72 Practical TCP/IP and Ethernet Networking spreads the laser light evenly over the core of the multimode fiber so the laser source looks more like a light emitting diode (LED) source. This spreads the light in a large number of rays across the fiber resulting in smoother spreading of the pulses, so less interference. This conditioned launch is done in the 1000Base-SX transceivers. The following table gives the maximum distances for full-duplex 1000Base-X repeaters. Table 4.4 Maximum fiber distances for 1000Base-X (full-duplex) 4.5.2 Gigabit repeater rules The cable distance and the number of repeaters, which can be used in a half-duplex 1000Base-T collision domain, depend on the delay in the cable and the time delay in the repeaters and NIC delays. The maximum round-trip delay for 1000Base-T systems is the time to transmit 512 bytes or 4096 bits and equals 4.096 µs. A frame has to go from the transmitter to the most remote node then back to the transmitter for collision detection within this round trip time. Therefore the one-way time delay will be half this. The maximum sized collision domain can then be determined by the following calculation: Repeater delays + Cable delays + NIC delays + Safety factor (5 bits minimum) < 2.048 µs The following Table 4.5 gives typical maximum one-way delays for various components. Repeater and NIC delays for your specific components can be obtained from the manufacturer. System Maximum collision diameter point-to-point Half-duplex Maximum collision diameter One repeater segment 1000Base-CX 25 m 50m 1000Base-T 100 m 200m 1000Base-SX or LX 316 m 220 m Table 4.5 Maximum one-way gigabit Ethernet component delays These calculations give the maximum collision diameter for IEEE 802.3z half-duplex Gigabit Ethernet systems. The maximum gigabit Ethernet network diameters specified by the IEEE are shown in Table 4.6. Fast and gigabit Ethernet systems 73 System Maximum collision diameter point-to-point Half-duplex Maximum collision diameter One repeater segment 1000Base-CX 25 m 50 m 1000Base-T 100 m 200 m 1000Base-SX or LX 316 m 220 m Table 4.6 Maximum half-duplex gigabit Ethernet network diameters Note half-duplex gigabit Ethernet repeaters are not available for sale. Use full duplex repeaters with the point-to-point cable distances between node and repeater or node and switch. 5 /TZXUJ[IZOUTZU:)6/6 5HPKIZO\KY When you have completed study of this chapter you should be able to: • Describe the origins of TCP/IP • Compare the OSI and DARPA (DOD) models • Describe the overall structure of the TCP/IP suite of protocols :NKUXOMOTYUL:)6/6 In the early 1960s The US Department Of Defense (DOD) indicated the need for a wide-area communication system, covering the United States and allowing the interconnection of heterogeneous hardware and software systems. In 1967 the Stanford Research Institute was contracted to develop the suite of protocols for this network, initially to be known as ARPANet. Other participants in the project included the University of Berkeley (California) and the private company BBN (Bolt, Barenek and Newman). Development work commenced in 1970 and by 1972 approximately 40 sites were connected via TCP/IP. In 1973 the first international connection was made and in 1974 TCP/IP was released to the public. Initially the network was used to interconnect governments; military and educational sites together. Slowly, as time progressed, commercial companies were allowed access and by 1990 the backbone of the Internet, as it was now known, was being extended into one country after the other. One of the major reasons why TCP/IP has become the de facto standard world-wide for industrial and telecommunications applications is the fact that the Internet was designed around it in the first place and that, without it, no Internet access is possible. /TZXUJ[IZOUTZU:)6/6 :NK'86'SUJKR\YZNK59/SUJKR Whereas the OSI model was developed in Europe by the International Standards Organization (ISO), the ARPA model (also known as the DoD or Department of Defense model) was developed in the USA by the Advanced Projects Research Agency. Although they were developed by different bodies and at different points in time, both serve as models for a communications infrastructure and hence provide ‘abstractions’ of the same reality. The remarkable degree of similarity is therefore not surprising. Whereas the OSI model has 7 layers, the ARPA model has 4 layers. The OSI layers map onto the ARPA model as follows: • The OSI session, presentation and applications layers are contained in the ARPA process