Network Guide Part 2 - Introducing the OSI Model
Introducing the OSI Model
The OSI model was designed to promote interoperability by creating a guideline for network data transmission between computers that have different hardware vendors, software, operating systems, and protocols. For example, look at the simple process of transferring a file. From a user’s perspective, a single operation has been performed to transfer the file. In reality, however, many different procedures had to take place behind the scenes to accomplish this seemingly simple task. Network data transmission (like the file transfer) is performed through the use of a protocol suite, also known as a protocol stack.
A protocol suite is most easily defined as a set of rules used to determine how computers communicate with each other. It is similar to language.
If I speak English and you speak English, then we can communicate. But if I speak only Spanish and you speak only English, we won’t be able to communicate.
The OSI model is used to describe what tasks a protocol suite performs as you explore how data moves across a network. Keep in mind that not all protocols map directly to the guideline provided for us through the OSI model, but there are enough similarities so that you can use the OSI model to examine how these protocols function. There are a myriad of protocol suites in use today, including IPX/SPX, NetBIOS, and TCP/IP. Each performs a specific function. Many of these functions that are provided through the use of a protocol stack and its components are standard functions performed by other components in other protocol stacks.
Note ISO is not an abbreviation for the International Organization for Standardization, but is instead derived from the Greek word isos , which means “equal,” and was adopted by the organization. For more information, go to www.iso.ch.
The most commonly referenced protocol model, the OSI model, was developed in 1977 by the International Organization for Standardization (ISO) to provide “common ground” when describing any network protocol
Tip You can use mnemonic devices to help you remember the order of the OSI model layers: APSTNDP (from top to bottom). The most popular mnemonic for this arrangement is All People Seem To Need Data Processing. A reverse mnemonic (from Physical to Application, bottom to top) is Please Do Not Throw Sausage Pizza Away. (Good advice, don’t you think?)
OSI model consists of seven layers. Each layer performs a specific function and then passes on the result to another layer. When a sending station has data to send, it formats a network request and then passes that request to the network protocol at the top layer, the Application layer. The protocol that runs at the Application layer performs an operation on the request and then passes it to the next, lower layer. Each protocol at each layer below the Application layer performs its own calculations and appends its own information to the data sent from the layer above it. At the receiving station, the process happens in reverse.
The OSI model is only a model; it is not a protocol. Nobody is running the “OSI protocol” (at least no one has developed one at the time of this writing). Let’s take a brief look at the layers of the OSI model and the basic protocol functions they describe. We’ll start at the top with the Application layer and work our way down to the Physical layer.
The Application Layer
The Application layer, the top layer of the OSI model, does not refer to applications such as word processors, but rather refers to a set of tools that an application can use to accomplish a task, such as a word processor application requesting a file transfer. This layer is responsible for defining how interactions occur between network services (applications) and the network. Services that function at the Application layer include, but are not limited to, file, print, and messaging services. The Application layer may also support error recovery.
The Presentation Layer
The Presentation layer is responsible for formatting data exchange. In this layer, character sets are converted, and data is encrypted. Data may also be compressed in this layer, and this layer usually handles the redirection of data streams.
The Session Layer
The Session layer defines how two computers establish, synchronize, maintain, and end a session. Practical functions, such as security authentication, connection ID establishment, data transfer, acknowledgments, and connection release, take place here. This list is not all-inclusive. Any communications that require milestones—or, put another way, require an answer to “Have you got that data I sent?”—are performed here. Typically these milestones are called checkpoints . Once a checkpoint has been crossed, any data not received needs retransmission only from the last good checkpoint. Adjusting checkpoints to account for very reliable or unreliable connections can greatly improve the actual throughput of data transmission.
The Transport Layer
The Transport layer is responsible for checking that the data was delivered error-free. It is also used to divide a message that is too long into smaller segments and, in the reverse, take a series of short messages and combine them into one longer segment. These smaller or combined segments must later be correctly reassembled. This is accomplished through segment sequencing (usually by appending a number to each of the segments).
This layer also handles logical address/name resolution. Additionally, this layer can send an acknowledgment that it got the data packet. Frequently you will see this referred to as an ACK, which is short for acknowledgment. This layer is responsible for the majority of error and flow control in network communications.
The Network Layer
The Network layer is responsible for logical addressing and translating logical names into physical addresses. A little-known function of the Network layer is prioritizing data. Not all data is of equal importance. Nobody is hurt if an e-mail message is delayed a fraction of a second. Delaying audio or video data a fraction of a second could be disastrous to the message. This prioritization is known as Quality of Service (QoS).
In addition, the Network layer controls congestion, routes data from source to destination, and builds and tears down packets. Most routing protocols function at this layer.
The Data Link Layer
The Data Link layer takes raw data from the Physical layer and gives it a logical structure. This logic includes information about where the data is meant to go, which computer sent the data, and the overall validity of the bytes sent. In most situations, after a data frame is sent, the Data Link layer then waits for a positive ACK. If one is not received or if the frame is damaged, another frame is sent.
The Data Link layer also controls functions of logical network topologies and physical addressing as well as data transmission synchronization and connection
The Physical Layer
The Physical layer is responsible for controlling the functional interface, such as transmission technique, pin layout, and connector type.
