8. Cisco Hierarchical Model

There are three layers to the Cisco hierarchical model

• The core (backbone) layer provides optimal transport between sites.
• The distribution layer provides policy-based connectivity.
• The local-access layer provides workgroup/user access to the network.

 Core Layer

• Responsible for transporting large amounts of traffic reliably and quickly.
• Only purpose is to switch traffic as fast as possible (speed and latency are factors).
• Failure at the Core layer can affect every user, design for fault tolerance at this level.
• Design specifications

Don't Do at this layer

• Don't use access lists, packet filtering, or VLAN Routing.
• Don't support workgroup access here.
• Don't expand (i.e. more routers), upgrade devices instead (faster with more capacity).

Do at this layer 

• Design for high reliability (FDDI, Fast Ethernet with redundant links, or ATM).
• Design for speed and low latency.
• Use routing protocols with low convergence times.

 Distribution Layer

• Also called workgroup layer, this is the communication point between the access and core layers.
• Primary functions include routing, filtering, WAN access, and determining how packets can access the Core layer if necessary.
• Determines fastest/best path and sends request to the Core layer. Core layer will then quickly transport the request to the correct service.
• Place to implement network policies.

 Distribution Layer Functions 

• Access lists, packet filtering, queuing.
• Security and network policies such as address translation and firewalling.
• Re-distribution between routing protocols including static routing.
• Routing between VLANs and other workgroup support functions.
• Departmental or workgroup access.
• Definition of broadcast and multicast domains.
• Any media transitions that need to occur.

 Access Layer

• Controls local end user access to internetwork resources.
• Also called desktop layer.
• The resources most users need will be available locally.
• Distribution layer handles traffic for remote services.
• Continued use of access lists and filters.
• Creation of separate collision domains (segmentation).
• Workgroup connectivity at Distribution layer.
• Technologies such as DDR and Ethernet switching are seen in the Access layer.
• Static routing is here.

7. Data Encapsulation

The sending and receiving of data from a source device to the destination device is possible with the help of networking protocols by using data encapsulation. The data is encapsulated with protocol information at each layer of the OSI reference model when a host transmits data to another device across a network. Each layer communicates with its neighbour layer on the destination. Each layer uses Protocol Data Units (PDUs) to communicate and exchange information.

Protocol Data Unit (PDU)

The Protocol Data Units contain the control information attached to the data at each layer. The information is attached to the header of the data field but can also be in end of the data field or trailer. PDUs are encapsulating by attached them to the data at each layer of the OSI reference model. Each Protocol Data Unit has a name depending on the information each header has. This PDU information is only read by the neighbour layer on the destination and then is stripped off and the data is handed to the next layer.

OSI Layer Model and PDUs

The seven layered Open System Interconnection (OSI) layered model is basically defined for reducing the complexity of the internetworking. The OSI model is then divided into two segments for more ease, Upper layers and Data Flow layers. The 7th, 6th and 5th layer of the OSI reference model are application layers also known as upper layers. The upper layers are directly related with user interface while the 4rth, 3rd, 2nd and 1st layer of the OSI model are also called data flow layers because they are related with the flow of the data. Each data flow layer has a Protocol Data Unit.

The Protocol Data Unit of each data flow layers is defined as follows:

• Transport Layer: Segment is the PDU of the Transport layer.
• Network Layer: Packet is the PDU of the Transport layer.
• Data Link Layer: Frame is the PDU of the Transport layer.
• Physical Layer: Bit is the PDU of the Transport layer.

Encapsulation and De-Encapsulation Process

The encapsulation and de-encapsulation of header control information on each layer of the OSI reference model is as follows:

Encapsulation

The data encapsulation process is defined as below:

TCP Header Encapsulation

The application-layers user data is converted for transmission on the network. The data stream is then handed down to the Transport layer, which sets up a virtual circuit to the destination. The data stream is then broken up, and a Transport layer header is created and called a segment. The header control information is attached to the Transport layer header of the data field. Each segment is sequenced so the data stream can be put back together on the destination exactly as transmitted. 

IP Header Encapsulation

Each segment is then handed to the Network layer for logical addressing and routing through a routed protocol, for example IP, IPX, Apple Talk and DECNET etc. The Network-layer protocol adds a header to the segment handed down to the Data Link layer. Remember that the 3rd and 4rth layers work together to rebuild a data stream on a destination host. However, they have no responsibility for placing their Protocol Data Units on a local network segment, which is the only way to get the information to host or router. 

