1 Ethernet Basics Chapter 5
2 Contents Describe the concept of EthernetDefine Ethernet cabling systems Explain the function of repeaters and bridges
3 Introduction
4 Ethernet Xerox created a network technology called Ethernet in 1973Digital Equipment Corporation (DEC) and Intel joined Xerox in 1979 to promote Ethernet as an industry standard – called DIX Ethernet Bus topology, coaxial cable, 10 Mbps IEEE controls the Ethernet standard today through its Ethernet committee Tech Tip Check out the details of Ethernet on the Internet at
5 Family of TechnologiesEthernet today is a family of technologies that share a bus topology, frame type, and network access method Tech Tip Defining Ethernet Providing a clear and concise definition of Ethernet provides one of the major challenges in teaching networking. The difficulty comes because nobody else has a good definition and Ethernet has changed over the years to incorporate new and improved technology. Here’s a stab at a current definition. Ethernet is a standard for a family of network technologies that share the same basic bus topology, frame type, and network access method. Because the technologies share these essential components, you can communicate between them just fine. The implementation of the network might be different, but the frames remain the same.
6 How Ethernet Works
7 How Ethernet Works Uses MAC addresses to identify computers on the network Uses CSMA/CD to determine which machine should access the wire at any given time
8 Physical Bus The first generations of Ethernet used both a physical and logical bus topology Each computer connected to a single coaxial cable Called the cable, segment, or bus
9 Ethernet Frames Data transmitted between computers is broken up into smaller pieces called frames Prevents any machine from monopolizing the shared bus cable Makes the process of retransmitting lost date more efficient Exam Tip The terms frame and packet are often used interchangeably, especially on exams! However, frames are based on MAC addresses; packets are generally associated with data assembled by the IP protocol at Layer 3 of the OSI Seven-layer model.
10 Ethernet Frames Preamble (64 bits) MAC addresses (48 bits each) LengthData (up to 1500 bytes) Pad (frame must carry at least 64 bytes of data) Frame Check Sequence (CRC) Exam Tip There are many situations where one computer might have two or more NICs so one system might have more than one node!
11 MAC Addresses All computers, or nodes, on the network examine every frame to see if it contains their MAC address If it does, the machine opens the frame and begins processing the data If it does not, the machine ignores the frame Any device connected to the network can potentially capture any data frame transmitted across the wire. Network diagnostic programs called sniffers can order a NIC to run in promiscuous mode. This causes the NIC to processes all the frames regardless of the MAC address. This may lead to security issues. Software programs, such as AntiSniff, may help administrators detect when sniffers are in use.
12 Frame Check Sequence Uses a cyclic redundancy check (CRC) to detect when data has been damaged in transit The sending station runs an algorithm on the frame to come up with a number – the CRC value The receiving station runs the same algorithm and should come up with the same value – if it doesn’t, the frame has been damaged
13 CSMA / CD CSMA/CD Carrier SenseEach node on the network examines the wire before sending a frame. If the node detects traffic, it will back off a random amount of time and try again. Multiple Access All machines have equal access to the wire. Access to the wire is on a first-come, first-serve basis. Collision Detection If two nodes transmit at the same time, a collision results. NICs may listen to detect a collision.
14 Carrier Sense A node on a network listens for traffic before it sends out a data frame.
15 Collisions Two nodes may listen to the wire at about the same time, detect that there is no traffic, and begin sending a frame The two signals may then collide causing a collision Collisions corrupt the signal
16 Collisions Ethernet nodes may detect a collision by seeing a spike in the voltage level on the wire Since we’re on the topic of collisions, a commonly used term in the Ethernet world is collision domain. A collision domain is a group of nodes that hear each other’s traffic. A segment is certainly a collision domain, but there are ways to connect segments together to create larger collision domains. If the collision domain gets too large, you’ll start running into traffic problems that manifest as general network sluggishness. That’s one of the reasons to break up networks into smaller groupings.
17 Collisions Following a collision, each node waits a random number of seconds and tries again
18 A Free-For-All! CSMA/CD will result in more collisions as the network gets busier Every node may attempt to access the wire at any time – sort of a “free-for-all” As the network gets busier, network performance degrades as more collisions are likely to occur An maximum of 10% collisions is generally acceptable
19 Reflection As a signal reaches the end of a cable, some of the energy reflects off the end of the wire and travels back up the wire
20 Reflection These reflections may look like a busy signal to the computers attached to the network
21 Termination A terminating resistor, or terminator, is used to absorb signal reflections at the end of the wire Both ends of the network bus cable must be properly terminated
22 Cable Breaks Cable breaks (or bad connections) on a CSMA/CD Ethernet network result in the network going down The bus cable to which computers on an Ethernet network connect is called a segment.