The OSI Model’s Lower Layers
The OSI Model’s Lower Layers
Now that you have a broad overview of the OSI model and its seven layers, you will now learn about the functions of each layer in a little more detail, starting with the lower layers. In addition to the concepts, you’ll read about some of the devices that operate at those layers and some of their installation concepts
The Physical Layer
The easiest way to think about the Physical layer is that it deals with measurable, physical entities. Any protocol or device that operates at the Physical layer deals with the physical concepts of a network.
Physical Layer Concepts
Generally speaking, Physical layer concepts deal with a network component that is tangible or measurable. For example, when a protocol at the Physical layer receives information from the upper layers, it translates all the data into signals that can be transmitted on a transmission medium. This process is known as signal encoding (or encoding , for short). With cable media (also called bounded media ), the protocols that operate at the Physical layer translate the ones and zeros of the data into electrical ons and offs.
Additionally, the Physical layer specifies how much of the media will be used (in other words, its signaling method ) during data transmission. If a network signal uses all available signal frequencies (or, to put it differently, the entire bandwidth), the technology is said to use baseband signaling. Most LAN technologies, such as Ethernet, use baseband signaling. On the other hand, if a signal uses only one frequency (or only part of the bandwidth), the technology is said to use broadband signaling. This means multiple signals can be transmitted on the media simultaneously. Television signals use broadband signaling .
Finally, the Physical layer specifies the layout of the transmission media (its topology, in other words). A physical topology describes the way the cabling is physically laid out (as opposed to a logical topology, discussed later in the section titled “The Data Link Layer”). The physical topologies include the following:
The Bus Topology
In a physical bus topology, every computer is directly connected to a common medium. A physical bus network uses one network cable that runs from one end of the network to the other. Workstations connect at various points along this cable. The main advantage to this topology is simplicity: Only one cable is used, and a physical bus topology typically requires less cable than other physical topologies. However, a cable fault can bring down the entire network, thus making a physical bus topology the least fault tolerant of all the physical topologies
The Star Topology
In a physical star topology, a cable runs from each network entity to a central device. This central device (called a hub ) allows all devices to communicate as if they were all directly connected. The main advantage to a physical star topology is its fault tolerance. If one node or cable malfunctions, the rest of the network is not affected. The hub simply won’t be able to communicate with the station attached to that port. An Ethernet 10BaseT network is one example of a network type that requires a physical star topology
The Ring Topology
A physical ring topology isn’t seen much in the computer-networking world. If you do see it, it’s usually in a wide area network (WAN) environment. In a physical ring topology, every network entity connects directly to only two other network entities (the one immediately preceding it and the one immediately following it). The complexity of the ring topology makes it a poor choice in most network environments
The Mesh Topology
A physical mesh topology is another physical topology that isn’t widely used in computer networks (except in special WAN cases). In a physical mesh topology, every computer is directly connected to every other computer in the network. The more computers that are on a mesh network, the more cables that make up the network. If a mesh network has n computers, there will be n (n– 1)/2 cables. With 10 computers, there would be 10(10–1)/2, or 45 cables. As you can see, this topology quickly becomes unmanageable with only a few computers
Physical Layer Devices
Several devices operate primarily at the Physical layer of the OSI model. These devices manipulate mainly the physical aspects of a network data stream (such as the voltages, signal direction, and signal strength). Let’s take a quick look at some of the most popular:
- NIC
- Transceivers
- Repeaters
- Hubs
- MAUs
The Network Interface Card (NIC)
Probably the most common component on any network is the network interface card (NIC). A NIC is the component that provides the connection between a computer’s internal bus and the network media. NICs come in many shapes and sizes. They vary by the type of bus connection they employ and their network media connection ports
The Transceiver
In the strictest definition, a transceiver is the part of any network interface that transmits and receives network signals (transmitter/receiver). Every network interface has a transceiver. The appearance and function of the transceiver vary with the type of network cable and topology in use.
Note Some network interface cards have an Attachment Unit Interface (AUI) port (typically a 15-pin DIN connector) that allows a different, external transceiver type to be used, thus changing the media types to which the NIC can connect. For example, if you are using an Ethernet 10Base2 network interface card with an AUI port, you can connect to an Ethernet 10BaseT network by using an external transceiver attached to the AUI port. A DIN connector meets the specification of the German national standards body, Deutsche Industrie Norm, or DIN.
The Repeater
The simplest of all the Physical layer devices is the repeater, which simply amplifies the signals it receives on one port and resends (or “repeats”) them on another. Repeaters are used to extend the maximum length of a network segment. They are often used if a few network stations are located far from the rest of the network.
