Guide to Networking Essentials Fifth Edition Chapter 7 Network - PowerPoint PPT Presentation

Guide to Networking Essentials Fifth Edition Chapter 7 Network Architectures

Objectives • Compare and contrast media access methods used in network architectures • Describe the operation of Ethernet • Differentiate between Ethernet standards and speeds • Explain the four Ethernet frame types and how they are used Guide to Networking Essentials, Fifth Edition 2

Objectives (continued) • Describe the token ring architecture and its components • Describe the AppleTalk network architecture • Explain the function of Fiber Distributed Data Interface • Describe other LAN and WAN architectures and their role in today’s networks Guide to Networking Essentials, Fifth Edition 3

Putting Data on the Cable: Access Methods • Given that network architectures communicate in a number of different ways, some factors in network communications must be considered – How computers put data on the cable – How they ensure that the data reaches its destination undamaged Guide to Networking Essentials, Fifth Edition 4

Function of Access Methods • How computers share the network medium must be defined • A collision results from two or more devices trying to use the channel (medium) at the same time – Splitting data in small chunks helps prevent collisions • Channel access methods specify when computers can access the cable/medium/data channel – Ensure that data reaches destination by preventing computers from sending messages that might collide – Every computer on a network must use the same access method Guide to Networking Essentials, Fifth Edition 5

Major Access Methods • Channel access is handled at the MAC sublayer of the Data Link layer in the OSI model • Five major types of channel access – Contention – Switching – Token passing – Demand priority – Polling Guide to Networking Essentials, Fifth Edition 6

Contention • In early networks based on contention, computers sent data whenever they had data to send • As networks grow, outgoing messages collide more frequently, must be sent again, and then collide again • To organize contention-based networks, two carrier access methods were created – CSMA/CD – CSMA/CA Guide to Networking Essentials, Fifth Edition 7

Switching • Switching: nodes are interconnected through a a switch, which controls access to the media – Contention occurs only when multiple senders ask to reach the same receiver simultaneously or when the simultaneous transmission requests exceed the switch’s capability to handle multiple connections • Advantages: fairer, centralized management (enables QoS), switch can have connection ports that operate at different speeds • Disadvantage: higher cost Guide to Networking Essentials, Fifth Edition 8

Token Passing Guide to Networking Essentials, Fifth Edition 9

Token Passing Illustrations Guide to Networking Essentials, Fifth Edition 10

More Token Passing Illustrations Guide to Networking Essentials, Fifth Edition 11

Demand Priority • Demand priority: channel access method used solely by the 100VG-AnyLAN 100 Mbps Ethernet standard (IEEE 802.12) – 100VG-AnyLAN runs on a star bus topology – Intelligent hubs control access to the network •Hub searches all connections in a round-robin fashion •When an end node has data to send, it transmits a demand signal to the hub •The hub then sends an acknowledgement that the computer can start transmitting its data – The major disadvantage of demand priority is price Guide to Networking Essentials, Fifth Edition 12

Demand Priority Illustration • Graphic from – www.microsoft.com/mspress/books/sampchap/4077.aspx#SampleChapter • Hub receives two frames to transmit (to the next hub) • Higher-priority frame is sent first Guide to Networking Essentials, Fifth Edition 13

Polling Guide to Networking Essentials, Fifth Edition 14

Choosing an Access Method Guide to Networking Essentials, Fifth Edition 15

Choosing an Access Method (continued) Guide to Networking Essentials, Fifth Edition 16

The Ethernet Architecture • 1960s – 1970s: many organizations worked on methods to connect computers and share data – ALOHA network at the University of Hawaii – 1972: Metcalfe and Boggs develop an early version •1975: 1st commercial version (3 Mbps, up to 100 computers, max. 1 km of total cable) • DIX developed standard based on Xerox’s Ethernet (10 Mbps) • 1990: IEEE defined the 802.3 specification – Defines how Ethernet networks operate at layers 1-2 Guide to Networking Essentials, Fifth Edition 17

Overview of Ethernet • Ethernet is the most popular network architecture – Advantages: easy to install, scalable, broad media support, and low cost – Supported transmission speeds: 10 Mbps to 10 Gbps – Uses the NIC’s MAC address to address frames – Ethernet variations are compatible with one another •Basic operation and frame formatting is the same •Cabling, speed of transmission, and method by which bits are encoded on the medium differ Guide to Networking Essentials, Fifth Edition 18

Ethernet Operation • Ethernet is a best-effort delivery system – It works at the Data Link layer of the OSI model •Relies on the upper-layer protocols to ensure reliable delivery of data • Understanding the following concepts is important: – How Ethernet accesses network media – Collisions and collision domains – How Ethernet handles errors – Half-duplex and full-duplex communications Guide to Networking Essentials, Fifth Edition 19

Accessing Network Media • Ethernet uses CSMA/CD in a shared-media environment (a logical bus) – Ethernet device listens for a signal or carrier (carrier sense) on the medium first – If no signal is present, no other device is using the medium, so a frame can be sent – Ethernet devices have circuitry that detects collisions and automatically resends the frame that was involved in the collision Guide to Networking Essentials, Fifth Edition 20

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Guide to Networking Essentials, Fifth Edition 21

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) • CSMA: – This is the "Carrier-Sense Multiple Access" part – Listen for prior transmission •if none, go ahead and transmit •if medium in use, "back off" and try again later • worst case: two hosts listen at the same time; neither hears anything; both begin to transmit at the same time • CD – This is the "Collision Detection" part – Listen to your own transmission •if interference is detected, "back off" and try again later • "Exponential Backoff": – If a host backs off more than once ( e.g. repeated collisions), wait up to twice as long after each time Guide to Networking Essentials, Fifth Edition 22

CSMA/CD simulation • In this screen capture, transmissions from node 1 and node 4 have just collided. – Node 3 is about to detect the collision – Nodes 2 and 4 will detect it next – The Senders, nodes 1 and 5, will be the last to notice • Source: – www.informatik.uni- mannheim.de/pi4/animations/e thernet/index.html Guide to Networking Essentials, Fifth Edition 23

Collisions and Collision Domains Guide to Networking Essentials, Fifth Edition 24

Ethernet Error Handling • Collisions are the only type of error for which Ethernet automatically attempts to resend the data • Errors can occur when data is altered in medium – Usually caused by noise or faulty media connections – When the destination computer receives a frame, the CRC is recalculated and compared against the CRC value in the FCS – If values match, the data is assumed to be okay – If values don’t match, the data was corrupted •Destination computer discards the frame •No notice is given to the sender Guide to Networking Essentials, Fifth Edition 25

Half-Duplex Versus Full-Duplex Communications • When half-duplex communication is used with Ethernet, CSMA/CD must also be used • When using a switched topology, a computer can send and receive data simultaneously (full-duplex communication) – The collision detection circuitry is turned off because collisions aren’t possible – Results in a considerable performance advantage Guide to Networking Essentials, Fifth Edition 26

Ethernet Standards • Each Ethernet variation is associated with an IEEE standard • The following sections discuss many of the standards, some of which are obsolete or had limited use • Keep in mind that Ethernet over UTP cabling has been the dominant technology since the early 1990s, and will likely to continue to be for the foreseeable future Guide to Networking Essentials, Fifth Edition 27

IEEE 802 Elements • From the online document standards.ieee.org/getieee802/download/802.2-1998.pdf Guide to Networking Essentials, Fifth Edition 28