errors

1. White noise

▪ Background hiss or static on radios and telephones. ▪ Not usually a problem unless being so strong that it obliterates the transmission. ▪ Caused by the thermal agitation of electrons. ▪ To prevent it, increase the signal-to-noise ratio.

2. Impulse noise

▪ Heard as a click or a crackling noise and can last as long as 1∕100 of a second. ▪ Not really affect voice communications, but it causes a burst error.

▪ At 1.5 Mbps, 15,000 bits would be changed by a spike of 1∕100 of a second.

▪ Caused by voltage changes in adjacent lines, lightning flashes during thunderstorms, fluorescent lights, and poor connections in circuits. ▪ To prevent it, shield the wires.

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4.3.1 Sources of Errors

3. Cross-talk

▪ During telephone calls, someone can hear other conversations in the background. ▪ Usually has a low signal strength that it is normally not bothersome. ▪ Occurs :

▪ when one circuit picks up signals in another ▪ between pairs of wires that are carrying separate signals, ▪ in multiplexed links carrying many discrete signals, ▪ In microwave links in which one antenna picks up a minute reflection from another antenna.

▪ Increased with :

▪ increased communication distance ▪ increased proximity of a two wires, ▪ increased signal strength, ▪ higher-frequency signals ▪ Wet or damp weather.

▪ To prevent it, shield the wires.

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4.3.1 Sources of Errors

4. Echoes

▪ Usually has a low signal strength that it is normally not bothersome.

▪ If the strength of the echo is strong enough to be detected, it causes errors.

▪ Occurs:

▪ when having poor connections that cause the signal to reflect back to the transmitting equipment. ▪ in fiber-optic cables when connections between cables are not properly aligned.

▪ To prevent it

▪ fix connection ▪ tune equipment

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4.3.1 Sources of Errors

• 5. Attenuation

▪ Attenuation is the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer. ▪ Some power is absorbed by the medium, or is lost before it reaches the receiver

▪ As the medium absorbs power, the signal becomes weaker, and the receiving equipment has less and less chance of correctly interpreting the data.

▪ The power loss affected by:

▪ transmission method ▪ circuit medium

▪ Increased with:

▪ High frequencies ▪ Decrease of the diameter of the wire

▪ To prevent it, use repeaters or amplifiers.

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4.3.1 Sources of Errors

6. Intermodulation noise (a special type of cross-talk)

▪ signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal (i.e., similar to harmonic music). ▪ Occurs when

▪ On a multiplexed line, many different signals are amplified together, ▪ Slight variations in the adjustment of the equipment can cause such noise ▪ A maladjusted modem may transmit a strong frequency tone when not transmitting data, thus producing this type of noise.

▪ To prevent it, shield the wires

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▪ In general, errors are more likely to occur in wireless, microwave, or satellite transmission than in transmission through cables.

▪ Therefore, error detection is more important when using radiated media than guided media.

▪ Impulse noise is the most frequent cause of errors in today’s networks.

▪ it could be very difficult to determine what caused this type of error.

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4.3.1 Sources of Errors

4.3.2 Error Prevention

1. Shielding

▪ Protecting wires by covering them with an insulating coating

▪ When increasing the shielding, the cable becomes more expensive and more difficult to be installed.

▪ One of the best ways to prevent impulse noise, cross-talk, and intermodulation noise.

2. Moving cables

▪ Locating cables away from sources of noise (especially power

sources)

▪ Reduces impulse noise, cross-talk, and intermodulation noise.

▪ For impulse noise:

▪ Avoiding lights and heavy machinery. ▪ Locating communication cables away from power cables.

▪ For cross-talk:

▪ Separating the cables physically from other communication cables.

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4.3.2 Error Prevention

• 3. Fixing multiplexing technique

▪ To prevent cross-talk and intermodulation noise.

▪ Changing multiplexing techniques (e.g., from FDM to TDM) ▪ Changing the frequencies or size of the guardbands in FDM.

• 4. Tuning the transmission equipment and

redoing the connections

▪ For many types of noise (e.g., echoes, white noise) ▪ Used to solve errors caused by

▪ poorly maintained equipment ▪ poor connections

▪ e.g., echo in fiber-optic cables

▪ splices among cables.