The main downfall of a repeater is that it repeats everything it receives on one port, including noise, to its other ports. This has the ultimate effect of limiting the number of repeaters that can practically be used on a network. The 5-4-3 Rule dictates how many repeaters can be used on a network and where they can be placed. According to this rule, a single network can have five network segments connected by four repeaters, with three of the segments populated. If this rule is violated, one station may not be able to see the rest of the network
The Hub
After the NIC, a hub is probably the most common Physical layer device found on networks today. A hub (also called a concentrator) serves as a central connection point for several network devices. At its basic level, a hub is nothing more than a multiport repeater. A hub repeats what it receives on one port to all other ports. It is, therefore, also subject to the 5-4-3 Rule
There are many classifications of hubs, but two of the most important are active and passive:
An active hub is usually powered and it actually amplifies and cleans up the signal it receives, thus doubling the effective segment distance limitation for the specific topology (for example, extending an Ethernet segment another 100 meters).
A passive hub is typically unpowered and makes only physical, electrical connections. Typically, the maximum segment distance of a particular topology is shortened because the hub takes some power away from the signal strength in order to do its job.
The Multistation Access Unit (MAU)
This Physical layer device is unique to Token Ring networks. Token Ring networks use a physical star topology, yet they use a logical ring topology (discussed later). The central device on an Ethernet star topology network is a hub, but on a Token Ring network, the central device is a Multistation Access Unit (MAU, sometimes called MSAU). The functionality of the MAU is similar to that of a hub, but the MAU provides the data path that creates the logical “ring” in a Token Ring network. The data can travel in an endless loop between stations. MAUs are chained together by connecting the Ring Out port of one MAU to the Ring In port of another and connecting the last Ring Out port to the Ring In of the first MAU in the chain, thus forming a complete loop. In a Token Ring network, you can have up to 33 MAUs chained together.
The Data Link Layer
The Data Link layer is actually made up of two sublayers:
The Media Access Control (MAC)
The Logical Link Control (LLC)
Data Link Layer Concepts
Protocols that operate at the Data Link layer have several responsibilities, including creating, transmitting, and receiving frames. Additionally, the Data Link layer is responsible for physical (MAC) addressing and logical link control (LLC) processing, creating logical topologies, and controlling media access.
Packets
At the Data Link layer, data coming from upper-layer protocols are divided into logical chunks called packets. A packet is a unit of data transmission. The size and format of these packets depend on the transmission technology
The Hardware (MAC) Address
Every network interface card has an address, typically assigned at the factory. This address is protocol-independent and is often called the hardware address. It’s technically accurate, however, to call it the MAC address because it exists at the MAC sublayer of the Data Link layer. This address is also called the Ethernet address or the physical address.
The MAC address itself is a 12-digit hexadecimal number. As you may remember, a hexadecimal uses all digits from 0 through 9 and A through F. Each two-digit set is separated by colons, like so:
07:57:AC:1F:B2:76
Normally, the MAC address of a network interface card is set at the factory and cannot be changed. For this purpose, all NIC manufacturers keep track of the MAC addresses they use so they don’t duplicate addresses between vendors. As of late, however, some manufacturers have started reusing their blocks of MAC addresses. This makes it necessary for administrators to be able to change the MAC addresses of the cards they receive (using a factorysupplied program), so if they discover a duplicated MAC address, they can resolve the conflict.
Logical Topology
In addition to these responsibilities, the Data Link layer can also dictate the logical topology of a network, or the way the packets move through a network. A logical topology differs from a physical topology in that the physical topology dictates the way the cables are laid out; the logical topology dictates the way the information flows. The types of logical topologies are the same as the physical topologies, except that the information flow specifies the type of topology to use.
Finally, the Data Link layer can describe the method of media access. The three main methods of media access are:
Contention, in which every station “competes” with other stations for the opportunity to transmit, and each has an equal chance at transmitting. If two stations transmit at the same time, an error, referred to as a collision, occurs, and the stations try again.
Polling, in which a central device, called a controller, polls each device, in turn, and asks if it has data to transmit. This type of media access virtually eliminates collisions.
Token passing, which uses a special data packet called a token. When a station has the token, it can transmit. If it doesn’t have the token, it can’t transmit. This media access technology also eliminates collision problems.
Media Access
With many stations on the same piece of network media, there has to be a way of vying for time on the cable. This process is called media access, and there are three main methods: CSMA/CD, token passing, and CSMA/CA.
CARRIER SENSE/MULTIPLE ACCESS WITH COLLISION DETECTION (CSMA/CD)
This media access technology with the extremely long acronym is probably the most common. When a protocol that uses CSMA/CD has data to transmit, it first senses if a signal is already on the wire (a carrier), indicating that someone is transmitting currently. That’s the “Carrier Sense” part. If no one else is transmitting, it attempts a transmission and then listens to hear if someone else tried to transmit at the same time. If someone else transmits at the exact same time, a collision occurs. Both senders “back off” and don’t transmit until some random period of time has passed. Then they both retry. That’s the “Collision Detection” part. The final part (“Multiple Access”) just means that more than one station can be on the network at the same time. CSMA/CD is the access method used in Ethernet and wireless Ethernet networks.
TOKEN PASSING
This media access method uses a special packet called a token. The first computer that is turned on creates the token. It then passes on the token to the next computer. The token passes around the network until a computer that has data to send takes the token off the network, modifies it, and puts it back on the network along with the data it has to send. Each station between the sender and the receiver along the network reads the destination address in the token. If the destination address doesn’t match its own, the station simply sends the package on its way. When the destination station recognizes its address in the destination address of the token, the NIC copies the data into the station’s memory and modifies the token, indicating it has received the data. The token continues around the network until the original sender receives the token again. If the original sender has more data to send, the process repeats itself. If not, the sender modifies the token to indicate that the token is “free” for anyone else to use. With this method, there are no collisions (as in CSMA/CD networks) because everyone has to have “permission” to transmit (via the token).
