Like the name suggests, LANs are limited to a local or small geographical area. An example of a LAN would be a network of individual computers or workstations that are connected in a single department. These users have shared access to resources such as data and network devices. Users on a LAN segment can share a network printer and communicate with one another via email. Also, they are governed by one authoritative administrator.

Given the size constraints, downsides of a LAN network are limited distance that data can travel and a limited number of computers that can be connected. An upside of a LAN is fast data transfer with data speed that can reach up to 10Gbps.

Xerox Corporation worked in collaboration with DEC and Intel to create ethernet, which is the most pervasive LAN architecture used today. Ethernet has evolved and has seen significant improvements in regard to speed and efficiency.

Other significant LAN technologies are Fiber Distributed Data Interface (FDDI) and token ring.

Local area networking uses switches, bridges and/or repeaters, and hubs to interconnect LANs and increase overall size. Routers are used to connect a LAN to a WAN or MAN. Both of these scenarios form an internetwork.

Figure 1.1 provides an example of a LAN. A single switch connects to all the peripheral network devices in this example.

Figure 1.1. Example of a LAN.

A MAN is larger than a LAN but smaller than or equal in size to a WAN. Think of it as the size of a city or college campus network, which can range anywhere from 5 to 50km in diameter. MANs are typically owned and managed by a single entity. This could be an ISP or telecommunications company that sells its services to end-users in that metropolitan area. For all intents and purposes, a MAN has the same characteristics as a WAN with distance constraints.

WANs cover more than one geographical area. This is ideal for a company that has offices in different cities around the country or even the world. Each office can connect to the other sites in the WAN via a router. Connectivity from router to router is a circuit leased from a telephone or communications company, such as AT&T to name one. The larger the circuit a company needs to transmit data, the more it costs to lease. The company also needs to pay close attention to the performance of its WAN connection because that cost can directly impact its ability to do business. It is important to keep an eye on the amount of traffic that is going over each circuit to ensure that you have sufficient throughput. Throughput refers to the amount of data transferred in a specified timeframe.

The following are WAN encapsulations that are reviewed in Chapter 17, “Wide Area Network Connections”:

Figure 1.2 provides an example of a WAN. In this example, a central router connects two LANs to create the WAN. Each LAN has a switch connected to its local end-user devices.

Figure 1.2. Example of a WAN.

SAN may be referred to as a subnetwork or special purpose network. Its special purpose is to allow users on a larger network to connect various data storage devices with clusters of data servers. Cisco offers this service with its Cisco MDS 9000 Series Multilayer SAN Switches. These switches provide scalable storage solutions for the end user.

VPN is a private network that can access public networks remotely. VPN uses encryption and security protocols to retain privacy while it accesses outside resources. When employed on a network, VPN enables an end user to create a virtual tunnel to a remote location. Typically, telecommuters use VPN to log in to their company networks from home.

Now that you know the layers in order, it is also important to know that the layers may also be referred to as the upper and lower layers. Primarily, the upper layers of the OSI model define communications between applications that reside on end-user stations. This is generally related to software communication. Table 1.2 provides a list of the layers considered as the upper layers.

Layer 7 provides an interface between a host’s communication software and any necessary external applications (such as email, file transfers, and terminal emulation). This layer can also evaluate what resources are necessary to communicate between two devices and determine their availability.

Layer 7 also provides the following functionality:

Table 1.3 provides a list of the protocols supported by the Application layer.

Layer 6 presents data to the Application layer and acts as a data format translator. Format translation is necessary to ensure that the data can be read by applications. Layer 6 also handles the structuring of data and negotiating data transfer syntax to Layer 7. Processes involved include data encryption, decryption, compression, and decompression.

Layer 6 protocols include the following:

Layer 5 is primarily concerned with dialog control among devices. This layer determines the beginning, middle, and end of a session or conversation that occurs between applications. In this way, the Session layer acts as an intermediary for those applications. Table 1.4 lists the Session layer protocols and their functionality.

The lower layers of the OSI model focus on data transport, which can be achieved via a router, switch, or a physical wire. They are listed in Table 1.5.

Layer 4 is responsible for end-to-end connections and data delivery between two hosts. The ability to segment and reassemble data is a key functionality of this layer. For example, when one system is sending data to another system, that data can be segmented into smaller data blocks and transmitted across the network. The receiving system can then reassemble the segmented data blocks at the Transport layer. Transmissions occur via logical connectivity between the sender and destination. Layer 4 provides transparent data transfer by hiding details of the transmission from the upper layers.

Layer 4 also provides the following functionality:

The Transport layer can provide reliable networking via acknowledgments, sequencing, and flow control.

Layer 4 protocols include the following:

Layer 3 is where the best path determination is made for packet delivery across the network. Routed protocols such as IP are used to determine logical addressing, which can identify the destination of a packet or datagram. The most common network device found at the Network layer is a router; however, Layer 3 switches may also be implemented.

