Department of Engineering Science ES465/CES 440, Intro. to Networking & Network Management Modulation & Modems (Ethernet & 802.3) http://www.sonoma.edu/users/k/kujoory References • “Computer Networks & Internet,” Douglas Comer, 6th ed, Pearson, 2014, Ch15, Textbook, 5th ed, slides by Lami Kaya (LKaya@ieee.org) with some changes. • “Computer Networks,” A. Tanenbaum, 5th ed., Prentice Hall, 2011, ISBN: 13:978013212695-3. • “Computer & Communication Networks,” Nader F. Mir, 2nd ed, Prentice Hall, 2015, ISBN: 13: 9780133814743. • “Data Communications Networking,” Behrouz A. Forouzan, 4th ed, Mc-Graw Hill, 2007 • “Data & Computer Communications,” W. Stallings, 7th ed., Prentice Hall, 2004. • “Computer Networks: A Systems Approach," L. Peterson, B. Davie, 4th Ed., Morgan Kaufmann 2007. Ali Kujoory 6/30/2016 Not to be reproduced without permission 1 Topics Covered • • • • • • • • • • • • • 15.1 Introduction 15.2 The Venerable Ethernet 15.3 Ethernet Frame Format 15.4 Ethernet Type Field & Demultiplexing 15.5 IEEE's Version of Ethernet (802.3) 15.6 LAN Connections & Network Interface Cards 15.7 Ethernet Evolution & Thicknet Wiring 15.8 Thinnet Ethernet Wiring 15.9 Twisted Pair Ethernet Wiring & Hubs 15.10 Physical & Logical Ethernet Topology 15.11 Wiring in an Office Building 15.12 Variants of Twisted Pair Ethernet & Speeds 15.13 Twisted Pair Connectors & Cables Ali Kujoory 6/30/2016 Not to be reproduced without permission 2 15.1 Introduction • This chapter – continues the discussion of LANs by focusing on wired LAN technologies – explains how concepts from previous chapters form the basis of Ethernet – shows the wired LAN technology that has proceeded to dominate all others References: • Ethernet frame https://en.wikipedia.org/wiki/Ethernet_frame • IEEE 802.3as – Ethernet Frame Expansion http://www.ieee802.org/3/as/public/0607/802.3as_overview.pdf Ali Kujoory 6/30/2016 Not to be reproduced without permission 3 15.2 The Venerable Ethernet • Although the hardware devices, cabling, & media used with Ethernet have changed dramatically – many of the fundamentals remain constant • One of the most interesting aspects of Ethernet evolution concerns the way newer versions of Ethernet remain backward compatible – a new version can sense an older form & automatically adapt to accommodate the older technology Ali Kujoory 6/30/2016 Not to be reproduced without permission 4 15.3 Ethernet Frame Format • The term frame format refers to the way a packet is organized – including details such as the size & meaning of individual fields • The main reason that older versions of Ethernet have remained compatible with newer versions – arises from the frame format, which has remained constant since the DIX standard was created in the 1970s Ali Kujoory 6/30/2016 Not to be reproduced without permission 5 15.3 Ethernet Frame Format • Fig. 15.1 illustrates the basic (original) format & the details of the frame header • As the figure shows – an Ethernet frame consists of a fixed-length header – a variable-length payload, & – a fixed-length CRC Fig. 15.1 Illustration of the Ethernet frame format & header details. Ali Kujoory 6/30/2016 Not to be reproduced without permission 6 15.4 Ethernet Type Field & Demultiplexing – When transmitting a datagram in an Ethernet frame, the sender assigns a type 0800 • The type field in an Ethernet frame provides multiplexing & demultiplexing that – allows a given computer to have multiple protocols operating simultaneously • The protocols used on the Internet send IP datagrams & ARP messages over Ethernet – Each is assigned a unique Ethernet type • When a frame arrives at its destination, the receiver – examines the type field, & – uses the value to determine which software module should process the frame • Fig. 15.2 illustrates the demultiplexing • Hex 0800 for IP datagrams & • Hex 0806 for ARP messages Ali Kujoory 6/30/2016 Not to be reproduced without permission 7 15.4 Ethernet Type Field & Demultiplexing Ali Kujoory 6/30/2016 Not to be reproduced without permission 8 15.5 IEEE's Version of Ethernet (802.3) • IEEE developed a standard for Ethernet (1983) & – attempted to redefine the Ethernet frame format • IEEE working group that produced the standard is numbered 802.3 – professionals often refer to it as 802.3 Ethernet • Major difference between conventional Ethernet & 802.3 Ethernet arises from – 802.3 standard • interprets the original type field as a packet length, & • adds 8-byte header that contains the packet type – The extra header is known as a Logical Link Control / Sub-Network Attachment Point (LLC/SNAP) header; • most professionals simply call it