23 Cable Breaks
24 Ethernet Cabling Systems
25 Thicknet or 10Base5 RG-8 coaxial cableCalled Thicknet, yellow cable, or frozen yellow garden hose Usually yellow outer jacket Used for 10 Mbps Ethernet
26 Cables may not be longer than 500 meters10Base5 10Base5 Speed 10 Mbps Signal Type Baseband A single signal on the cable Distance Cables may not be longer than 500 meters
27 Baseband vs. Broadband Baseband BroadbandA single signal or channel on the cable Transceivers that transmit and receive signals on the cable need only distinguish between a one, zero, or idle Broadband Multiple signals on the same cable Each signal or channel is at a different frequency Uses frequency division multiplexing Used for cable TV Cable modems are the lone common devices that use broadband signaling.
28 Distance Limitations 10Base5 segments cannot be longer than 500 meters from end to end A segment is the single length of cable to which computers on an Ethernet network connect
29 AUI Connector 10Base5 nodes use a NIC with a 15-pin female DB connector, or AUI connector, to connect to an external transceiver
30 Distances Black bands are spaced every 2.5 metersExternal transceivers must be placed exactly on one of those bands The cable between the NIC and the transceiver may be up to 50 meters long A maximum of 100 nodes on a segment The fact that you must place a drop on one of the 2.5 meter intervals does not mean you must put a drop at every 2.5 meter interval.
31 10Base5 transceiver connected to a NIC drop cable10Base5 Cabling 10Base5 uses drop cables to connect NICs to the segment, typically installed in the ceiling 10Base5 transceiver connected to a NIC drop cable
32 10Base5 Summary Speed: 10 Mbps Signal type: BasebandDistance: 500 meters/segment No more than 100 nodes per segment Nodes must be spaced at 2.5 meter intervals 10Base5 is an historical technology that is rarely, if ever, seen today. But you need to know about it for the Network+ exam.
33 10Base5 Summary Cables marked with a black band every 2.5 meters to ease installation The thick coaxial cable used for 10Base5 is almost always yellow (although nothing in the standard requires that color) Expensive cost per foot compared to other cabling systems Known as Thick Ethernet or Thicknet
34 Cables may not be longer than 185 meters10Base2 10Base2 Speed 10 Mbps Signal Type Baseband A single signal on the cable Distance Cables may not be longer than 185 meters Exam Tip Be sure to know the distance limitation is 185 meters (not 200 meters).
35 10Base2 Uses RG-58 coaxial cable with BNC connectorsAlso called Thinnet Costs less than Thicknet (10Base5) Computers may be spaced at any interval greater than 0.5 meters Limited to 30 computers per segment
36 10Base2 Connectors Use built-in transceivers Uses BNC connectorsBNC connectors keep the center wire and the shield from touching
37 Poorly crimped cable causing a short circuitCrimping BNC connectors are attached to the cable using a crimping tool that bends the metal of the connector around the wire Care must be taken not to let the inner wire and the outer shielding to touch, which would cause a short circuit Tech Tip If you find yourself in the (increasingly rare) position where you need a custom 10Base2 cable but have no crimping tool, all is not lost! For a quick connection on an RG58 cable, you can dispense with the whole retro crimping scene and get a modern twist-on BNC connector. They install in seconds and require no tools other than a pair of hands. What’s in a name? The origins of the initials BNC have been lost. Some of the possible things it could stand for include Bayonet Nut Connector, Bayonet Navy Connector, British Naval Connector, Bayonet Neil Cofflin (purported inventor), and according to a long-time manufacturer of the devices, BNC stands for Bayonet Nut Coupling. My advice: If you can recognize a BNC connector, know what it’s for, and know how to use one, then you don’t need to worry about the initials! Poorly crimped cable causing a short circuit
38 T-Connectors T-connectors are used in 10Base2 networks as shown
39 Terminators The figure shows a terminator as required at each end of the cable All BNC connectors are locked into place by turning the locking ring Unlocked Locked Terminator
40 BNC Connectors & NICs BNC connectors should never be attached directly to the NIC Try This Simulating 10Base2 Cable Break or Termination Malfunction If you have access to a functioning 10Base2 network, you can apply what you learned in the first part of this chapter, on the importance of termination and the effects of improper termination. Pull the terminating resistor off the end of the 10Base2 network cable. What happens when you try to access the network on the machine on the end of the cable? What about other machines on the network?