CARRIER SENSE/MULTIPLE ACCESS WITH COLLISION AVOIDANCE (CSMA/CA)
This technology works almost identically to CSMA/CD, but instead of sending the whole data chunk and then listening to hear if it was transmitted, the sender transmits a request to send (RTS) packet and waits for a clear to send (CTS) before sending. When it receives the CTS, the sender sends the chunk. AppleTalk networks use this method of media access. The difference between CSMA/CD and CSMA/CA has been described like this: Say you want to cross a busy street and you want to use one of these protocols to cross it. If you are using CSMA/CD, you just cross the street. If you get hit, you go back to the curb and try again. If you’re using CSMA/CA, you send your little brother across. If he makes it, it’s probably OK for you to go.
Project 802
One of the major components of the Data Link layer is the result of the Institute of Electrical and Electronics Engineers’ (IEEE) 802 subcommittees and their work on standards for local area and metropolitan area networks (LANs/MANs). The committee met in February 1980, so they used the “80” from 1980 and the “2” from the second month to create the name Project 802. The designation for an 802 standard always includes a dot (.) followed by either a single or a double digit. These numeric digits specify particular categories within the 802 standard. Currently, there are 12 standards.
IEEE 802 Networking Standards
802.1
LAN/MAN Management (and Media Access Control Bridges)
802.2
Logical Link Control
802.3
CSMA/CD
802.4
Token Bus
802.5
Token Ring
802.6
Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)
802.7
Broadband Local Area Networks
802.8
Fiber-Optic LANs and MANs
802.9
Integrated Services (IS) LAN Interface
802.10
LAN/MAN Security
802.11b
Wireless LAN
802.12
Demand Priority Access Method
Note Some standards have a letter to further distinguish the standard (e.g., 802.11b). The letters usually refer to different versions or interpretations of the standard
The 802.1 LAN/MAN Management (and Media Access Control Bridges)
IEEE 802.1 discusses standards for LAN and MAN management, as well as for MAC bridges. One of the derivatives of 802.1 is the spanning tree algorithm for network bridges (bridges are discussed later in this chapter). The spanning tree algorithm helps to prevent bridge loops in a multibridge network.
The 802.2 Logical Link Control
This standard specifies the operation of the Logical Link Control (LLC) sublayer of the Data Link layer of the OSI model. The LLC sublayer provides an interface between the MAC sublayer and the Network layer. The 802.2 standard is used by the IEEE 802.3 Ethernet specification (discussed next), but not by the earlier Ethernet 2 specifications (used in early implementations of Ethernet).
The 802.3 CSMA/CD
This standard specifies a network that uses a bus topology, baseband signaling, and a CSMA/CD network access method. This standard was developed to match the Digital, Intel, and Xerox (DIX) Ethernet networking technology. So many people implemented the 802.3 standard, which resembles the DIX Ethernet, that people just started calling it Ethernet. It is the most widely implemented of all the 802 standards because of its simplicity and low cost.
Recently the 802.3u working group updated 802.3 to include Ethernet 100BaseT implementations.
The 802.4 Token Bus
This standard specifies a physical and a logical bus topology that uses coaxial or fiber-optic cable and a token-passing media access method. It is used mainly for factory automation and is seldom used in computer networking. It most closely resembles the Manufacturing Automation Protocol (MAP), developed by General Motors and used by many manufacturing companies. Some people think that the IEEE 802.4 standard is for a technology known as the Attached Resource Computer Network (ARCnet). That is an incorrect assumption. Although the technologies are similar, the IEEE 802.4 standard more closely resembles MAP, not ARCnet.
The 802.5 Token Ring
This standard is one example of a commonly used product becoming a documented standard. Typically, a standard is developed and then products are written to conform to the standard. Token Ring was developed by IBM in 1984, and the 802.5 standard soon followed. The 802.5 standard and Token Ring are almost identical.
Like Ethernet, Token Ring can use several cable types. Most often, it is installed using twisted-pair cabling, which can be either shielded or unshielded. Shielding adds to the cable investment but offers the advantage of resistance to unwanted electrical signals that could impair the network signal.
Possible transmission rates for Token Ring have increased with time; after 4Mbps Token Ring came 16Mbps Token Ring. Token Ring uses a physical star, logical ring topology with token-passing media access. If you install 4Mbps NICs on a network that otherwise uses 16Mbps NICs, your entire ring speed is reduced to 4Mbps. Unlike Ethernet, a computer cannot talk unless it has a token. This can cause some grief if a token gets “stuck.”
Unlike ARCnet, Token Ring is still used in a number of locations for two reasons:
IBM made sure that Token Ring did a fine job of talking to IBM mainframes, which are still commonly used.
Token Ring network performance “degrades with grace.”
