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3 Chapter 4. Learning Objectives Understand the major components of LANsUnderstand shared Ethernet and switched Ethernet topologiesUnderstand Ethernet media access controlUnderstand Ethernet error controlBe familiar with Ethernet message delineationBe familiar with how data is transmitted through physical circuitsUnderstand the best practice recommendations for LAN design
4 Chapter 4. Outline Introduction Topology Media Access Control Shared Ethernet, Switched EthernetMedia Access ControlError ControlError Detection, Error CorrectionMessage DelineationCodes, Frame Layout, Frame SizeData Transmission in the Physical Layer10BaseT, 100BaseT and 100BaseF, 1GbE, 10GbEThe Best Practice LANEffective Data Rates, Costs, Recommendations
6 IntroductionOver 95% of LANs in use today use the Ethernet protocol standardized as IEEELANs generally have two hardware layers: the data link layer and the physical layer.The data link layer performs three basic functions: message delineation, medium access control and error control.Ethernet can detect and correct errors, but since errors are rare in modern LANs, error correction is uncommon.10BaseT and 100BaseT are example of physical layer Ethernet protocols (10 stands for 10 Mbps, T stands for twisted pair).
8 Topology Topology refers to the geometric layout of the network. A logical topology is how the network works conceptually, rather like the logical DFD in systems analysis.Physical topology refers to how the network is physically connected.Ethernet’s two forms shared and switched Ethernet, use bus and star logical topologies, respectively.
9 Shared Ethernet (Figure 4-1) In addition to the computers, shared Ethernet LANs contains three basic components:Network Interface CardsCablesHubs
11 Network Interface Cards Network interface cards, also called network cards and network adapters include a cable socket allowing computers to be connected to the network.NICs are part of both the physical and data link layer and include a unique data link layer address (sometimes called a MAC address), placed in them by their manufacturer.Before sending data onto the network, the network card also organizes data into frames and then sends them out on the network.Notebook computers often use NICs that are plugged into the PCMCIA port.
12 CablesEach computer is physically connected to the network using a cable.The cables used on Ethernet LANs are either twisted-pair or optical fiber cables.Data can flow through cables in one of three modes (Figure 4-2):One way only (simplex)Both ways, one way at a time (half-duplex)Both ways at the same time (full-duplex)
13 Figure 4-2 Simplex, half-duplex, and full-duplex transmissions
14 Hubs (Figure 4-3)Hubs act as junction boxes, linking cables from several computers on a network. Hubs are usually sold with 4, 8, 16 or 24 ports.Some hubs allow connection of more than one kind of cabling, such as UTP and coax.Hubs also repeat (reconstruct and strengthen) incoming signals. This is important since all signals become weaker with distance.The maximum LAN segment distance for a cable can therefore be extended using hubs.
16 Shared Ethernet Topology (Figure 4-4) Shared Ethernet’s logical topology is a bus topology.This means all computers on the network receive messages from all other computers, whether the message is intended for those computers or not.When a frame is received by a computer, the first task is to read the frame’s destination address to see if the message is meant for it or not.Ethernets today use a physical star topology, with the network’s computers linked into hubs.It is also common to link use multiple hubs to form more complex physical topologies, enabling the networks to span longer distances (Figure 4-5).
18 Figure 4-5 An example of an Ethernet local area network with two hubs
19 Switched Ethernet Topology Switched Ethernet uses switches instead of hubs.Switches that make switching decisions based on data link layer addresses are called workgroup or layer-2 switches.While a hub broadcasts frames to all ports, the switch reads the destination address of the frame and only sends it to the corresponding port.The effect is to turn the network into a group of point-to-point circuits and to change the logical topology of the network from a bus to a star (Figure 4-6).
21 Basic Switch Operation Switches have onboard chips that make forwarding decisions using forwarding tables (similar to routing tables).When a frame is received, the switch reads its [data link layer] destination address and sends the frame out the corresponding port in its forwarding table.When a switch is first turned on, its forwarding table is empty. Switches learn which ports correspond to which computers by reading the source addresses of the incoming frames and noting the port number the frame arrived on.If the switch’s forwarding table does not have the destination address of the frame, it broadcasts the frame to all ports.Thus, a switch starts by working like a hub and then works more and more as a switch as it fills its forwarding table.
22 Managing LAN traffic using switches Switch differ from hubs in two important ways.First, unlike a hub, which broadcasts incoming frames out all the ports of the hub, a switch only sends an incoming frames out the port corresponding to its destination computer. This greatly reduces network traffic and prevents most collisionsSecond, switches use a store-and-forward approach to managing LAN traffic. If two frames arrive at a switch at the same time, the second frame is temporarily stored in memory until the switch is done processing the first frame.