MAC Header Encapsulation

The Data Link layer receives the packets from the Network layer and placing them on the network medium such as cable or wireless media. The Data Link layer encapsulates each packet in a frame, and the MAC header carries the source Mac address and destination Mac address. If the device is on a different network, then the frame is sent to a router to be routed through an internetwork. 

Physical Layer Encapsulation

Once the frame gets to the destination network, a new frame is used to get the packet to the destination host. To put this frame on the network, it must first be put into a digital signal. Since a frame is really a logical group of 1s and 0s, the Physical layer of the OSI model is responsible for encapsulating these digits into a digital signal, which is read by devices on the same local network. 

De-Encapsulation

On destination side, the receiving devices will synchronize on the digital signal and extract the 1s and 0s from the digital signal. At this point the devices build the frames, run a Cyclic Redundancy Check (CRC), and then check their output against the output in the Frame Check Sequence (FCS) field of the data frame. If the information matches then the packet is pulled from the frame, and the frame is discarded. This process is known as de-encapsulation. The packet then transfers to the Network layer, where the IP address is checked. If the IP address matches then the segment is pulled from the packet, and the packet is discarded. The data is processed at the Transport layer that rebuilds the data stream and acknowledges to the transmitting station that it received each piece of segment. It then happily transfers the data stream to the upper layer application. 

At a transmitting device, the data encapsulation method works as follows:

• User information is converted into data for transmission on the network.
• Data is converted into segments and a reliable or unreliable connection is set up between the source and destination devices using connection oriented and connectionless protocols.
• Segments are converted into packets using a logical address such as IP datagram using an IP address.
• Packets are converted into frames for transmission on the local network. Media Access Control (MAC) addresses or Ethernet addresses are commonly used to uniquely identify hosts on a local network segment.
• Frames are converted into bytes and bits, and a digital encoding and clocking or signalling method is used.

 

6. Ethernet Cabling

Ethernet has become the de facto standard for LAN implementations. At one time, there were three competing technologies: Ethernet, Token Ring, and FDDI. Because of the cost of FDDI, it was never really implemented on a wide scale but used only for

backbone connections. Token Ring, however, was designed and heavily promoted by IBM. When Cisco purchased IBM’s networking division, this stopped any further development on Token Ring technology. Therefore, most companies that used other LAN media types have converted to Ethernet. This section will provide a review from Chapter 2 on Ethernet, as well as the types of connections and cables you’ll use to attach Ethernet devices together. 

Ethernet cables are available in different types. These are:

• Straight-through cable
• Crossover cable
• Rolled cable

(i) Straight-Through Cable

The straight-through cable is used to connect

• Host to switch or hub
• Router to switch or hub

 Four wires are used in the straight-through cable to connect Ethernet devices. It is relatively simple to create this type; Figure shows the four wires used in a straight-through Ethernet cable.

Notice that only pins 1,2,3, and 6 are used. Just connect to 1 to 1, 2 to 2, 3 to 3 , and 6 to 6 and you’ll be up and networking in no time. However, remember that this would be an Ethernet-only cable and wouldn’t work with voice, Token Ring, ISDN, and so on.

(ii) Crossover Cable

The crossover cable can be used to connect

• Switch to switch
• Hub to hub
• Host to host
• Hub to switch
• Router direct to host

 The same four wires are used in this cable as in the straight-through cable; we just connect different pins together. Figure shows how the four wires are used in a crossover Ethernet cable. 

 Notice that instead of connecting 1 to 1, 2 to 2, and so on. Here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

(iii) Rolled Cable

Although rolled cable isn’t used to connect any Ethernet connections together, you can use a rolled cable to connect a host to a router console and communication (com) port.

If you have a Cisco router or switch, you would use this cable to connect your PC running Hyper Terminal to the Cisco hardware. Eight wires are used in this cable to connect serial devices, although not all eight are used to send information, just as in Ethernet networking. Figure shows the eight wires used in a rolled cable. 

These are probably the easiest cable to make because you just cut the end off on one side of a straight-through cable, turn it over, and put it back on( with a new connector). 

Once you have the correct cable connected from your PC to Cisco router or switch, you can start HyperTerminal to create a console connection and configure the device.