The latter means that as network traffic increases, the network slowly gets slower, because the single token, which can travel in only one direction, gets busy carrying all that traffic. Ethernet, on the other hand, can become so flooded as network traffic increases that the entire network fails. Now, suppose you were wiring a computerized fire alarm system for a large building. Which would you rather use: Ethernet or Token Ring? To increase performance, some Token Ring technologies implement early token release, whereby the sending station doesn’t hog the token. It simply grabs the token, sends its data, and frees the token.
In Token Ring, just as in all ARCnet and most Ethernet schemes, there is a central device to which stations connect. It isn’t, however, called a hub. IBM calls it a MAU. IBM often has a different name for things. Even their name for Token Ring cabling is different. In telephone and computer networks, twisted cable is rated by categories. IBM rates Token Ring cable by type.
One final difference between Token Ring and the others is the regeneration process. Data signals are read, amplified, and repeated by every device on the network, to reduce degradation. This includes MAUs and NICs and is one reason that Token Ring is fairly expensive. An average Token Ring NIC is upward of Rs. 10000, whereas a similar Ethernet card can be less than Rs 800.
The 802.6 Distributed Queue Dual Bus (DQDB) Metropolitan Area Network
In some ways, asking what defines a metropolitan area network (MAN) is like asking how long a rope is. We can safely say that a MAN reaches beyond the area of a LAN. The interesting question is “When does a MAN become a WAN?” Sorry to say, there is no easy answer. Like a WAN, a MAN can support many computers. How many miles a MAN can cover has more to do with regulations than with geography. For example, from a geographical standpoint, Portland, Oregon, and Vancouver, Washington, are separated by nothing more than several hundred feet of water. From a political standpoint, they are in different states, and, therefore, different telecommunication regulations apply to each city. This could mean that no MANs can connect Portland and Vancouver. For our purposes, we need to know only that a MAN generally encompasses a city-sized area and can support many-to-many connections. Transmission speeds vary with the size of an enterprise’s bank account. The standard recommends the use of Distributed Queue Dual Bus (DQDB) technologies for MANs.
The 802.7 Broadband Local Area Networks
Don’t let the fancy phrasing fool you. You have already used broadband if you have seen cable TV. When one cable carries multiple signals, that is broadband. The most common method for separating signals is to have them on different frequencies, which is called Frequency Division Multiplexing (FDM). For example, each channel on a TV uses a different frequency. It is as simple as that. Maybe you can win a beer from some friends by seeing if they can explain Frequency Division Multiplexing. If they can’t, collect your reward and tell them that is how all those TV channels get into their TV from one cable. The alternative to sending a set of signals this way is to use the entire cable for one signal. This is known as baseband and is used by standards such as Ethernet.
The 802.8 Fiber-Optic LANs and MANs
As the name implies, this working group handed down guidelines for fiberoptic usage on networks defined by 802.3 through 802.6, which includes Fiber Distributed Data Interface (FDDI) as well as 10BaseFL. 10BaseFL defines Ethernet over fiber-optic cable. As you can see, some of the 802 definitions have more to do with your day-to-day work than others do.
The 802.9 Integrated Services (IS) LAN Interface
For a while, it seemed that this definition would have a profound effect on daily networking, because it laid out how Integrated Services Digital Network (ISDN) behaves. Late in 1998, however, many industry watchers began to call for the slow death of ISDN, because both cable modems and asymmetrical digital subscriber line (ADSL) had overtaken ISDN with lesscomplicated setup, higher performance, and lower cost.
The 802.10 LAN/MAN Security
This standard provides a secure pathway for data across a shared path. An implementation of this standard is using the public Internet as a backbone for a private interconnection between locations. The term for this form of connecting is known as virtual private networking (VPN). Because VPN costs less than direct private connections, VPN is likely to become popular in the near future.
The 802.11 Wireless LAN
Wireless networking usually requires a higher up-front investment than cable-based networking. Still, the cost can be justified if an office is rearranged with any regularity or must be moved from location to location to satisfy business requirements. A famous example of this is the Red Cross. This agency would not be effective if it had to wire computers together before assisting at each disaster area.
Recently, 802.11 was updated to include the 802.11b standard, which specifies higher wireless speeds (11Mbps instead of 1Mbps for the original 802.11 standard). This demonstrates that the 802 standards have not been static for 20 years; instead, they’ve been a dynamic set of rules that continue to be updated as technology moves forward.
The 802.12 Demand Priority Access Method
First developed by Hewlett-Packard, this standard combines the concepts of Ethernet and ATM. The communication scheme used is called Demand Priority (thus, the name of the standard). It uses “intelligent” hubs that allocate more bandwidth to frames that have been assigned a higher priority by the sending computer. The hub scans its ports and then allocates bandwidth according to each frame’s priority. This is extremely valuable for real-time audio and video transmissions.
The 802.12 standard is also known as 100VG (Voice Grade), 100VGAnyLAN, 100BaseVG, and AnyLAN. The 100 is short for 100Mbps, or 10 times faster than the original Ethernet speeds. Other manufacturers didn’t buy into the ideas of 100VG, perhaps in part because of the higher overhead of demand priority due to port scanning. Instead, they updated the original Ethernet to Fast Ethernet, which also supports 100Mbps while maintaining the 802.3 standards.