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4.3.2 Error Prevention

5. Using Repeaters or Amplifiers

▪ For telephone circuits, use repeaters or amplifiers spaced throughout their length. ▪ The distance between repeaters/amplifiers depends on the amount of power lost per unit length of the transmission line. ▪ Amplifiers:

▪ Commonly used on analog circuits ▪ An amplifier takes the incoming signal, increases its strength, and retransmits it on the next section of the circuit

▪ On analog circuits, the noise and distortion are also amplified, along with the signal. This means some noise from a previous circuit is regenerated and amplified each time the signal is amplified.

▪ Repeaters:

▪ Commonly used on digital circuits. ▪ A repeater receives the incoming signal, translates it into a digital message, and retransmits the message.

▪ Because the message is recreated at each repeater, noise and distortion from the previous circuit are not amplified ▪ This provides a much cleaner signal and results in a lower error rate for digital circuits.

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4.3.3 Error Detection

▪ The way to do error detection is to send extra data with each message. ▪ These error-detection data are added to each message by the data link layer of the sender on the basis of some mathematical calculations performed on the message. ▪ As the amount of error-detection data is increased, the throughput

• f useful data is reduced, because more of the available capacity is

used to transmit these error-detection data and less is used to transmit the actual message itself. ▪ Three well-known error-detection methods are:

▪ parity checking ▪ Checksum ▪ cyclic redundancy checking.

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4.3.3 Error Detection

▪ Parity Checking:

▪ One additional bit is added to each byte in the message

▪ Based on the number of 1s in each byte transmitted ▪ A parity bit is set to make the total number of 1s in the byte (including the parity bit) either an even number or an odd number

▪ odd parity or even parity

▪ The probability of detecting an error, given that one has occurred, is

• nly about 50%.

▪ can detect errors only when an odd number of bits have been switched

▪ Many networks today do not use parity because of its low error- detection rate.

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▪ Checksum

▪ With the checksum technique, a checksum (typically 1 byte) is added to the end of the message. ▪ [Sender Side] The checksum is calculated by

1. adding the decimal value of each character in the message 2. dividing the sum by 255 3. using the remainder as the checksum.

▪ [Receiver Side] The receiver calculates its own checksum in the same way and compares it with the transmitted

• checksum. If the two values are equal, the message is

presumed to contain no errors. ▪ Use of checksum detects close to 95% of the errors for multiple-bit burst errors.

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4.3.3 Error Detection

▪ Cyclic Redundancy Check

▪ It adds 8, 16, 24, or 32 bits to the message. ▪ [Sender Side]

▪ A message is treated as one long binary number, which is divided by a preset number, and the remainder is used as the CRC code. ▪ The preset number is chosen so that the remainder will be either 8 bits, 16 bits, 24 bits, or 32 bits.

▪ [Receiver Side] The receiving divides the received message by the same preset number, which generates a remainder. The receiving checks if the received CRC matches the locally generated remainder. If it does not, the message is assumed to be in error. ▪ CRC-16 will detect about 99.998% of all burst errors longer than 16 bits. ▪ CRC-32 will detect about 99.99999998% of all burst errors longer than 32 bits.

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4.3.3 Error Detection

4.3.4 Error Correction via Retransmission

▪ The simplest, most effective, least expensive, and most commonly used method for error correction is retransmission. ▪ Interestingly, transport layer (layer 4) is responsible for retransmission.

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4.3.5 Forward Error Correction

▪ Forward error correction uses codes containing sufficient redundancy to prevent errors

▪ by detecting and correcting them at the receiving end without retransmission of the original message. ▪ The redundancy, or extra bits required, varies with different schemes.

▪ It ranges from a small percentage of extra bits to 100% redundancy

▪ Forward error correction is commonly used in satellite transmission.

▪ Error rates can fluctuate depending on the condition of equipment, sunspots, or the weather. ▪ Compared with satellite equipment costs, the additional cost of forward error correction is insignificant.

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4.3.6 Error Control in Practice

▪ In the OSI model, error control is defined to be a layer-2 function. ▪ Most data link layer software used in LANs (i.e., Ethernet) is configured to detect errors, but not correct them.

▪ Any time a packet with an error is discovered, it is simply discarded.

▪ error correction is commonly done by the transport layer using continuous automatic repeat reQuest (ARQ).

▪ Wireless LANs and some Wide Area Networks (WANs), where errors are more likely, still perform both error detection and error correction.

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▪ The PDU at this layer is called a frame.

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• All layers except the physical layer create a new

Protocol Data Unit (PDU) as the message passes through them.

• The PDU contains information that is needed to

transmit the message through the network.