and application layer (nowadays referred to by the Internet community as the application level) • The OSI transport layer maps onto the ARPA host-to-host layer (nowadays referred to by the Internet community as the host level) • The OSI network layer maps onto the ARPA Internet layer (nowadays referred to by the Internet community as the network level) • The OSI physical and data link layers map onto the ARPA network interface layer The relationship between the two models is depicted in Figure 5.1. Figure 5.1 OSI vs ARPA models :NK:)6/6VXUZUIURY[OZK\YZNK'86'SUJKR TCP/IP, or rather – the TCP/IP protocol suite – is not limited to the TCP and IP protocols, but consist of a multitude of interrelated protocols that occupy the upper three layers of the ARPA model. TCP/IP does NOT include the bottom network access layer, but depends on it for access to the medium. :NKTKZ]UXQOTZKXLGIKRG_KX The network interface layer is responsible for transporting data (frames) between hosts on the same physical network. It is implemented in the network interface card or NIC, using both hardware and ‘firmware’ (i.e. software resident in read only memory). 6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM The NIC employs the appropriate medium access control methodology, such as CSMA/CA, CMSA/CD, token passing or polling, and is responsible for placing the data received from the upper layers within a frame before transmitting it. The frame format is dependent on the system being used, for example Ethernet or frame relay, and holds the hardware address of the source and destination hosts as well as a checksum for data integrity. RFCs that apply to the network interface layer include: • Asynchronous transfer mode (ATM), described in RFC 1438 • Switched multimegabit data service (SMDS), described in RFC 1209 • Ethernet, described in RFC 894, • ARCNET, described in RFC 1201 • Serial line internet protocol (SLIP), described in RFC 1055 • Frame relay, described in RFC 1490 • Fiber distributed data interface (FDDI), described in RFC 1103 (Note: Any Internet-related specification is originally submitted as a request for comments or RFC. As time progresses an RFC may become a standard, or a recommended practice, and so on. Regardless of the status of an RFC, it can be obtained from various sources on the Internet such as http://www.rfc-editor.org. :NK/TZKXTKZRG_KX This layer is primarily responsible for the routing of packets from one host to another. The emphasis is on ‘packets’ as opposed to frames, since at this level the data has not yet been placed in a frame for transmission. Each packet contains the address information needed for its routing through the Internet work to the receiving host. The dominant protocol at this level is the IP (as in TCP/IP), namely the Internet protocol. There are, however, several other additional protocols required at this level. These protocols include: • Address resolution protocol (ARP), RFC 826. This is a protocol used for the translation of an IP address to a hardware (MAC) address, such as required by Ethernet. • Reverse address resolution protocol (RARP), RFC 903. This is the complement of ARP and translates a hardware address to an IP address. • Internet control message protocol (ICMP), RFC 792. This is a protocol used for sending control or error messages between routers or hosts. One of the best-known applications here is the ping or echo request that is used to test a communications link. :NKNUYZZUNUYZRG_KX This layer is primarily responsible for data integrity between the sender host and receiver host regardless of the path or distance used to convey the message. Communications errors are detected and corrected at this level. It has two protocols associated with it, these being: • User data protocol (UDP). This is a connectionless (unreliable) protocol used for higher layer port addressing. It offers minimal protocol overhead and is described in RFC 768 • Transmission control protocol (TCP). This is a connection-oriented protocol that offers vastly improved protection and error control. This . 72 Practical TCP/IP and Ethernet Networking spreads the laser light evenly over the core of the multimode fiber. Berkeley (California) and the private company BBN (Bolt, Barenek and Newman). Development work commenced in 1970 and by 1972 approximately 40 sites were connected via TCP/IP. In 1973 the first. Maximum one-way gigabit Ethernet component delays These calculations give the maximum collision diameter for IEEE 802.3z half-duplex Gigabit Ethernet systems. The maximum gigabit Ethernet network