41 10Base2 Summary Speed: 10 Mbps Signal type: BasebandDistance: 185 meters/segment No more than 30 nodes per segment Nodes must be spaced at least 0.5 meters apart Tech Tip Distances in Meters and Feet Although the network standards for Ethernet cabling lengths are commonly written in meters, I find that the distances make a lot more sense to most American students when they have the Standard English equivalents at hand. So here you go: 185 meters = approximately 607 feet 500 meters = approximately 1640 feet 1000 meters = approximately 6/10ths of a mile
42 10Base2 Summary RG-58 coaxial cable with BNC connectors connect to T connectors on each node The nodes on the ends of the bus must have a terminator installed on one side of the T connector Inexpensive cost per foot compared to 10Base5 Known as Thin Ethernet, Thinnet, and sometimes Cheapernet
43 Extending the Network Repeaters and Bridges
44 Extending the Network 10Base2 and 10Base5 have limitations in distance, number of computers, fault tolerance, and cannot be combined Special devices called repeaters and bridges may be used in the network to address limitations
45 Repeater A repeater is a device that takes frames it receives from one Ethernet segment and retransmits (or repeats) them on another segment Amplifiers boost signals – including the flaws and noise Repeaters regenerate the signals Repeaters only operate at layer 1 of the OSI model, the Physical layer.
46 Repeater Benefits Extend the distance that a network can coverFault tolerance by limiting the impact of cable breaks to the segment Link segments of different Ethernet types
47 Extending the DistanceTwo 10Base2 segments connected by a repeater can cover 370 meters (2x185)
48 Fault Tolerance Fault tolerance is the ability of the network to continue functioning even after some part of the system has failed Cable breaks affect only the segment on which they occur
49 Connecting 10Base5 & 10Base2 A repeater can connect Ethernet segments that use different types of cabling and connectors
50 Collision Domains Repeaters repeat every frame they hear, so CSMA/CD applies to the entire network Repeaters extend the part of the network where collisions may occur Repeaters extend the size of a collision domain
51 Repeater Summary Repeaters increase total network cable distanceRepeaters provide a measure of fault tolerance Repeaters enable interoperability between different Ethernet cabling systems Repeaters operate only at the Physical layer (Layer 1) of the OSI model. Repeaters do not help reduce or manage network traffic, but their other attributes make them important tools for network technicians and architects.
52 Bridges Bridges are devices that link together Ethernet segments to form larger networks Bridges, unlike repeaters, can filter traffic between segments based on MAC addresses To filter means to stop it from crossing over to the other segment Bridges may be stand-alone devices that look similar to a hub or repeater, a PC with two NICs running special bridging software, or part of a device that performs functions in addition to bridging.
53 How Bridges Function A bridge monitors and records network traffic – making it more intelligent than a repeater When a bridge receives a frame, it records the MAC address of the source device Eventually the bridge learns the location of each device on the network Using that knowledge it can decide whether a frame that arrives on a port needs to be forwarded to a different segment or dropped Bridges become more efficient as they become more knowledgeable about the network If a bridge does not know the location of a device, it forwards the frame to all segments
54 Bridge Table Bridges keep track of the MAC addresses of devices in a table called the bridge table that tracks the device name, MAC address, and segment Bridges copy the frame exactly to put it on another segment Machines on both sides of the bridge are totally unaware of the bridge Terminology Alert! To be absolutely precise, the type of bridging described here is transparent bridging. Some documentation, especially documentation that deals with networking theory, will refer to translational bridges, which can translate between different frame formats. Translational bridges rarely, if ever, appear in Ethernet or Token Ring networks. You can assume the term bridge refers to the transparent type unless specifically told otherwise.
55 Benefits and LimitationsBridges preserve bandwidth by breaking larger networks up into smaller ones – reducing the size of collision domains Bridges cannot connect dissimilar networks or take advantage of multiple routes Bridges operate at the Data Link layer, Layer 2 of the OSI model. Why? Because they work with MAC addresses. Bridges also operate at layer 1, the Physical layer, by providing physical connections.
56 Bridge Summary Bridges filter or forward traffic based on the MAC addresses contained in each data frame. Bridges operate at the Data Link layer of the OSI model. Bridges can only connect two networks using the same type of data frames (for example, Ethernet to Ethernet, or Token Ring to Token Ring). Bridges learn the MAC addresses of machines on each network by listening to the cable. Bridges cannot be used to provide multiple routes between machines.
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