a SNAP header • Fig. 15.3 illustrates the format – interpretation of the type field Ali Kujoory 6/30/2016 Not to be reproduced without permission 9 15.5 IEEE's Version of Ethernet (802.3) 4 bytes 802.1Q tag (optional) Ali Kujoory 6/30/2016 2 bytes Ethertype or length Not to be reproduced without permission 10 15.5 IEEE's Version of Ethernet (802.3) • The overall frame size in 802.3 Ethernet remains the same as conventional Ethernet: 1518 bytes – IEEE reduced the maximum payload from 1500 bytes to 1492 bytes – SNAP header occupies the first 8 bytes of the payload Ali Kujoory 6/30/2016 • To keep the two versions of Ethernet compatible, a convention is used: – If bytes 13-14 of a frame contain a numeric value less than 1500 • the field is interpreted as a packet length & the 802.3 standard applies, otherwise • the field is interpreted as a type field & the original Ethernet standard applies Not to be reproduced without permission 11 15.6 LAN Connections & Network Interface Cards • NIC appears to be an I/O device – it is connected in the same manner as a disk or video device • A NIC handles – – – – address recognition CRC computation frame recognition sending/receiving frames Ali Kujoory 6/30/2016 • A NIC consists of a circuit board with a plug on one side • Most computers come with a NIC already installed • The NIC is independent from the rest of the computer, & – a user can choose to replace the NIC without making other changes Not to be reproduced without permission 12 15.7 Ethernet Evolution & Thicknet Wiring • Ethernet has undergone several major changes – with the most significant changes in media & wiring • Hardware used with Thicknet was divided into two major parts • The original Ethernet wiring scheme was informally called thick wire Ethernet or Thicknet – because the medium consisted of a heavy coaxial cable – the formal term for the wiring is 10Base5 The 10 refers to its transmission speed of 10 Mbit/s. The BASE is short for baseband signalling as opposed to broadband,& the 5 stands for the maximum segment length of 500 metres. Ali Kujoory 6/30/2016 – A NIC handled the digital aspects of communication – A separate electronic device called a transceiver connected to the Ethernet cable that handles • carrier detection • conversion of bits into appropriate voltages for transmission, & • conversion of incoming signals to bits Not to be reproduced without permission 13 15.7 Ethernet Evolution & Thicknet Wiring • A physical cable known as an Attachment Unit Interface (AUI) connected a transceiver to a NIC in a computer • A transceiver was usually remote from a computer – E.g., in an office building, transceivers might attach to an Ethernet in a hallway ceiling • Fig. 15.4 illustrates how the original Thicknet wiring used an AUI cable to connect a computer to a transceiver Ali Kujoory 6/30/2016 Not to be reproduced without permission 14 15.7 Ethernet Evolution & Thicknet Wiring https://en.wikipedia.org/wiki/10BASE5 Ali Kujoory 6/30/2016 Not to be reproduced without permission 15 15.8 Thinnet Ethernet Wiring • A 2nd generation of Ethernet used a thinner coaxial cable that was more flexible than Thicknet – Formally named 10Base2 & informally known as Thinwire Ethernet or Thinnet • Thinnet integrates a transceiver directly on the NIC – runs a coaxial cable from one computer to another • Fig. 15.5 illustrates Thinnet wiring Ali Kujoory 6/30/2016 https://en.wikipedia.org/wiki/10BASE2 Not to be reproduced without permission 16 15.8 Thinnet Ethernet Wiring • Thinnet had advantages & disadvantages • The primary advantages – Lower overall cost & ease of installation – No external transceivers were needed – Thinnet cable could be installed in a convenient path • The chief disadvantage – because the entire network was vulnerable • If a user unplugged a segment of the network to relocate wires or move a computer, the entire network would stop working Ali Kujoory 6/30/2016 Not to be reproduced without permission 17 15.9 Twisted Pair Ethernet Wiring & Hubs • A 3rd generation of Ethernet wiring made a dramatic shift: – In place of coaxial cable • it uses a central electronic device separate from the computers attached to the network – Thus, when someone now refers to Ethernet, they are referring to twisted pair Ethernet • The electronic device that served as the central interconnection was known as a hub – Instead of heavy, shielded cabling • it uses twisted pair wiring • The technology is informally known as twisted pair Ethernet, & – Hubs were available in a variety of sizes, – has replaced other versions • with the cost proportional to size, but • recently replaced