Data Link Layer Devices
Two main devices manipulate data at the Data Link layer:
Bridges
Switches
They are more complex than their Physical layer counterparts and thus are more expensive and more difficult to implement. But they each bring unique advantages to the network.
The Bridge
A bridge is a network device, operating at the Data Link layer, that logically separates a single network into two segments, but it lets the two segments appear to be one network to higher layer protocols. The primary use for a bridge is to keep traffic meant for stations on one side of the bridge and not let it pass to the other side. For example, if you have a group of workstations that constantly exchange data on the same network segment as a group of workstations that don’t use the network much at all, the busy group will slow down the performance of the network for the other users. If you put in a bridge to separate the two groups, however, only traffic destined for a workstation on the other side of the bridge will pass to the other side. All other traffic stays local
Tip Bridges can connect dissimilar network types (for example, Token Ring and Ethernet) as long as the bridge operates at the LLC sublayer of the Data Link layer. If the bridge operates only at the lower sublayer (the MAC sublayer), the bridge can connect only similar network types (Token Ring to Token Ring and Ethernet to Ethernet).
The Switching Hub
In the past few years, the switching hub has received a lot of attention as a replacement for the standard hub. The switching hub is more intelligent than a standard hub in that it can actually understand some of the traffic that passes through it. A switching hub (or switch for short) operates at the Data Link layer and is also known as a Layer 2 Switch. Layer 2 switches build a table of the MAC addresses of all the connected stations
When two stations attached to the switch want to communicate, the sending station sends its data to the switch. This part of the process is similar to the way a standard hub functions. However, when the switch receives the data, rather than broadcasting it over all its other ports as a hub would, the switch examines the Data Link header for the MAC address of the receiving station and forwards it to the correct port. This opens a virtual pipe between ports that can use the full bandwidth of the topology.
Switches have received a lot of attention because of this ability. If a server and several workstations were connected to the same 100Mbps Ethernet switch, each workstation would need a dedicated 100Mbps channel to the server, and there would never be any collisions.
The OSI Model’s Middle Layers
The OSI Model’s Middle Layers
As you move up the OSI model, the protocols at each successive layer get more complex and have more responsibilities. At the middle are the Network and Transport layers, which perform the bulk of the work for a protocol stack. You’ll see why in the sections to follow.
The Network Layer
The Network layer of the OSI model defines protocols that ensure that the data arrive at the correct destination. This is probably the most commonly discussed layer of the OSI model.
The most important Network layer concepts are:
Logical network addressing
Routing
Logical Network Addressing
In the above section, you learned that every network device has an address (the MAC address) assigned at the factory and that this address is protocolindependent. But, as you know, most networks communicate using protocols that must have their own addressing scheme. If the MAC address is the Data Link layer physical address, the protocol-addressing scheme at the Network layer defines the logical address.
Note If IP addresses are duplicated on Windows 95/98 workstations, the first station that is assigned an address gets to use it. Any other station that has that address receives error messages about duplicated IP addresses. The address is then unassigned. The first station receives error messages as well, but it can continue to function.
Each logical network address is protocol-dependent. For example, a TCP/ IP address is not the same as an IPX address. Additionally, the two protocols can coexist on the same computer without conflict. However, two different stations using the same protocol cannot have the same logical network address on the same network. If that happens, neither station can be seen on the network
Note Address conflicts can be common with TCP/IP because an administrator often needs to assign IP addresses. IPX addresses don’t suffer from conflict nearly as often, because they use the MAC address as part of the IPX address. The MAC address is unique and can’t be changed. For more information on network addresses
Real World Scenario: Using Network Address Formats
Whenever you have to set up a network or add a station, it is important to have an understanding of how network addresses work. Every network address in either TCP/IP or IPX has both a network portion and a node portion. The network portion is the number that is assigned to the network segment to which the station is connected. The node portion is the unique number that identifies that station on the segment. Together, the network portion and the node portion of an address ensure that a network address will be unique across the entire network.
IPX addresses use an eight-digit hexadecimal number for the network portion. This number, called the IPX network address, can be assigned randomly by the installation program or manually by the network administrator. The node portion is the 12-digit hexadecimal MAC address assigned by the manufacturer. A colon separates the two portions.
TCP/IP addresses, on the other hand, use a dotted decimal notation in the format xxx.xxx.xxx.xxx
The address consists of four collections of eight-digit binary numbers (or up to three decimal digits) called octets, separated by periods. Each decimal number in an IP address is typically a number in the range of 1 through 254. Which portion is the network and which portion is the node depends on the class of the address and the subnet mask assigned with the address. A subnet mask is also a dotted decimal number with numbers in the range of 0 through 255. If a subnet mask contains 255 in any position ( corresponding to a binary number of all ones), the corresponding part of the IP address is the network address. For example, if you have the mask 255.255.255.0, the first three octets are the network portion, and the last portion is the node.
