Network Protocols Part Two

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The Data-Link layer (the second OSI layer) is often divided into two sublayers ; the Logical Link Control (LLC) and the Medium Access Control (MAC). The IEEE also defines standards at the data-link layer. The ISO standards for the MAC layer, or lower half of the data-link layer, were taken directly from the IEEE 802.x standards.

Medium Access Control, as its name suggests, is the protocol that determines which computer gets to use the cable (the transmission medium) when several computers are trying. For example, 802.3 allows packets to collide with each other, forcing the computers to retry a transmission until it is sent successfully. 802.4 and 802.5 limit conversation to the computer with the token. Remember, this is done in fractions of a second, so even when the network is busy, users don't wait very long for access on any of these three network types.

The upper half of the data-link layer, the LLC, provides reliable data transfer over the physical link. In essence, it manages the physical link.

The IEEE splits the data-link layer in half because the layer has two jobs to do. The first is to coordinate the physical transfer of data. The second is to manage access to the physical medium. Dividing the layer allows for more modularity and therefore more flexibility. The type of medium access control has more to do with the physical requirements of the network than the actual management of data transfer. In other words, the MAC layer is closer to the physical layer than the LLC layer. By dividing the layer, a number of MAC layers can be created, each corresponding to a different physical layer, but just one LLC layer can handle them all. This increases flexibility and gives the LLC an important role in providing an interface between the various MAC layers and the higher-layer protocols. The role of the data-link's upper layer is so crucial, the IEEE gave it a standard of its own: 802.2 LLC.

Besides 802.2, other protocols can perform the LLC functions. High-level Data-Link Control (HDLC) is a protocol from ISO, which also conforms to the OSI model. IBM's Synchronous Data-Link Control (SDLC) does not conform to the OSI model but performs functions similar to the data-link layer. Digital Equipment's DDCMP or Digital Data Communications Protocol provides similar functions.

Three Transport Protocols

The ISO has established protocol standards for the middle layers of the OSI model. The transport layer, at layer four, ensures that data is reliably transferred among transport services and users. Layer five, the session layer, is responsible for process-to-process communication. The line between the session and transport layers is often blurred.

As of yet, no ISO transport or session layer has been implemented on a widespread basis, nor has the complete OSI protocol stack been established. To make matters more confusing, most middle-layer protocols on the market today do not fit neatly into the OSI model's transport and session layers, since many were created before the ISO began work on the OSI model.

The good news is many existing protocols are being incorporated into the OSI model. Where existing protocols are not incorporated, interfaces to the OSI model are being implemented. This is the case for TCP/IP, and IPX, which are the major middle-layer protocols available today.

In the PC LAN environment, NetBIOS has been an important protocol. IBM developed NetBIOS (or Network Basic Input/Output System) as an input/output system for networks. NetBIOS can be considered a session-layer protocol that acts as an application interface to the network. It provides the tools for a program to establish a session with another program over the network. Many programs have been written to this interface.

NetBIOS does not obey the rules of the OSI model in that it does not talk only to the layers above and below it. Programs can talk directly to NetBIOS, skipping the application and presentation layers. This doesn't keep NetBIOS from doing its job; it just makes it incompatible with the OSI model. The main drawback of NetBIOS is that it is limited to working on a single network.

TCP/IP or Transmission Control Protocol/Internet Protocol is actually several protocols. TCP is a transport protocol. IP operates on the network layer. TCP/IP traditionally enjoyed enormous support in government, scientific, and academic internetworks and in recent years has dominated the commercial networking environment, too. Part of the explanation is that corporate networks began to approach the size of networks found in the government and in universities, which drove corporations to look for internetworking protocol standards. They found TCP/IP to be progressively more useful as it became more widespread. Many people once viewed TCP/IP as an interim solution until OSI could be deployed, but no one seriously believes that the OSI protocols will ever have more than a niche role in the future.

Often when TCP/IP is discussed, the subjects of SMTP, FTP, Telnet, and SNMP are also raised. These are application protocols developed specifically for TCP/IP. SMTP or the Simple Mail Transfer Protocol is the electronic mail relay standard. FTP stands for File Transfer Protocol and is used to exchange files among computers running TCP/IP. Telnet is remote log-in and terminal emulation software. SNMP or the Simple Network Management Protocol is the most widely implemented network management protocol. The figure shows the protocols of TCP/IP.

Novell traditionally used IPX/SPX as its native transport protocols, though the company introduced a "native" implementation of TCP/IP in place of IPX/SPX. Internetwork Packet Exchange (IPX) and Sequenced Packet Exchange (SPX) are both variants of Xerox's XNS protocol. IPX provides network layer services, while SPX is somewhat rarely employed by applications that need transport layer services. Because IPX implementations prior to the introduction of NetWare Link Services Protocol (NLSP) in NetWare 4 caused a great deal of broadcast traffic and required frequent transmission acknowledgements, which can cause problems in a WAN, Novell also supported TCP/IP with gateways prior to its native TCP/IP implementation.

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The TCP/IP stack includes protocols that provide services equivalent to the OSI stack.

Other transport layer protocols include XNS and NetBEUI. XNS or Xerox Network System was one of the first local area network protocols used on a wide basis, mainly for Ethernet networks. 3Com's 3+ used a version of it. NetBEUI is IBM's transport protocol for its PC networking products. (The legacy of IBM's long-deceased partnership with Microsoft lives on in Microsoft's default implementations of NetBEUI in Windows for Workgroups, Windows 95/98, and Windows NT.)

Protocol Babel

If the number of available protocols seems like senseless confusion, it is and it isn't. Certain protocols have different advantages in specific environments. No single protocol stack will work better than every other in every setting. NetBIOS works well in small PC networks but is practically useless for communicating with WANs; APPC works well in peer-to-peer mainframe environments; TCP/IP excels in internetworks and heterogeneous environments.

On the other hand, much more is made about the differences in protocols than is warranted. Proprietary protocols can be perfect solutions in many cases. Besides, if proprietary protocols are sufficiently widespread, they become de facto standards, and gateways to other protocols are built. These include DEC's protocol suite, Sun Microsystems' Network Filing System and other protocols, and Apple's AppleTalk protocols. While these enjoy widespread use, that use is based on the computers these companies sell and not the proliferation of the protocols throughout the networking industry.

Whether it's a proprietary or standard protocol, users are faced with difficult choices. These choices are made slightly easier by the shakeout and standardization that has occurred at the physical and data-link layers. There are three choices: Token Ring, Ethernet, or FDDI. At the transport layers, IPX/SPX and TCP/IP emerged as the dominant protocols. [Editor's note: As of 2000, practically every new local network installation uses some form of Ethernet and TCP/IP, while the installed base remnants of Token Ring, FDDI, and IPX are diminishing irretrievably.]

This tutorial, number 3, by Aaron Brenner, was originally published in the October 1988 issue of LAN Magazine/Network Magazine.

 
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Network Tutorial
Lan Tutorial With Glossary of Terms: A Complete Introduction to Local Area Networks (Lan Networking Library)
ISBN: 0879303794
EAN: 2147483647
Year: 2003
Pages: 193

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