with switches https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair Originallly by StarLAN in AT&T Ali Kujoory 6/30/2016 Not to be reproduced without permission 18 15.9 Twisted Pair Ethernet Wiring & Hubs Ali Kujoory 6/30/2016 Not to be reproduced without permission 19 15.9 Twisted Pair Ethernet Wiring & Hubs • A hub emulates a physical cable – making the entire system operate like a conventional Ethernet • A system that uses a hub in CSMA/CD receives a copy of each frame & – uses the address to determine • Twisted pair Ethernet retains the same frame format as the previous versions • In fact, software on a computer cannot distinguish between thick Ethernet, thin Ethernet, & twisted pair Ethernet – The NIC handles the details & hides any differences • whether to process or ignore the frame Ali Kujoory 6/30/2016 Not to be reproduced without permission 20 15.10 Physical & Logical Ethernet Topology • Twisted pair Ethernet appears to follow a star topology • To understand Ethernet topology – In fact, the term hub arose to clarify the concept of a central interconnection point – However, because a hub emulates a physical cable • the system appears to perform as if computers attach to a cable – we must distinguish between logical & physical topologies – Logically • twisted pair Ethernet employs a bus topology – Physically • twisted pair Ethernet forms a star-shaped topology – In fact, professionals joked that a hub really provided a bus in a box Ali Kujoory 6/30/2016 Not to be reproduced without permission 21 15.11 Wiring in an Office Building • The styles of wiring used for LANs make little difference in a machine room or laboratory • The type of wiring makes a major difference in terms of – – – – type number of wires needed distance spanned cost • Fig. 15.7 depicts wiring on a floor of an office building – Note that twisted pair Ethernet requires many individual cables to go between offices & a central point (wiring closet) – Thus, twisted pair Ethernet requires careful labeling of cables • The three versions of Ethernet wiring illustrate the three principal forms that LANs use Ali Kujoory 6/30/2016 Not to be reproduced without permission 22 Fig. 15.7 Illustration of various LAN wiring schemes that have been used in an office building (Part I) Ali Kujoory 6/30/2016 Not to be reproduced without permission 23 Fig. 15.7 Illustration of various LAN wiring schemes that have been used in an office building (Part II) Compare the ThinNet topology with the twisted pair using a hub, considering material, equipment, & person-power costs, & maintenance & repairs efficiency. Ali Kujoory 6/30/2016 Not to be reproduced without permission 24 15.12 Variants of Twisted Pair Ethernet & Speeds • Significant improvements have been made in the quality & shielding available in twisted pair cables – the data rate used on twisted pair Ethernet has increased • Fig. 15.8 summarizes the three types of twisted pair Ethernet & the cable used with each • Higher-speed Ethernet technologies use an electronic device known as a switch rather than a hub Ali Kujoory 6/30/2016 • To remain backward compatible – standards for the higherspeed versions specify that interfaces automatically sense the speed at which a connection can operate, & – slow down to accommodate older devices – E.g., if one plugs an Ethernet cable between an old device that uses 10BaseT & a new device that uses 1000BaseT • the new device will autosense Not to be reproduced without permission 25 15.12 Variants of Twisted Pair Ethernet & Speeds 1000BaseT up to 100 meters, https://en.wikipedia.org/wiki/Gigabit_Ethernet Ali Kujoory 6/30/2016 Not to be reproduced without permission 26 15.13 Twisted Pair Connectors & Cables • Twisted pair Ethernet uses RJ45 connectors – which are larger versions of the RJ11 connectors used to connect telephones • What is straight & crossed Ethernet cable? • A straight cable, used between a computer & a switch – connects each pin of the RJ45 attached to one end of the cable directly to the corresponding pin on the RJ45 at the other end Ali Kujoory 6/30/2016 • A crossed cable, which is used to connect two switches – connects a pin on one end to a different pin on the other end • To help technicians make the correct connections – individual wires in a Category 5 or Category 6 cable are coated with colored plastic • Fig. 15.9 lists the color codes used with a straight cable Straight-through & Cross-over cables http://www.home-network-help.com/straight.html Not to be reproduced without permission 27 15.13 Twisted Pair Connectors & Cables Ali Kujoory 6/30/2016 Not to be reproduced without permission 28