Routing
Routing is the process of moving data throughout a network, passing through several network segments using devices called routers, which select the path the data takes. Placing routers on a network to connect several smaller routers turns a network into an entity known as an internetwork. Routers get information about which paths to take from files on the routers called routing tables. These tables contain information about which router network interface (or port) to place information on in order to send it to a particular network segment. Routers will not pass unknown or broadcast packets. A router will route a packet only if it has a specific destination.
Information gets into routing tables in two ways:
Through static routing
Through dynamic routing
In static routing, the network administrator manually updates the router’s routing table. The administrator enters every network into the routing table and selects the port on which the router should place data when the router intercepts data destined for that network. Unfortunately, on networks with more than a few segments, manually updating routing tables is timeintensive and prohibitive.
Note When using a Windows NT server as a router, use the ROUTE command to add, change, or remove static routes.
Dynamic routing, on the other hand, uses route discovery protocols (or routing protocols for short) to talk to other routers and find out which networks they are attached to. Routers that use dynamic routing send out special packets to request updates of the other routers on the network as well as to send their own updates. Dynamic routing is the most popular routing technology.
With dynamic routing, the two categories of route discovery protocols are distance vector and link state. Older route discovery protocols, such as Routing Information Protocol (RIP) for TCP/IP and RIP for IPX, use the distance vector method. In distance vector routing, a router sends out its routing table when the router is brought online and the contents of its routing tables every 30 seconds thereafter. When another router receives the contents of the other router’s table, it adds 1 to the hop count of each route in the list of routes and then rebroadcasts the list. A hop is one pass through a router. This process typically takes place every 30 seconds.
The main downside to distance vector route discovery is the overhead required in broadcasting the entire routing table every 30 seconds. Link state route discovery is more efficient. Routers using link state route discovery routers send out their routing table via a multicast, not a broadcast packet, every five minutes or so. If there is an update, only the update is sent. NetWare Link Services Protocol (NLSP) for IPX and Open Shortest Path First (OSPF) for TCP/IP are two link state route discovery protocols.
Several protocols can be routed, but a few protocols can’t be routed. It is important to know which protocols are routable and which aren’t so that you can choose the appropriate protocol when it comes time to design an internetwork
Note When setting up routing on your network, you may have to configure a default gateway. A default gateway, when configured on a workstation, is the router that all packets are sent to when the workstation doesn’t know where the station is or can’t find it on the local segment. TCP/IP networks sometimes have multiple routers as well and must use this parameter to specify which router is the default. Other protocols don’t have very good routing functions at the workstation, so they must use this feature to “find” the router.
Network Layer Devices
Three devices operate at the Network layer:
The Router
The router is the device that connects multiple networks or segments to form a larger internetwork. It is also the device that facilitates communication within this internetwork. It makes the choices about how best to send packets within the network so that they arrive at their destination.
Several companies manufacture routers, but probably the two biggest names in the business are Bay Networks and Cisco. Bay Networks is a conglomeration of smaller networking companies bought out by networking giant Synoptics. Cisco has always been a built-from-the-ground-up router company. Both companies make other products, to be sure, but their bread and butter is routing technologies.
Routers have many functions other than simply routing packets. Routers can connect many small segments into a network, as well as connect networks to a much larger network, such as a corporate WAN or the Internet. Routers can also connect dissimilar lower-layer topologies. For example, you can connect an Ethernet and a Token Ring network using a router. Additionally, with added software, routers can perform firewall functions and packet filtering.
Routers are probably the most complex devices on a network today. Consequently, they are likely to be the most expensive. But simple low-end routers have been introduced by Bay Networks, Cisco, and other companies in the sub-Rs 35000 range that make Internet connectivity more affordable. Hub vendors have begun to introduce basic intranetwork routing functionality into their products as well. You will learn more about that later in this chapter when we discuss switches.
The Brouter
The brouter is a unique device that combines the functionality of a bridge and a router. It routes most packets, but if it can’t route a particular packet, it will try and bridge it. Unfortunately, if you try to use a brouter as either a bridge or a router, it will fall short in the functionality of either.
The brouter was mainly used to connect different network topologies and to bridge them, but it is not used much anymore
Layer 3 Switches
A fairly new Network layer device that has received much media attention of late is the Layer 3 Switch. The Layer 3 part of the name corresponds to the Network layer of the OSI model. It performs the multiport, virtual LAN, data-pipelining functions of a standard Layer 2 Switch, but it can also perform basic routing functions between virtual LANs. In some workgroups, a Layer 3 Switch can replace a workgroup router.
The Transport Layer
The Transport layer defines the protocols for structuring messages and checks the validity of transmissions
The Transport layer reminds me of what my old Net Tech instructors used to pound into my head: “Reliable end-to-end error and flow control.” The Transport layer does other things as well, but the protocols that operate at the Transport layer mainly ensure reliable communications between upper peer layers.
The Connection Type
To provide error and flow control services, protocols at the Transport layer use connection services. The two types of connection services are:
Connection-oriented
Connectionless
Connection-oriented connection services use acknowledgments and responses to establish a virtual connection between sending and receiving stations. The acknowledgments are also used to ensure that the connection is maintained.
