Ethernet Networking- P2 ppsx

18 Introduction The number of bits that can travel together at the same time represents the bandwidth of the transmission medium. If we can increase the bandwidth, we can increase the throughput without changing the maximum physical transfer speed of bits down the wire. Fiber optic cabling, for example, is very fast not only because each bit can travel at the speed of light, but be- cause so many tiny glass fibers can be bound together into a single cable to provide a high bandwidth. Ethernet Standards The types of Ethernet about which you have just read are defined in a set of standards prepared by the Institute of Electrical and Electronic Engi- neers (IEEE). The committee in charge of the standards for LANs is known as IEEE LAN 802, and the group within it that handles media access con- trois standards as 802.3. Each 802.3 standard describes a method for media access control and the transmission media that should be supported. Note: Although the name of the IEEE may not suggest that the organization has anything to do with computing, keep in mind that the IEEE predates computers. It has evolved to encompass a wide range of computing standards and applications. Although in most cases you won't be concerned directly with the specifi- cations themselves and the rather strange numbering scheme that goes along with them, you may find that equipment and cable vendors use the standard numbers to identify the type of Ethernet for which a product is ap- propriate. You should therefore at least be familiar with the type of Ether- net each standard represents. This book identifies the standards that accompany each type of Ethernet cabling as we explore hardware details in the following chapters. A Bit of Ethernet History 19 A Bit of Ethernet History Originally, Ethernet was the brainchild of one person: Robert Metcalfe. In the early 1970s, while working at Xerox PARC on the "office of the future" project, Metcalfe was intrigued by a radio network in Hawaii known as AlohaNet. One problem faced by AlohaNet's media access control was that its maximum effeciency was 17 percent: That is, a maximum of 17 percent of the transmission units sent actually reached their destination. According to Metcalfe, the unreceived portions of the transmissions were "lost in the ether." Metcalfe developed an alternative media access control method that al- lowed up to 90 percent of the transmission units to reach their destination. Originally known as "experimental Ethernet," it transferred up to 3 Mbps. As you can see in Metcalfe's original drawing in Figure 1-6, he refers to the cabling along with data travel as "the ether," hence the name Ethernet. Figure 1-6: Metcalfe) Bob Metcalfe' s original drawing for Ethernet (Courtesy of Bob Note: Bob Metcalfe went on to found the 3Com Corpora- tion and currently is a networking pundit and guru. His columns appear in InfoWorld and elsewhere. 20 Introduction The first Ethernet specifications were published in 1980 by a consortium of commercial hardware vendors ~ Digital Equipment Corporation (now a part of Compaq Corp.), Intel, and Xerox (DIX). By that time, the trans- mission speed had been increased tO 10 Mbps. The IEEE adopted Ethernet as a LAN standard and published its initial specifications as 10BASE5 in 1983. Later, Ethernet was also endorsed as a standard by the ISO. Ethernet is therefore an international standard for one way in which nodes on a LAN can gain access to transmission media. Throughout its history, Ethernet has moved to faster and faster standards: 1986: The standard for 10BASE2 was approved, still running at 10 Mpbs. 1991: The standard for 10BASE-T was approved. Although still running at 10 Mpbs, it used copper wiring, making it much easier to handle than earlier standards. Note: For more information on these earlier Ethernet standards, see Appendix A. 1995: The standard for 100 Mpbs Ethemet was approved. This is the slowest speed in general use today. 1998: The standard for 1000 Mbps (Gigabit) Ethernet using fi- ber optic cable was approved. 1999: The standard for 1000 Mbps Ethemet using copper wire was approved. 2002: The standard for 10,000 Mbps (10 Gigabit) Ethemet was approved. This type of Ethemet is for wide area rather than lo- cal area networks. As of early 2007, standards committees were beginning to explore the pos- sibilities for 40 Gigabit and 100 Gigabit Ethernet, although speeds beyond 1 Gigabit currently aren't designed for use in local area networks. How TCP/IP and Ethernet Work Regardless of the type of Ethernet you choose, the basic way in which data are packaged to travel over the network and the way in which devices gain access to the network media remain the same. In this chapter we will there- fore look at both the packaging of the data and the way that Ethernet pro- vides media access control. However, before we can look at the physical layer in depth, you need to know how the upper layers of the TCP/IP protocol stack operate. This knowledge forms the basis for understanding how devices such as switches and routers determine where to send packets of information. 