*Multiplexing Basics

Multiplexing is a process in which multiple data channels are combined into a single data or physical channel at the source. Multiplexing can be implemented at any of the OSI layers. Conversely, demultiplexing is the process of separating multiplexed data channels at the destination. One example of multiplexing is when data from multiple applications is multiplexed into a single lower-layer data packet. Figure illustrates this example. 

Figure  Multiple Applications Can Be Multiplexed into a Single Lower-Layer Data Packet.

Another example of multiplexing is when data from multiple devices is combined into a single physical channel (using a device called a multiplexer). Figure illustrates this example.

Figure  Multiple Devices Can Be Multiplexed into a Single Physical Channel.

A multiplexer is a physical layer device that combines multiple data streams into one or more output channels at the source. Multiplexers demultiplex the channels into multiple data streams at the remote end and thus maximize the use of the bandwidth of the physical medium by enabling it to be shared by multiple traffic sources. 

Some methods used for multiplexing data are time-division multiplexing (TDM), asynchronous time-division multiplexing (ATDM), frequency-division multiplexing (FDM), and statistical multiplexing. 

In TDM, information from each data channel is allocated bandwidth based on preassigned time slots, regardless of whether there is data to transmit. In ATDM, information from data channels is allocated bandwidth as needed by using dynamically assigned time slots. In FDM, information from each data channel is allocated bandwidth based on the signal frequency of the traffic. In statistical multiplexing, bandwidth is dynamically allocated to any data channels that have information to transmit.

*Error-Checking Basics

Error-checking schemes determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination. Error checking is implemented at several of the OSI layers. 

One common error-checking scheme is the cyclic redundancy check (CRC), which detects and discards corrupted data. Error-correction functions (such as data retransmission) are left to higher-layer protocols. A CRC value is generated by a calculation that is performed at the source device. The destination device compares this value to its own calculation to determine whether errors occurred during transmission. First, the source device performs a predetermined set of calculations over the contents of the packet to be sent. Then, the source places the calculated value in the packet and sends the packet to the destination. The destination performs the same predetermined set of calculations over the contents of the packet and then compares its computed value with that contained in the packet. If the values are equal, the packet is considered valid. If the values are unequal, the packet contains errors and is discarded.

*Flow Control Basics

Flow control is a function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data. A high-speed computer, for example, may generate traffic faster than the network can transfer it, or faster than the destination device can receive and process it. The three commonly used methods for handling network congestion are buffering, transmitting source-quench messages, and windowing.

Buffering is used by network devices to temporarily store bursts of excess data in memory until they can be processed. Occasional data bursts are easily handled by buffering. Excess data bursts can exhaust memory, however, forcing the device to discard any additional datagrams that arrive.

Source-quench messages are used by receiving devices to help prevent their buffers from overflowing. The receiving device sends source-quench messages to request that the source reduce its current rate of data transmission. First, the receiving device begins discarding received data due to overflowing buffers. Second, the receiving device begins sending source-quench messages to the transmitting device at the rate of one message for each packet dropped. The source device receives the source-quench messages and lowers the data rate until it stops receiving the messages. Finally, the source device then gradually increases the data rate as long as no further source-quench requests are received. 

Windowing is a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted. With a window size of 3, the source requires an acknowledgment after sending three packets, as follows. First, the source device sends three packets to the destination device. Then, after receiving the three packets, the destination device sends an acknowledgment to the source. The source receives the acknowledgment and sends three more packets. If the destination does not receive one or more of the packets for some reason, such as overflowing buffers, it does not receive enough packets to send an acknowledgment. The source then retransmits the packets at a reduced transmission rate.

*Addresses Versus Names

Internetwork devices usually have both a name and an address associated with them. Internetwork names typically are location-independent and remain associated with a device wherever that device moves (for example, from one building to another). Internetwork addresses usually are location-dependent and change when a device is moved (although MAC addresses are an exception to this rule). As with network addresses being mapped to MAC addresses, names are usually mapped to network addresses through some protocol. The Internet uses Domain Name System (DNS) to map the name of a device to its IP address. For example, it's easier for you to remember www.cisco.com instead of some IP address. Therefore, you type www.cisco.com into your browser when you want to access Cisco's web site. Your computer performs a DNS lookup of the IP address for Cisco's web server and then communicates with it using the network address.