23 Figure 4-7 802.3 Ethernet versus switched Ethernet
25 Media Access ControlEthernet’s medium access control protocol, CSMA/CD, is contention-based.This means two devices can transmit at the same time. When they do, their frames collide, become garbled and need to be retransmitted.Stands for: Carrier Sense Multiple Access w/ Collision DetectCarrier Sense: devices listen to see if another the network is busy before transmitting.Multiple Access: multiple devices have network access.Collision Detect: if a collision is detected, the devices sending the frames wait a random amount of time before resending the frame (It has to be random in order to avoid another collision).
27 Error ControlData communications are prone to errors, such as noise spikes from power surges, crosstalk from wires being too close together, and echoes due to faulty connections.A common way to prevent errors is to use shielded cables.Error control combines the techniques for detecting errors with those for correcting them.
28 Error Detection with Ethernet Ethernet’s error detection scheme is called Cyclic Redundancy Check (CRC).CRC is computed by treating a message as one long binary number, P.Before transmission, the data link layer hardware divides P by a fixed binary number, G, resulting in a whole number, Q, and a remainder, R/G.The remainder, R, is written into the outgoing frame and then recalculated by the receiver.CRC-16 (99.998% effective for errors greater than 16 bits in length) and CRC-32 ( %for errors greater than 32 bits long) are in common use today.
29 Error Correction via Retransmission The default setting for Ethernet is to use error detection without error correction. Bad frames are simply discarded.When error correction is used, however, errors are corrected by retransmitting the frames found to be in error.The process of requesting a frame be resent is called an Automatic Repeat Request or ARQ.When error correction is turned on, Ethernet uses stop-and-wait ARQ.
30 Stop-and-Wait ARQ (Figure 4-8) With stop-and-wait ARQ, the sender first sends a packet, then waits to hear from the receiver.If the packet is received without error, an acknowledgement (ACK) is sent back by the receiver and the next packet is sent.If the receiver detects an error in the packet that was just sent, the receiver sends back a negative acknowledgement (NAK) and the sender resends the packet again.
33 CodingWritten languages use symbols, but computers send signals in 1s and 0s (bits).Each written character needs a bit code in order to be used by a computer. A set of these codes for a language is called a coding scheme.The mostly commonly used coding scheme is ASCII: the American Standard Code for Information Interchange, originally used a 7-bit code (27 = 128 combinations).An 8-bit version is now in use (an 8-bit coding scheme has 28 = 256 combinations).Ethernet usually uses 8-bit ASCII.
34 Frame Layout (Figure 4-9) The latest version of Ethernet frame format, defined by the IEEE 802.3ac standard, consists of four parts: preamble, MAC header, LLC protocol data unit and the MAC trailer.The frame begins with an 8-byte preamble. When the receiver sees the preamble’s bit pattern, it knows a frame is beginning. The 8th byte is the start of frame delimiter ( ).The last section of the frame is the 4-byte frame check sequence (FCS). This is where the CRC-32 value for the frame is stored.
35 MAC Header (Figure 4-9)The section following the preamble in the Ethernet frame is the MAC header.The first two fields of the MAC header are the 6-byte source and destination addresses.The last field is the 2-byte field length, which gives the length of the data field. Because Ethernet has variable length data fields, their length needs to be known. Without it, the receiver will not know where the frame ends.The third field is the 4-byte virtual LAN (VLAN) tag. When in use, the first two bytes are set to (an impossible field length) which tells the receiver that the VLAN tag field is being used.
36 LLC Protocol Data Unit (Fig. 4-9) The LLC protocol data unit follows the MAC header.The first two fields are the Destination Service Access Point (DSAP) and the Source Service Access Point (SSAP), which function like the port number in TCP; i.e., telling the internetwork layer which software to use to process the frame.The control field holds frame sequence numbers and ACKs and NAKs and is used for error control.The data field contains the message being transmitted. It is typically an IP packet. It must be at least 43 bytes long and must not be more than 1497 bytes in length.
39 Analog and Digital Data A fundamental physical layer distinction is between digital and analog forms of data.Computers produce digital data such a binary code which is either on or off or a zero or one.Telephones produce continuously varying electrical signals. This is an example of analog data.Most computers transmit data in digital form, but can convert it to analog form by using a modem.
40 Transmission ModesData can also be sent either in serial or in parallel modes:Parallel mode (Figure 4-10a): uses several wires, each wire sending one bit at the same time as the other wires in its cable.A parallel printer cable sends 8 bits together.Your computer’s processor and motherboard also use parallel busses to move data around.Serial Mode (Figure 4-10b): sends one bit after another over a single line. Serial mode is slower than parallel, but can be used over longer distances because the bits stay in the order they were sent, while bits sent in parallel mode tend to spread out over long distances.