21 22 How TCP/IP and Ethernet Work Network Data Transmission The data that travel over a network can be serial or parallel. With serial data transmission, each bit (a 0 or 1 value) travels single file. Parallel data transmission sends rows of bits, 32, 64, 128, or more at a time. As you may remember from Chapter 1, the bandwidth of a data communications chan- nel relates to the number of bits per unit time (usually a second) that arrive at their destination, thus the term bits per second for the speed of a data communications network. It might seem at first that parallel transmission is faster than serial transmission~and it is~but we use serial transmission over data com- munications networks because there is a major drawback to parallel transmission~interference that gets worse over distance. Let's assume that you have a cable designed to carry 32 bits in parallel. Because each wire in the cable can carry only one bit at a time, you need to bundle 32 wires together to obtain the desired bandwidth. (If they aren't close togeth- er, it will be next to impossible to fit a connector to them.) Unfortunately, the wires in the cable tend to leak signals to one another. The closer the wires are bound and the longer they get, the worse the inter- ference. Therefore, parallel transmission of this type (using a flat ribbon cable) is only good for very short distances, such as a few feet. Today we use it most commonly for connecting peripherals such as disk drives inside a system box. The speed of a serial transmission~the speed at which data reach their destination~is affected by many factors, including the following: The maximum physical speed that the wire can carry a signal. Note: When we speak of "wire" in this context, we mean copper wire and fiber optics. The speed at which a new signal can be placed on the wire. This is an effect of the equipment that places signals on the wire, as well as the method for giving hardware control of the wire. The ratio of overhead bits to data bits. (The more overhead bits you have, the lower the data throughput.) Major TCP/IP Protocols 23 Major TCP/IP Protocols In a practical sense, you don't need to know anything about networking protocols to plug the right wires into the fight interconnection hardware. However, if you really want to know how your equipment works, then you'll want to understand the material in this section. It looks at how pro- tocols stacks work in general and how the major TCP/IP protocols work specifically. The Operation of a Protocol Stack The protocols in a protocol stack are organized so that protocols that pro- vide similar functions are grouped into a single layer. As you saw in Chap- ter 1, the original TCP/IP provided four layers. (It has no physical layer.) However, the lower two layers of the original four have been replaced with protocols that were originally part of the OSI protocol stack. The exchange of bits occurs only at the Physical layer. The remaining lay- ers are software protocols. Conceptually, each layer communicates with the matching layer on the machine with which it is exchanging messages, as in Figure 2-1. However, because bits flow between machines only at the Physical layer, the actual communication is down one protocol stack, across the Physical layer, and up the receiving protocol stack (see Figure 2-2). The top three layers in the TCP/IP protocol stack are independent of the hardware a network is using. The remaining layers, however, are hard- ware-dependent, often meaning that there will be multiple sets of protocol specifications corresponding to different types of hardware. As a message moves down the protocol stack on the sending machine, it is encapsulated: Each software layer below the Application layer adds a header (and possibly a trailer) to the message before passing it down. On the receiving end, each layer strips off the header (and trailer, if present) before passing the message up to the next layer. By the time the message reaches the Application layer on the destination machine, it has been re- stored to is original state. 24 How TCP/IP and Ethernet Work Application layer Transport layer Internet layer _ 1Logical_U .nk _Con~_.ol__ . yer _ Physical layer v = , v Application layer Transport layer Internet layer ._ _.Logical_ U n.nk _Con_.