*Address Assignments

 Addresses are assigned to devices as one of two types: static and dynamic. Static addresses are assigned by a network administrator according to a preconceived internetwork addressing plan. A static address does not change until the network administrator manually changes it. Dynamic addresses are obtained by devices when they attach to a network, by means of some protocol-specific process. A device using a dynamic address often has a different address each time that it connects to the network. Some networks use a server to assign addresses. Server-assigned addresses are recycled for reuse as devices disconnect. 

A device is therefore likely to have a different address each time that it connects to the network. 

 

*Hierarchical Versus Flat Address Space

Internetwork address space typically takes one of two forms: hierarchical address space or flat address space. A hierarchical address space is organized into numerous subgroups, each successively narrowing an address until it points to a single device (in a manner similar to street addresses). A flat address space is organized into a single group (in a manner similar to U.S. Social Security numbers).

 

Hierarchical addressing offers certain advantages over flat-addressing schemes. Address sorting and recall is simplified using comparison operations. For example, "Ireland" in a street address eliminates any other country as a possible location. Figure illustrates the difference between hierarchical and flat address spaces. Figure Hierarchical and Flat Address Spaces Differ in Comparison Operations.

*Network Layer Addresses

A network layer address identifies an entity at the network layer of the OSI layers. Network addresses usually exist within a hierarchical address space and sometimes are called virtual or logical addresses. 

The relationship between a network address and a device is logical and unfixed; it typically is based either on physical network characteristics (the device is on a particular network segment) or on groupings that have no physical basis (the device is part of an AppleTalk zone). End systems require one network layer address for each network layer protocol that they support. (This assumes that the device has only one physical network connection.) Routers and other internetworking devices require one network layer address per physical network connection for each network layer protocol supported. For example, a router with three interfaces each running AppleTalk, TCP/IP, and OSI must have three network layer addresses for each interface. The router therefore has nine network layer addresses. Figure 1-16 illustrates how each network interface must be assigned a network address for each protocol supported.

Figure Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported.

*Mapping Addresses

 Because internetworks generally use network addresses to route traffic around the network, there is a need to map network addresses to MAC addresses. When the network layer has determined the destination station's network address, it must forward the information over a physical network using a MAC address. Different protocol suites use different methods to perform this mapping, but the most popular is Address Resolution Protocol (ARP). 

Different protocol suites use different methods for determining the MAC address of a device. The following three methods are used most often. Address Resolution Protocol (ARP) maps network addresses to MAC addresses. The Hello protocol enables network devices to learn the MAC addresses of other network devices. MAC addresses either are embedded in the network layer address or are generated by an algorithm. 

Address Resolution Protocol (ARP) is the method used in the TCP/IP suite. When a network device needs to send data to another device on the same network, it knows the source and destination network addresses for the data transfer. It must somehow map the destination address to a MAC address before forwarding the data. First, the sending station will check its ARP table to see if it has already discovered this destination station's MAC address. If it has not, it will send a broadcast on the network with the destination station's IP address contained in the broadcast. Every station on the network receives the broadcast and compares the embedded IP address to its own. Only the station with the matching IP address replies to the sending station with a packet containing the MAC address for the station. The first station then adds this information to its ARP table for future reference and proceeds to transfer the data.  

When the destination device lies on a remote network, one beyond a router, the process is the same except that the sending station sends the ARP request for the MAC address of its default gateway. It then forwards the information to that device. The default gateway will then forward the information over whatever networks necessary to deliver the packet to the network on which the destination device resides. The router on the destination device's network then uses ARP to obtain the MAC of the actual destination device and delivers the packet. 

The Hello protocol is a network layer protocol that enables network devices to identify one another and indicate that they are still functional. When a new end system powers up, for example, it broadcasts hello messages onto the network. Devices on the network then return hello replies, and hello messages are also sent at specific intervals to indicate that they are still functional. Network devices can learn the MAC addresses of other devices by examining Hello protocol packets. 

Three protocols use predictable MAC addresses. In these protocol suites, MAC addresses are predictable because the network layer either embeds the MAC address in the network layer address or uses an algorithm to determine the MAC address. The three protocols are Xerox Network Systems (XNS), Novell Internetwork Packet Exchange (IPX), and DECnet Phase IV.