Connection-oriented connections are similar to phone calls. You dial the intended recipient, and the recipient picks up and says hello. You then identify yourself and say that you’d like to talk about something, and the conversation begins. If you hear silence for a while, you might ask “Are you still there?” to make sure the recipient is still on the line. When finished, you both agree to end the connection by hanging up. Connection-oriented services work in the same way, except that instead of mouths, phones, and words, they use computers, NICs, and special packets
Connectionless services, on the other hand, don’t have error and flow control. They do have one simple advantage: speed. Because connectionless services don’t have the overhead of maintaining the connection, the sacrifice in error control is more than made up for in speed. To make another analogy, connectionless services are similar to a postcard. Each message is considered singular and not related to any other. So, if one part of the message is lost, it can simply be resent.
Name Resolution
The Transport layer also handles logical address–to–logical name resolution. In some protocols, a node address, such as 185.45.2.23, isn’t the best way to reference a host. Some protocol stacks (TCP/IP and IPX/SPX, for example) can use Transport layer logical names for hosts in addition to their Network layer logical addresses. These logical names make it easier for you to find hosts on the network.
At the Transport layer, various protocol stacks implement a protocol to translate Network layer addresses into Transport layer logical names.
Transport Layer Implementations
Before we discuss the other layers of the OSI model, let’s take a look at the IPX/SPX, TCP/IP, and NetBEUI implementations of the Transport layer
The IPX/SPX Protocol
As far as the connection services of IPX/SPX are concerned, there are two transport protocols: IPX and SPX. IPX is connectionless and thus enjoys the benefits of connectionless transports, including increased speed. SPX, on the other hand, uses connection-oriented services.
IPX/SPX has no name resolution system by default. That functionality is employed when a NetWare server is running Novell Directory Services (NDS) and the NDS directory requester (which runs at the Session, Presentation, and Application layers) can make requests of an NDS database.
The TCP/IP Protocol
Like the IPX/SPX protocol stack, the TCP/IP protocol stack has two transport protocols:
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
TCP is connection-oriented, and UDP is connectionless. Some upper-layer protocols, such as FTP and HTTP, require reliable connection-oriented service and, therefore, use TCP. Other upper-layer protocols, such as Trivial File Transfer Protocol (TFTP) and Network File System (NFS), require increased speed and will trade reliability for that speed. They, therefore, use UDP.
Note For network address—to–name resolution, TCP/IP uses Domain Name Services (DNS). It is my belief that the name resolution of the OSI model’s Transport layer was designed for DNS. Many operating systems use DNS for name resolution, but Unix (whose networking is based on TCP/IP) uses DNS almost exclusively. DNS is probably the most cross-platform name resolution method available
The NetBEUI Implementation
Because it is based on the NetBIOS protocol, NetBEUI (NetBIOS Enhanced User Interface) has datagram support and, thus, has support for connectionless transmission. It doesn’t, however, have support for connection-oriented services. NetBIOS does allow hosts to have logical names (using WINS), but the naming service, as with NDS, functions at the upper layers of the OSI model.
The OSI Model’s Upper Layers
The OSI Model’s Upper Layers
The upper layers of the OSI model deal with less esoteric concepts. Even though we’re still discussing computer networking, the top three layers (Session, Presentation, and Application) seem easier to understand. Because the Network+ exam doesn’t cover the upper layers (and many times these top three layers are grouped together), the following sections will give only a brief overview.
The Session Layer
Protocols that operate at the Session layer of the OSI model are responsible for establishing, maintaining, and breaking sessions, or dialogs. This is different from the connection services provided at the Transport layer, because the Session layer operates at a higher level and looks at the bigger picture—the entire conversation, not just one sentence. Many gateways operate at the Session layer.
The Presentation Layer
The Presentation layer does what you might think it does: It changes the look, or presentation, of the data from the lower layers into a format that the upper-layer processes can work with. Among other services, the Presentation layer deals with encryption, data compression, and network redirectors.
In addition, the Presentation layer deals with character-set translation. Not all computer systems use the same table to convert binary numbers into text. Most standard computer systems use the American Standard Code for Information Interchange (ASCII). Mainframe computers (and some IBM networking systems) use the Extended Binary Coded Decimal Interchange Code (EBCDIC). The two are totally different. Protocols at the Presentation layer can translate between the two
The Application Layer
Now I know what you might be thinking: “This layer is for my programs, right?” Wrong. The Application layer defines several standard network services that fall into categories such as file transfer, print access, and e-mail relay. The applications that access these network services are located above the Application layer (although some people say that applications are an extension of the Application layer).
Upper-Layer Devices
There are only a few upper-layer devices, none of which operate at any specific layer. Because they perform a range of functions for the network, they fall into the class of devices known as gateways. A gateway translates one type of network data into another. Gateways can be either hardware or software, but the most popular way to run a gateway is as a software program on a dedicated computer.
There are many, many types of gateways, but the one most people think of is an e-mail gateway. E-mail gateways translate e-mail messages from one type of e-mail system so that they can be transmitted on another (for example, from GroupWise e-mail to SMTP mail for the Internet).
Re: Network Guide Part 2 - Introducing the OSI Model
I've learned more from this article than my own txt book. Thank YOU very much. Please keep spreading the knowledge >^..^<