41 Figure 4-10a Serial transmission of an 8 bit code Figure 4-10b Parallel transmission of an 8 bit code
42 Ethernet Physical Layer Standards The three most commonly used forms of Ethernet today are:10BaseT (10 = 10Mbps, T = twisted pair and base = baseband, meaning one channel)Fast Ethernet which includes 100BaseT and 100BaseF (F = fiber)Gigabit Ethernet which includes 1 Gigabit Ethernet (1 GbE) and 10 GbE (10 Gbps). Even faster versions are being developed.
43 The 10BaseT Ethernet Standard 10BaseT uses twisted-pair cable.By far the most common form today is using 8-wire, category 5 cables with RJ-45 connectors (similar to phone jacks but larger).Twisted pair cables can be either unshielded (UTP) or shielded (STP).Shielding reduces interference from outside sources that may cause transmission errors.The maximum possible cable length is 100 meters.
44 10BaseT Data Transmission (Figure 4-13) To transmit at 10Mbps, 10BaseT uses a 10 MHz signaling rate, sending a bit every 100 nanoseconds.To preserve the synchronization of the signal between sender and receiver, 10BaseT uses Manchester encoding in which the bit value is defined by a mid-bit transition.The data is not defined in terms of whether the signal is at a high or low voltage, but by whether the mid-bit transition goes from a high to a low value (binary 0) or from a low to a high value (defining a binary 1).Figure 4-13 shows a an example of Manchester encoding.
46 Fast Ethernet: 100BaseTThere are two forms of fast Ethernet, 100BaseT and 100BaseF. 100BaseT uses twisted-pair cable.Signals are sent using a combination of 4B5B coding and MLT-3 transmission.With 4B5B coding, five bits are sent together, four data bits followed by a fifth synchronization bit.Multi-Level Transmission 3 uses three levels of voltage to send these bits, +1, 0, and –1. Data is encoded by changing the voltage to the next level.To transmit a binary 1, the voltage is changed to an adjacent level, such as from –1 to 0 or from 0 to +1.
47 100BaseF100BaseF uses fiber optic cable. Light created by an LED (light-emitting diode) or laser is sent down a thin glass or plastic fiber.Fiber optic cable structure (from center):Core (v. small, 5-50 microns, ~ the size of a single hair)Cladding, that reflects the signal back into the coreProtective outer jacket
48 Fiber Optic Cable Types Types of Optical Fiber (Figure 4-14):Multimode (MMF) is cheap, but the signal spreads out over short distances (up to ~500m).Graded index MMF reduces the spreading problem by changing the refractive properties of the fiber to periodically refocus the signal. Can be used over distances of up to about 1000 meters.Single mode is expensive because it is more difficult to manufacture, but the signal can be sent over longer distances (up to 100 kilometers) without spreading.Three most commonly used fiber types for LANs are: 62.5/125 MMF, 50/125 MMF and 10/125 SMF, (the numbers are the core and cladding diameters, respectively).
49 100BaseF Signaling100BaseF uses a pair of 62.5/125 MMF fibers, one strand for sending, one for receiving.Almost always used with switched topologies.Maximum segment length (distance from computer to hub or switch) is 412 meters.Transmits using a bright light to indicate a 1 and a dim light to indicate a zero.This is called non-return to zero, since the signal never goes completely off.
51 Gigabit Ethernet (1GbE) Gigabit Ethernet is the newest family of Ethernet protocols running at speeds of 1 Gbps and above.Can use either twisted pair or fiber.1000BaseT uses cat-5 UTP, using all four wire pairs operating in parallel.Gigabit Ethernet uses the same 125 MHz clock as 100BaseT. It is able to achieve the 1Gbps data rate by sending data over four lines simultaneously. This enables data to be sent at a rate of 500Mbps.1000Mbps is achieved by sending 2 bits per interval. The technique used by 1000BaseT for this is called PAM-5.
52 PAM-5 encoding (Figure 4-15) 100BaseT encodes data using a technique called pulse amplitude modulation 5.PAM-5 uses 5 different voltage levels, four to send data and one as a control bit. The data bits are defined as follows:00 volts01 volts10 +0.5 volts11 +1.0 voltsBecause the NIC has to distinguish between finer differences between signals, 1GbE is more susceptible to noise. Many organizations opt to stick with 100BaseT instead.
54 Fiber-based Gigabit Ethernet 2 types of Gigabit Ethernet use fiber optic cables:1000BaseSX uses MMF and has a maximum segment length of 220 or 550 meters depending on the cable1000BaseLX which uses MMF or SMF which can have a segment length of up to 5 kilometers for SMF.Both use 8B10B encoding, meaning data is sent in 10 bit groups with two overhead bits.This data is sent at 1.25 GHz, resulting in a net 1 Gbps data rate.