 

*MAC Addresses

Media Access Control (MAC) addresses consist of a subset of data link layer addresses. MAC addresses identify network entities in LANs that implement the IEEE MAC addresses of the data link layer. As with most data-link addresses, MAC addresses are unique for each LAN interface. Figure 1-14 illustrates the relationship between MAC addresses, data-link addresses, and the IEEE sublayers of the data link layer.  

Figure MAC Addresses, Data-Link Addresses, and the IEEE Sublayers of the Data Link Layer Are All Related.

MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the Organizationally Unique Identifier (OUI). The last 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. MAC addresses sometimes are called burned-in addresses (BIAs) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the interface card initializes.

 Figure  illustrates the MAC address format. 

Figure  The MAC Address Contains a Unique Format of Hexadecimal Digits

*Data Link Layer Addresses

A data link layer address uniquely identifies each physical network connection of a network device. Data-link addresses sometimes are referred to as physical or hardware addresses. Data-link addresses usually exist within a flat address space and have a pre-established and typically fixed relationship to a specific device.  

End systems generally have only one physical network connection and thus have only one data-link address. Routers and other internetworking devices typically have multiple physical network connections and therefore have multiple data-link addresses. Figure illustrates how each interface on a device is uniquely identified by a data-link address.

Figure Each Interface on a Device Is Uniquely Identified by a Data-Link Address.

5. Internetwork Addressing

Internetwork addresses identify devices separately or as members of a group. Addressing schemes vary depending on the protocol family and the OSI layer. Three types of internetwork addresses are commonly used: data link layer addresses, Media Access Control (MAC) addresses, and network layer addresses.

4. Internwtoerking devices

Internetworking devices which include Repeaters, Hubs, Routers, Switches, and Bridges. We started by defining each device followed by the various types and lastly how they work. Links were also provided for more information about each device. 

Like any type of technology, LANs have certain limitations. These limitations include the geographical distances that they cover, the number of nodes that can be connected, or the need to run more than one protocol. The devices within a network and the functions they support make a significant impact on the limitations of the network. Various types of hardware devices can be used to extend LAN functionality, each offering a specific function in exchange for added cost and complexity.

There are a variety of devices used to extend LANs, from a simple Network Interface Card (NIC) that allows you to connect to a network to a sophisticated router that can move information across the Internet. However, each device performs a specific function that, when combined with the functions of others, enables the LAN to transfer information from Point A to Point B.

•  Network Interface Cards (NICs)

The Network Interface Card (NIC) is a circuit board that is physically installed within an active network node, such as a computer, server, or printer. The NIC is an adapter that controls the exchange of information between the network and the user.  

Repeaters/Hubs

A repeater extends the length of a network cabling system by amplifying the signal and then re-transmitting it. Repeaters operate at Physical Layer 1. As a result, they do not look at the data at all. Any information coming into one port is simply repeated out all other ports.

A hub is another Layer 1 device that is typically used as a central point for connecting segments in a LAN. While passive hubs simply pass packets from one port to another, the signal may be regenerated in active hubs called multi-port repeaters. Hubs are becoming increasingly intelligent, enabling them to support network management and minimal path selection functions.

Bridges

As networks grow larger, they are often divided into smaller LANs to reduce traffic drain on the network. A bridge is a Data Link Layer 2 device that provides a connection between separate LAN segments. The bridge monitors packets as they move between segments, keeping track of the MAC addresses that are associated with various ports. As they gain more knowledge of the nodes connected to each network, they are better able to manage traffic flow.

Switches

Switches are becoming a more common way to connect networks together because they are simply faster and more intelligent than bridges. Advances in technology spawned a new generation of networking devices known as LAN switches, which included bridging as one of several functions. Switches have replaced bridges for two reasons: superior performance and lower price per port.

Routers

Routers were originally invented to solve some of the problems that weren't addressed by bridges. Like bridges, routers are used to segment a LAN in order to reduce excess broadcast traffic and latency. In addition, routers make internetworking possible by interconnecting both local and wide area networks.

The function of a router is to direct data along the most efficient and economical route to the destination device. Routers operate at Network Layer 3, which means they examine the logical network address (for example, 191.29.21.100) and not the physical hardware address (MAC). 

Routers are smarter than bridges because they know about routing protocols, different address schemes, different frame sizes and different data rates in order to make the best decision on which path to choose. The best path is determined by using routing tables and algorithms.