~ol _ . ~ ~~~yer Physical layer Figure 2-1" Logical protocol communication Application layer Transport layer Internet layer ._ __Logical Link Control MAC layer Physical layer , , , Application layer Transport layer Internet layer _ _Logi~_.].l Link _controls ' _ MAC layer Physical layer Figure 2-2: The actual path for protocol communication The Application Layer The Application layer handles the interaction with the end user. All mes- sages originate there. Commonly, the Application layer sends a string of text down to the Transport layer, which begins the encapsulation process. .Frequently used Application layer protocols are summarized in Table 2-1. In most cases, the specifications for a protocol include the syntax and com- mands to be used when formulating the message. For example, to retrieve Major TCP/IP Protocols 25 Table 2-1" Frequently Used TCP/IP Application Layer Protocols Acronym Name Purpose HTTP Hypertext Transport Protocol SMTP Simple Mail Transport Protocol MIME Multipurpose Internet Mail Extensions POP3 Post Office Protocol DNS FTP NNTP Domain Name Server Manage the interaction between Web clients (browsers) and Web servers Transfer e-mail messages between client (e- mail client software) and e-mail server as well as between servers Provide format conversation for e-mail extensions so they can travel over a TCP/IP network Handle e-mail transfer Manage the mapping of domain names to IP addresses telnet Remote system login File Transfer Protocol Network News Transfer Protocol Transfer files Exchange Internet news articles between servers and clients a Web page, a Web browser formats a GET command, which includes the URL of the page to be retrieved Users rarely interact with the application layer protocol directly. Instead, applications present a more user-friendly interface to the user and then for- mulate the communications command out of sight. The Transport Layer The Transport layer contains two protocols" TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). They are fundamentally dif- ferent in the way in which they operate. TCP provides a virtual connection between the communicating Transport layers and is suitable for long mes- sages; UDP does not provide a virtual connection and is used mostly for short messages. 26 How TCP/IP and Ethernet Work Transmission Control Protocol TCP is known as a connection-oriented protocol because it establishes a logical circuit between sender and recipient that stays intact for the dura- tion of a communications session. It is also known as a reliable protocol because it provides both error correction and detection. The heart of TCP's operation is its three-way handshake for establishing a connection, which works in the following manner: 1. The sender transmits a segments with a SYN (Synchronization of Se- quence Numbers) request (a request to open a virtual connection be- tween the two machines). The sender chooses an ISN (Initial Sequence Number), either a 0 or some random number, that it sends in the initial SYN request. 2. The destination replies with a SYN containing the sender' s original se- quence number and an ACK (Acknowledge) containing the sender's original sequence number plus 1. (The segments may not arrive at the destination in the correct order, so the sequence numbers are essential to reassembling the message. They are also unique identifiers for each segment.) 3. The source responds with an ACK and the connection is established. A similar process gives TCP its reliability and error correction ability. Each segment that TCP sends is acknowledged by the recipient with an ACK segment. This ensures reliability; if the sender doesn't receive the ACK message within a specified amount of time, it retransmits the seg- ment. This also provides error correction for segments dropped when other layers and/or protocols detected errors in them. The beauty of having TCP handle the error correction is that lower level protocols need to worry only about error detection. Because each segment received must be acknowledged, TCP is a verbose protocol, at least compared to UDP. It also is not a particularly fast protocol compared to UDP because it requires an extra exchange of messages. When TCP receives a message from the Application layer, it attaches a header to the message, creating a segment. You can find the structure of a segment in Figure 2-3. The application layer message appears in the Data field; the rest of the segment is the header. The header fields are summa- rized in Table 2-2. Major TCP/IP Protocols 27 0 15 31 Source port I Destination port Sequence number Acknowledgment number Data Offset I Reserved I Flags I Window Checksum [ Urgent pointer Options I Padding Data Figure 2-3" The structure of a TCP segment Table 2-2: Fields in a TCP header Field Size Contents Source Port 16 bits Destination Port 16 bits Sequence Number 32 bits Acknowledgment 32 bits Number The TCP software port originating the message (for example, port 80 for the Web). The TCP software port to which the message is being sent. A number indicating the segment's position in the set of segments that comprise the entire message. TCP counts the number of octets a in the data field of the entire message and assigns each segment a sequence number that represents the number of the first data octet in that sequence. The recipient uses the sequence numbers to reassemble a message into the correct order, even if the segments are received out of order. A number acknowledging the receipt of a segment. It is set to the number of the next octet the recipient expects to receive. [...]