4.4.1 Asynchronous Transmission

▪ Asynchronous transmission is often referred to as start–stop transmission.

▪ It is typically used on point-to-point full-duplex circuits

▪ so media access control is not a concern.

▪ If you connect to a UNIX or Linux computer using Telnet, chances are you are using asynchronous transmission.

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4.4.1 Asynchronous Transmission

▪ Asynchronous transmission (Cont.)

▪ With asynchronous transmission, each character is transmitted independently of all other characters. ▪ To separate the characters and synchronize transmission, a start bit and a stop bit are put on the front and back of each individual character.

▪ For example, if we are using 7-bit ASCII with even parity, the total transmission is 10 bits for each character (1 start bit, 7 bits for the letter, 1 parity bit, 1 stop bit).

▪ The start bit and stop bit are the opposite of each other. Typically, the start bit is a 0 and the stop bit is a 1. ▪ There is no fixed distance between characters because the terminal transmits the character as soon as it is typed, which varies with the speed of the typist.

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4.4.2 Synchronous Transmission

▪ With synchronous transmission, all the letters or data in one group of data are transmitted at one time as a block of data. This block of data is called a frame. ▪ For example, a terminal or personal computer will save all the keystrokes typed by the user and transmit them only when the user presses a special “transmit” key. In this case, the start and end of the entire frame must be marked, not the start and end of each letter. ▪ Synchronous Transmission is often used on both point-to- point and multipoint circuits. ▪ For multipoint circuits, each frame must include a destination address and a source address, and media access control is important.

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4.4.2 Synchronous Transmission

▪ The start and end of each frame (synchronization) sometimes are established by adding synchronization characters (SYN) to the start of the frame.

▪ After the SYN characters, the transmitting computer sends a long stream of data that may contain thousands of bits.

▪ We discuss four commonly used synchronous data link protocols.

1. Synchronous Data Link Control (SDLC) 2. High-Level Data Link Control (HDLC) 3. Ethernet 4. Point-to-Point Protocol (PPP)

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Synchronous Data Link Control

▪ Synchronous Data Link Control (SDLC) is a mainframe protocol developed by IBM in 1972 that is still in use today.

▪ It uses a controlled-access media access protocol.

▪ If you use a 3270 protocol, you’re using SDLC.

▪ SDLC Frame Components:

▪ The Flag fields: e.g., 01111110 (a fixed pattern) – identifies the start and end

• f a frame

▪ The address field identifies the destination. ▪ The control field (the kind of frame that is being transmitted)

▪ An information frame ▪ A supervisory frame:

▪ ACKs and NAKs

▪ The frame check sequence field is a 32-bit CRC code ▪ The message field.

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High-Level Data Link Control

▪ High-level Data Link Control (HDLC) is a formal standard developed by ISO often used in WANs. ▪ HDLC is essentially the same as SDLC,

▪ except that the address and control fields can be longer.

▪ It uses a controlled-access media access protocol. ▪ A version of HDLC called Cisco HDLC (cHDLC)

▪ includes a network protocol field.

▪ cHDLC and HDLC have gradually replaced SDLC.

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Ethernet

▪ Ethernet is a very popular LAN protocol, conceived by Bob Metcalfe in 1973 and developed jointly by Digital, Intel, and Xerox in the 1970s.

▪ Since then, Ethernet has been further refined and developed into a formal standard called IEEE 802.3ac. ▪ There are several versions of Ethernet in use today. ▪ Ethernet uses a contention media access protocol. ▪ Preamble: a repeating pattern of ones and zeros (10101010)

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▪ The VLAN tag: The Ethernet frame uses this field

• nly when VLANs (virtual LAN, i.e., VPN) are in

use;

▪ otherwise, the field is omitted, and the length field immediately follows the source address field. ▪ When the VLAN tag field is in use, the first 2 bytes are set to the number 24,832 (hexadecimal 81-00), which is obviously an impossible packet length.

▪ The DSAP and SSAP are often used to indicate the type of network layer protocol the packet contains (e.g., TCP/IP or IPX/SPX)

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Ethernet

▪ The control field is used to hold the frame sequence numbers and ACKs and NAKs

▪ The last 2 bits in the first byte are used to indicate the type of control information being passed ▪ and whether the control field is 1 or 2 bytes

▪ (e.g., if the last 2 bits of the control field are 11, then the control field is 1 byte in length).

▪ In most cases, the control field is 1-byte long.