Information Formats

The data and control information that is transmitted through internetworks takes a variety of forms. The terms used to refer to these information formats are not used consistently in the internetworking industry but sometimes are used interchangeably. Common information formats include frames, packets, datagrams, segments, messages, cells, and data units.  

A frame is an information unit whose source and destination are data link layer entities. A frame is composed of the data link layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the data link layer entity in the destination system. Data from upper-layer entities is encapsulated in the data link layer header and trailer. Figure illustrates the basic components of a data link layer frame. Figure  Data from Upper-Layer Entities Makes Up the Data Link Layer Frame

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A packet is an information unit whose source and destination are network layer entities. A packet is composed of the network layer header (and possibly a trailer) and upper-layer data. The header and trailer contain control information intended for the network layer entity in the destination system. Data from upper-layer entities is encapsulated in the network layer header and trailer. Figure illustrates the basic components of a network layer packet.

 Figure Three Basic Components Make Up a Network Layer Packet

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The term datagram usually refers to an information unit whose source and destination are network layer entities that use connectionless network service. The term segment usually refers to an information unit whose source and destination are transport layer entities.  

A message is an information unit whose source and destination entities exist above the network layer (often at the application layer). A cell is an information unit of a fixed size whose source and destination are data link layer entities. Cells are used in switched environments, such as Asynchronous Transfer Mode (ATM) and Switched Multimegabit Data Service (SMDS) networks.  

A cell is composed of the header and payload. The header contains control information intended for the destination data link layer entity and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long.

The length of the header and the payload fields always are the same for each cell.
Figure  depicts the components of a typical cell.

Figure Two Components Make Up a Typical Cell

Data unit is a generic term that refers to a variety of information units. Some common data units are service data units (SDUs), protocol data units, and bridge protocol data units (BPDUs). SDUs are information units from upper-layer protocols that define a service request to a lower-layer protocol. PDU is OSI terminology for a packet. BPDUs are used by the spanning-tree algorithm as hello messages.

 

3. OSI Layers

1. OSI Model Physical Layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications. Figure  illustrates some common LAN and WAN physical layer implementations.

Figure Physical Layer Implementations Can Be LAN or WAN Specifications.

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2. OSI Model Data Link Layer 

The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time. 

The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). Figure illustrates the IEEE sublayers of the data link layer. Figure  The Data Link Layer Contains Two Sublayers.

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The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link. The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.

3. OSI Model Network Layer 

The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.

 4.OSI Model Transport Layer 

The transport layer accepts data from the session layer and segments the data for transport across the network. Generally, the transport layer is responsible for making sure that the data is delivered error-free and in the proper sequence. Flow control generally occurs at the transport layer. 

Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur. The transport protocols used on the Internet are TCP and UDP.

 5. OSI Model Session Layer 

The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.

 6. OSI Model Presentation Layer 

The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes. 

Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types of computer systems. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly deciphered at the destination. 

Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding. 

Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF). GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding format for graphic images.

 7.  OSI Model Application Layer 

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. 

This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication.

When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit.

When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.  

Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).

Information Exchange Process

The information exchange process occurs between peer OSI layers. Each layer in the source system adds control information to data, and each layer in the destination system analyzes and removes the control information from that data. 

If System A has data from a software application to send to System B, the data is passed to the application layer. The application layer in System A then communicates any control information required by the application layer in System B by prepending a header to the data. The resulting information unit (a header and the data) is passed to the presentation layer, which prepends its own header containing control information intended for the presentation layer in System B. The information unit grows in size as each layer prepends its own header (and, in some cases, a trailer) that contains control information to be used by its peer layer in System B. At the physical layer, the entire information unit is placed onto the network medium. 

The physical layer in System B receives the information unit and passes it to the data link layer. The data link layer in System B then reads the control information contained in the header prepended by the data link layer in System A. The header is then removed, and the remainder of the information unit is passed to the network layer. Each layer performs the same actions: The layer reads the header from its peer layer, strips it off, and passes the remaining information unit to the next highest layer. After the application layer performs these actions, the data is passed to the recipient software application in System B, in exactly the form in which it was transmitted by the application in System A.

OSI Model Layers and Information Exchange

The seven OSI layers use various forms of control information to communicate with their peer layers in other computer systems. This control information consists of specific requests and instructions that are exchanged between peer OSI layers. 