55 10GbE A 10 gigabit version of Ethernet is now being developed. 10GbE now has two forms, one for LANs and one for WANs. Interconnect LAN/WAN networks will be possible using 10GbE.The LAN form of 10GbE runs over four parallel MMF or SMF fibers.Each fiber uses 8B10B coding at a clock speed of GHz, yielding x .8 x 4 = 10 Gbps.The WAN form operates using 64B66B (64 data bits + 2 overhead bits) on a GHz line, yielding a data rate of 9.95 Gbps.
57 Effective Data RatesThe effective data rate is the maximum practical speed hardware layers can expect to provide. It depends on 4 factors:nominal data rate (e.g., 10Mbps for 10BaseT)error rate, since this determines the frame retransmission rateefficiency of the data link protocol which depends on the percentage of the transmission devoted to overheadefficiency of the media access control protocol meaning how effective is the protocol at making use of the nominal data rate.
58 Data Link Protocol Efficiency Based on the amount of overhead per frame and so is the same for both shared and switched Ethernet.Since Ethernet frames have variable length, it also depends on the traffic being sent over the network.For small frames (~150 bytes) have an efficiency of about 82% while for large, 1,500 byte frames, it is high, about 98%.Typical network traffic combines Web pages with and usually means sending one small frame with many large frames in the response.For a small packet followed by 20 large packets, the average data link protocol efficiency would be 97%.
59 Media Access Control Protocol Efficiency Shared and switched Ethernet differ in their MAC protocols. Shared Ethernet is very sensitive to network traffic levels (see Figure 4-16).For traffic levels below 50% response time delays are minimalFor traffic levels between 50-80% of capacity, response time delays are significantAbove 80%, response time delays increase exponentially.Thus, shared Ethernet network capacity is effectively limited to just under 50% of capacityAt 97% data link protocol efficiency, 10BaseT can carry 0.97 x 0.5 x 10mbps = 4.85Mbps.
61 Eff. Data Rates for Shared Ethernet The 4.85 Mbps estimated in the previous slide is a shared capacity that must be divided amongst all of the active network users.In a low traffic environment with only two active users on a network, this would correspond to 2.5 Mbps/user (see Fig. 4-17)In moderate traffic, with 10 users, this would mean only 500 kbps/user.100BaseT with 10 active users provides about 7.5 Mbps/user.
62 Eff. Data Rates for Switched Ethernet Switched Ethernet dramatically improves performance since each computer appears to have its own dedicated circuit and collisions and congestion are no longer a problem.Experts believe switched Ethernet users can effective utilize 95% of network capacity.For a 10BaseT switched LAN, each computer would have an effective capacity = 0.97 x 0.95 x 10Mbps ~ 9 Mbps.For 100BaseT switched LAN, each computer would have an effective capacity = 0.97 x 0.95 x 100Mbps ~ 92Mbps.
63 Eff. Data Rates for Gigabit Ethernet Gigabit Ethernet is typically implemented in a full-duplex switched environment.This means it provides 1 Gbps in both directions at once.This makes the effective data rate for sending data in one direction is0.97 x 0.95 x 1 Gbps ~ 900 MbpsSince 1GbE is implemented as a full-duplex network, traffic can be sent simultaneously in both directions, so the effective capacity is really double this value or 1.8 Gbps.
64 Figure 4-17 Effective data rates for Ethernet 1. Effective Data Rate per UserTechnologyLowTrafficModerate TrafficHighShared 10BaseT2.5 Mbps1 Mbps500 kbpsShared 100BaseT37.5 Mbps15 Mbps7.5 MbpsSwitched 10BaseT9 MbpsSwitched 100BaseT90 MbpsFull Duplex 1 GbE1.8 GbpsFull Duplex 10 GbE18 GbpsAssumptions:Most frames are 1500 bytes of largerNo transmission errors occurLow traffic means 2 active users, moderate trafficmeans 5 active users, high traffic means 10 active usersFigure 4-17 Effective data rates for Ethernet
65 LAN RecommendationsSeveral best practice recommendations for LANs are shown in Figure 4-18.Since network traffic almost always increase, the best practice design is for the worst case.For most networks, switched 10BaseT is the best optionFor small networks, such as home networks, shared 10BaseT over cat-5 cable is best.For networks with high traffic levels, switched 100BaseT or 1 GbE over MMF is best.In most LANs, the circuit to and from the server is the network bottleneck.The solution for this is to use a 10/100 switch and then connect the server using a 100Mbps circuit.
66 Most networks: Switched 10Base-T Ethernet over category 5 cables Very small networks (e.g., home networks): traditional shared Ethernet over category 5 cables or category 3 cablesNetworks with high demands (e.g., multimedia networks): Switched 100Base-T Ethernet over category 5 cables or full duplex 1 GbE over multimode fiberFigure 4-18: Best practice LAN recommendations