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Ethernet

▪ Ethernet II is another commonly used version of Ethernet.

▪ The type field is used to specify an ACK frame or the type of network layer packet the frame contains (e.g., IP). ▪ Ethernet II has an unusual way of marking the end of a frame.

▪ When the frame ends, the sending computer transmits no signal for 96 bits (i.e., neither a 0 or a 1). ▪ After these 96 bits have been on no signal, the sending computer then transmits the next frame, which starts with a preamble, and so on.

▪ It is possible that in the time that the computer is sending no signal, some other computer could jump in and begin transmitting.

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Ethernet

▪ Newer versions of these two types of Ethernet permit jumbo frames

▪ with up to 9,000 bytes of user data in the message field.

▪ Some vendors are experimenting with super jumbo frames

▪ That can hold up to 64,000 bytes.

▪ Jumbo frames are common for some types of Ethernet such as gigabit Ethernet (see Chapter 6).

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Ethernet

Point-to-Point Protocol

▪ Point-to-Point Protocol (PPP) was developed in the early 1990s and is often used in WANs.

▪ It is designed to transfer data over a point-to-point circuit but provides an address so that it can be used

• n multipoint circuits.

▪ The control field is typically not used. ▪ The protocol field indicates what type of data packet the frame contains (e.g., an IP packet). ▪ The data field is variable in length and may be up to 1,500 bytes.

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▪ Transmission efficiency is defined as the total number of information bits (i.e., bits in the message sent by the user) divided by the total bits in transmission (i.e., information bits plus overhead bits).

▪ For example, with asynchronous transmission, only 70% of the data rate is available for the user;

▪ 30%is used by the transmission protocol. ▪ If we have a communication circuit using a dial-up modem receiving 56 Kbps, the user sees an effective data rate (or throughput) of 39.2 Kbps.

▪ This is very inefficient.

▪ With SDLC: if the message portion of the frame contains 100 information characters and we are using 8-bit code. Then there are 100*8 = 800 bits of

• information. The total number of bits is the 800 information bits plus the
• verhead bits that are inserted for delineation and error control. SDLC has

a beginning flag (8 bits), an address (8 bits), a control field (8 bits), a frame check sequence (assume that we use a CRC-32 with 32 bits), and an ending flag (8 bits).This is a total of 64 overhead bits.

▪ efficiency is 800∕(800 + 64) = 92.6%. ▪ If the circuit provides a data rate of 56 Kbps, then the effective data rate available to the user is about 51.9 Kbps.

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▪ Synchronous networks are usually more efficient than asynchronous networks and that some protocols are more efficient than others. ▪ The longer the message (1,000 characters as

• pposed to 100), the more efficient the protocol.

▪ However, anytime a frame is received containing an error, the entire frame must be retransmitted. ▪ Furthermore, the probability that a frame contains an error increases with the size of the frame;

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▪ Thus, in designing a protocol, there is a trade-

• ff between large and small frames.

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▪ Throughput is the total number of information bits received per second,

▪ after taking into account the overhead bits and the need to retransmit frames containing errors.

▪ Generally speaking,

▪ small frames provide better throughput for circuits with more errors, ▪ whereas larger frames provide better throughput in less- error-prone networks.

▪ Fortunately, in most real networks, the curve shown in Figure 4-10 is very flat on top,

▪ meaning that there is a range of frame sizes that provide almost optimum performance.

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▪ Frame sizes vary greatly among different networks,

▪ But the ideal frame size tends to be between 2,000 and 10,000 bytes.

▪ So why are the standard sizes of Ethernet frames about 1,500 bytes?

▪ Because Ethernet was standardized many years ago, ▪ when errors were more common.

▪ Jumbo and super jumbo frame sizes emerged from higher speed, highly error-free fiber-optic networks.

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▪ The MAC address is assigned to a computer network interface card in a factory

▪ and is therefore hardcoded on the network interface card (NIC) and cannot be changed.

▪ MAC address filtering will create a list of MAC addresses that are allowed to connect to a Wi-Fi network or to a switch in corporate networks.

▪ This feature allows for some degree of security.

▪ However, MAC address filtering can offer a false sense of security because of MAC address spoofing.

▪ MAC address spoofing is a software-enabled technique that can change the hardcoded MAC address to any MAC address and thus overcome MAC address filtering.

▪ Keep in mind that while MAC address spoofing is not illegal, what you do with it may be.

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