Control information typically takes one of two forms: headers and trailers. Headers are prep ended to data that has been passed down from upper layers. Trailers are appended to data that has been passed down from upper layers. An OSI layer is not required to attach a header or a trailer to data from upper layers. 

Headers, trailers, and data are relative concepts, depending on the layer that analyzes the information unit. At the network layer, for example, an information unit consists of a Layer 3 header and data. At the data link layer, however, all the information passed down by the network layer (the Layer 3 header and the data) is treated as data.

In other words, the data portion of an information unit at a given OSI layer potentially
can contain headers, trailers, and data from all the higher layers. This is known as encapsulation. Figure shows how the header and data from one layer are encapsulated into the header of the next lowest layer.

Figure  Headers and Data Can Be Encapsulated During Information Exchange

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*OSI Layer Services

One OSI layer communicates with another layer to make use of the services provided by the second layer. The services provided by adjacent layers help a given OSI layer communicate with its peer layer in other computer systems. Three basic elements are involved in layer services: the service user, the service provider, and the service access point (SAP). 

In this context, the service user is the OSI layer that requests services from an adjacent OSI layer. The service provider is the OSI layer that provides services to service users. OSI layers can provide services to multiple service users. The SAP is a conceptual location at which one OSI layer can request the services of another OSI layer.

Figure illustrates how these three elements interact at the network and data link layers.

Figure  Service Users, Providers, and SAPs Interact at the Network and Data Link Layers

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*Interaction Between OSI Model Layers

A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, and its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B. Figure illustrates this example.

Figure  OSI Model Layers Communicate with Other Layers

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*OSI Model and Communication Between Systems

Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B, the application program in System A will pass its information to the application layer (Layer 7) of System A. The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application program to complete the communication process.

*Protocols

The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. In the context of data networking, a protocol is a formal set of rules and conventions that governs how computers exchange information over a network medium. A protocol implements the functions of one or more of the OSI layers.

 A wide variety of communication protocols exist. Some of these protocols include LAN protocols, WAN protocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layers of the OSI model and define communication over the various LAN media. WAN protocols operate at the lowest three layers of the OSI model and define communication over the various wide-area media. Routing protocols are network layer protocols that are responsible for exchanging information between routers so that the routers can select the proper path for network traffic. Finally, network protocols are the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for operation. For example, many routing protocols use network protocols to exchange information between routers. This concept of building upon the layers already in existence is the foundation of the OSI model. 

*Characteristics of the OSI Layers

The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers. 

The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model. 

The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium. 

Figure  illustrates the division between the upper and lower OSI layers.

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2. Open System Interconnection Reference Model

The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions. The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for inter computer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers. The following list details the seven layers of the Open System Interconnection (OSI) reference model:

         •Layer 7—Application 

         •Layer 6—Presentation 

         •Layer 5—Session 

         •Layer 4—Transport 

         •Layer 3—Network 

         •Layer 2—Data link 

         •Layer 1—Physical 

Note A handy way to remember the seven layers is the sentence "All people seem to need data processing." The beginning letter of each word corresponds to a layer.

•All—Application layer 

•People—Presentation layer 

•Seem—Session layer 

•To—Transport layer 

•Need—Network layer 

•Data—Data link layer

•Processing—Physical layer  

 

*History of Internetworking

The first networks were time-sharing networks that used mainframes and attached terminals. Such environments were implemented by both IBM's Systems Network Architecture (SNA) and Digital's network architecture. 

Local-area networks (LANs) evolved around the PC revolution. LANs enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers. 

Wide-area networks (WANs) interconnect LANs with geographically dispersed users to create connectivity. Some of the technologies used for connecting LANs include T1, T3, ATM, ISDN, ADSL, Frame Relay, radio links, and others. New methods of connecting dispersed LANs are appearing everyday. 

Today, high-speed LANs and switched internetworks are becoming widely used, largely because they operate at very high speeds and support such high-bandwidth applications as multimedia and videoconferencing.

Internetworking evolved as a solution to three key problems: isolated LANs, duplication of resources, and a lack of network management. Isolated LANs made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did separate support staff. This lack of network management meant that no centralized method of managing and troubleshooting networks existed.

 

1. Internetworking

1. Introduction

An internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. Internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks. Figure illustrates some different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork. 

Figure displays Different Network Technologies Can Be Connected to Create an Internetwork

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