qr code generator vb net open source Figure 1319 Fast Ethernet topology in Software

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Figure 1319 Fast Ethernet topology
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Fast Ethernet implementation at the physical layer can be categorized as either two-wire or four-wire The two-wire implementation can be either category 5 UTP (lOOBase-TX) or fiber-optic cable (lOOBase-FX) The four-wire implementation is designed only for category 3 UTP (l00Base-T4) See Figure 1320
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Fast Ethernet implementations
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Two wires category 5 UTP
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SECTION 134
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Encoding
Manchester encoding needs a 200-Mbaud bandwidth for a data rate of 100 Mbps, which makes it unsuitable for a medium such as twisted-pair cable For this reason, the Fast Ethernet designers sought some alternative encoding/decoding scheme However, it was found that one scheme would not perform equally well for all three implementations Therefore, three different encoding schemes were chosen (see Figure 1321)
Encoding for Fast Ethernet implementation
100Base-TX 100Base-FX 4 x 25 Mbps 4x 25 Mbps 4 x 25 Mbps
4 x 25 Mbps
Station
Two UTP category 5
Station
Two fibers
100Base-T4 100 Mbps 100 Mbps
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Station
4 category 3 UTP
lOOBase-TX uses two pairs oftwisted-pair cable (either category 5 UTP or STP)
For this implementation, the MLT-3 scheme was selected since it has good bandwidth performance (see 4) However, since MLT-3 is not a self-synchronous line coding scheme, 4B/5B block coding is used to provide bit synchronization by preventing the occurrence of a long sequence of Os and Is (see 4) This creates a data rate of 125 Mbps, which is fed into MLT-3 for encoding lOOBase-FX uses two pairs of fiber-optic cables Optical fiber can easily handle high bandwidth requirements by using simple encoding schemes The designers of 100Base-FX selected the NRZ-I encoding scheme (see 4) for this implementation However, NRZ-I has a bit synchronization problem for long sequences of Os (or Is, based on the encoding), as we saw in 4 To overcome this problem, the designers used 4B/5B
WIRED LANs: ETHERNET
block encoding as we described for IOOBase-TX The block encoding increases the bit rate from 100 to 125 Mbps, which can easily be handled by fiber-optic cable A 1OOBase-TX network can provide a data rate of 100 Mbps, but it requires the use of category 5 UTP or STP cable This is not cost-efficient for buildings that have already been wired for voice-grade twisted-pair (category 3) A new standard, called lOOBase-T4, was designed to use category 3 or higher UTP The implementation uses four pairs of UTP for transmitting 100 Mbps Encoding/decoding in 100Base-T4 is more complicated As this implementation uses category 3 UTP, each twisted-pair cannot easily handle more than 25 Mbaud In this design, one pair switches between sending and receiving Three pairs of UTP category 3, however, can handle only 75 Mbaud (25 Mbaud) each We need to use an encoding scheme that converts 100 Mbps to a 75 Mbaud signal As we saw in 4, 8B/6T satisfies this requirement In 8B/6T, eight data elements are encoded as six signal elements This means that 100 Mbps uses only (6/8) x 100 Mbps, or 75 Mbaud
Summary
Table 132 is a summary of the Fast Ethernet implementations Table 132 Summary of Fast Ethernet implementations
Characteristics lOOBase-TX lOOBase-FX 100Base-T4
Media Number of wires Maximum length Block encoding Line encoding
Cat 5 UTP or STP
Fiber
Cat 4 UTP 4 100m
100m 4B/5B MLT-3
100m 4B/5B
NRZ-I
8B/6T
GIGABIT ETHERNET
The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps) The IEEE committee calls the Standard 8023z The goals of the Gigabit Ethernet design can be summarized as follows:
1 2 3 4 5 6
Upgrade the data rate to 1 Gbps Make it compatible with Standard or Fast Ethernet Use the same 48-bit address Use the same frame format Keep the same minimum and maximum frame lengths To support autonegotiation as defined in Fast Ethernet
MAC Sublayer
A main consideration in the evolution of Ethernet was to keep the MAC sublayer untouched However, to achieve a data rate 1 Gbps, this was no longer possible Gigabit Ethernet has two distinctive approaches for medium access: half-duplex and full-duplex
SECTION 135
GIGABIT ETHERNET
Almost all implementations of Gigabit Ethernet follow the full-duplex approach However, we briefly discuss the half-duplex approach to show that Gigabit Ethernet can be compatible with the previous generations Full-Duplex Mode In full-duplex mode, there is a central switch connected to all computers or other switches In this mode, each switch has buffers for each input port in which data are stored until they are transmitted There is no collision in this mode, as we discussed before This means that CSMAlCD is not used Lack of collision implies that the maximum length of the cable is determined by the signal attenuation in the cable, not by the collision detection process
In the full-duplex mode of Gigabit Ethernet, there is no collision; the maximum length of the cable is determined by the signal attenuation in the cable
Half-Duplex Mode Gigabit Ethernet can also be used in half-duplex mode, although it is rare In this case, a switch can be replaced by a hub, which acts as the common cable in which a collision might occur The half-duplex approach uses CSMAlCD However, as we saw before, the maximum length of the network in this approach is totally dependent on the minimum frame size Three methods have been defined: traditional, carrier extension, and frame bursting Traditional In the traditional approach, we keep the minimum length of the frame as in traditional Ethernet (512 bits) However, because the length of a bit is 11100 shorter in Gigabit Ethernet than in lO-Mbps Ethernet, the slot time for Gigabit Ethernet is 512 bits x 111000 JlS, which is equal to 0512 JlS The reduced slot time means that collision is detected 100 times earlier This means that the maximum length of the network is 25 m This length may be suitable if all the stations are in one room, but it may not even be long enough to connect the computers in one single office Carrier Extension To allow for a longer network, we increase the minimum frame length The carrier extension approach defines the minimum length of a frame as 512 bytes (4096 bits) This means that the minimum length is 8 times longer This method forces a station to add extension bits (padding) to any frame that is less than 4096 bits In this way, the maximum length of the network can be increased 8 times to a length of 200 m This allows a length of 100 m from the hub to the station Frame Bursting Carrier extension is very inefficient if we have a series of short frames to send; each frame carries redundant data To improve efficiency, frame bursting was proposed Instead of adding an extension to each frame, multiple frames are sent However, to make these multiple frames look like one frame, padding is added between the frames (the same as that used for the carrier extension method) so that the channel is not idle In other words, the method deceives other stations into thinking that a very large frame has been transmitted
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