... dial-up modems) 33 The Ethernet MAC Protocol The Ethernet MAC Protocol Ethernet is really a MAC protocol and the media specifications that go with it The MAC protocol includes details of how the data should be formatted when traveling over the wire and how devices should gain control of the wire to transmit Ethernet Frames To transmit a message across an Ethernet, a device constructs an Ethernet frame, a... TCP/IP refer to their units of transmission as "packets," Ethernet frames are also often called packets There are two general types of frame The first carries meaningful data (the content of messages two devices want to exchange) The second carries network management information Nonetheless, the general structure of both types of frame is identical An Ethernet frame varies in size from 64 bytes to 1529 bytes... network management information Frame Check Sequence bits 1 i SFD i , Preamble i i ',address i , , , ! , ~~ Address assignments bit Individuallmulticast address bit Figure 2-5" An Ethernet frame (IEEE 802.3 standard) 34 How TCP/IP and Ethernet Work Preamble: The preamble contains a group of 64 bits that are used to help the hardware synchronize itself with the data on the network If a few bits of the preamble... example, the author's printer has an Internet layer address of 192.168.1.105 and an Ethernet address of 00:C0:B0:02:15:75.) One job of data communications protocols is therefore to translate between hardware and software addresses TCP/IP, for example, uses Address Resolution Protocol (ARP) to map TCP/IP addresses onto Ethernet addresses Source address: The 48 bits of the source address field contain... and some type of transmission error has occurred Note: FCS error checking will not catch all errors, but it is certainly more effective than having no error checking at all/ Ethernet Media Access Whenever a device connected to an Ethernet network wants to send a message, it places that message in one or more frames However, only one frame can be transmitted on any given network segment at a time because... out (its topology) Originally, all Ethernet networks used a bus topology, a layout in which the devices all connect to a single network transmission line As you can see in Figure 2-6, the ends of the bus are unconnected Each device simply taps into the bus, which is conceptually~although not necessarily physically~a single unbroken transmission pathway How TCP/IP and Ethernet Work 36 File server m ~... checks again Note: The use of the term carrier in this case is not the same as the carrier signal used by modems A modem carrier is The Ethernet MAC Protocol 37 a tone of a known frequency, which is raised or lowered during data transmission to indicate patterns of Os and l s Ethernet uses the term merely to indicate the presence of a signal on the network If the network is idle (no carrier is detected),... in the network It may mean that you need to reexamine the way in which the How TCP/IP and Ethernet Work 38 network is laid out; breaking it into smaller collision domains or upgrading the type of Ethemet in use may be warranted Because only one signal can be on the transmission medium at one time, the original Ethernet networks were half-duplex (transmission in only one direction at a time) This is... Connectivity An Ethernet can be as small as connecting a couple of computers and a printer, or it can contain hundreds of devices in interconnected segments This part of this book describes how network are designed and the hardware that connects small segments into larger networks This Page Intentionally Left Blank Fast and Gigabi? Etherne? Media and S?andards The Fast (100 Mbps) and Gigabit (1000 Mbps) Ethernet. .. heavy traffic network links either within a building or as a network backbone between buildings UTP Cabling The most common cabling for Fast and Gigabit Ethernet today is twistedpair wire It is inexpensive and easy to set up and maintain 41 Fast and Gigabit Ethernet Media and Standards 42 Twisted-pair wire cables contain one or more pairs of copper wires that are twisted in a spiral manner For example, . accompany each type of Ethernet cabling as we explore hardware details in the following chapters. A Bit of Ethernet History 19 A Bit of Ethernet History Originally, Ethernet was the brainchild. Mbps. The IEEE adopted Ethernet as a LAN standard and published its initial specifications as 10BASE5 in 1983. Later, Ethernet was also endorsed as a standard by the ISO. Ethernet is therefore. and 100 Gigabit Ethernet, although speeds beyond 1 Gigabit currently aren't designed for use in local area networks. How TCP/IP and Ethernet Work Regardless of the type of Ethernet you