A router at the Network layer follows these general steps to ensure proper data transport:

A routing table on a router contains the following information:

There are two packet types utilized at Layer 3:

Layer 3 routed protocols include the following:

Layer 2 ensures reliable data transfer from the Network layer to the Physical layer for transmission across the network.

Two domains determine data transport reliability:

Data received from the Network layer is formatted into frames to be transmitted to the Physical layer. Physical addressing or hardware addressing (rather than logical addressing) ensures that data is delivered to the appropriate node on the LAN. This layer is also responsible for error notification (not correction), network topology, and flow control.

This is the only layer of the OSI model that has sublayers. The two sublayers in question define the IEEE Ethernet 802.3 frame, which in turn provides physical addressing and flow control. Also, routed protocol information (IP, IPX, AppleTalk, and so on) is provided to the upper layers.

The IEEE Ethernet 802.3 sublayers are Media Access Control (MAC) and Logical Link Control (LLC), and are described in the following sections.

Logical Link Control (LLC)

The LLC sublayer complements the MAC sublayer in the ethernet model; the LLC is responsible for framing, error, and flow control. LLC provides a service access point (SAP) identifier in the frame. The SAP field of the frame consists of one byte that identifies an upper layer protocol (for example, 06 = IP, whereas E0 = IPX). The LLC inserts a destination SAP (DSAP) and a Source SAP (SSAP) in the frame. Figure 1.3 provides an example of an ethernet frame.

Figure 1.3. Example of an ethernet frame.

Two devices are used at the Data Link layer:

• Bridges—. Bridges connect two segments in a single network or two networks together. They simply forward data between those segments/networks without performing an analysis or redirection of the data.

• Switches—. At Layer 2, switches are multi-port bridges that utilize Application Specific Integrated Circuit (ASIC) to forward frames. Each port of the switch has a dedicated bandwidth.

Exam Alert

Dedicated bandwidth enables the switch port to guarantee the speed assigned to that port. For example, 100Mbps port connections get 100Mbps transmission rates.

Although both devices create a separate collision domain for each connected device, all the devices connected to either are a part of the same broadcast domain. Remember that broadcast domains are segmented at the Network layer by routers.

Switches and bridges identify MAC addresses by scanning for the source MAC address of each frame received.

Note

Bridging and switching are discussed in more detail in Chapter 3.

Layer 1 moves bits between nodes. Electrical, mechanical, procedural, and functional requirements are defined at the Physical layer to assist with the activation, maintenance, and deactivation of physical connectivity between devices.

Other attributes of Layer 1 include the following:

Devices at the Physical layer include hubs and repeaters. Hubs and repeaters extend a network, whereas Layer 2 and Layer 3 devices segment a network.

Now that you have reviewed all seven layers of the OSI model, it is a good time to see how those layers communicate with each other. Each layer passes information to adjacent layers by using Protocol Data Units (PDUs). The PDU includes both the message and the protocol/control information from the forwarding layer. That control information can be in the form of a header or trailer. The process of adding a header or trailer to the PDU at each layer of the OSI is called encapsulation.

Each layer has an associated control information name, which is listed in Table 1.6.

Based on this chart, you can see how information is encapsulated as it travels down through the various layers. The correct order for data encapsulation is data, segment, packet, frame, and bit.

This layer combines functionalities of the three top layers of the OSI model and may also be called the Process/Application layer. Also, some of the most popular applications (email, file transport, and so on) interface with this layer to communicate with other applications on the network.

If you’ll remember, the description of the Application layer of the OSI model included a list of application protocols and their primary functions. (Refer to Table 1.3.) These applications are also relative to the Application layer of the TCP/IP model.

Table 1.9 provides a quick list of the protocols at their respective layers of the TCP/IP model.

The Transport layer corresponds with the Transport layer of the OSI model and is also known as the Host-to-Host layer. Not only is this layer responsible for reliable data delivery, but it can also make certain that data arrives in the proper order. You will see two transport layer protocols on the CCENT exam. These protocols are TCP and UDP. The following sections cover each protocol and its related applications.

The Internet layer corresponds with the Network layer of the OSI model.

The following protocols relate to the logical transmission of packets:

IP

IP uses logical or virtual addressing to get a packet from a source to its destination. IP addresses are used by routers to make forwarding decisions.

Some key characteristics of IP addresses include the following:

• Addresses are allocated by the Internet Assigned Numbers Authority (IANA).

• IPv4 IP addresses are 32 bits, divided into four octets (8 bits each). An example of an IP address in dotted decimal format would be 172.16.122.204.

• The minimum value (per octet) is 0 and the maximum value is 255.

• IPv6, which is the future of IP addresses, is 128 bits.

Figure 1.6 shows the data fields that make up an IP datagram.

Figure 1.6. IP datagram.

Note

IP addressing is a topic discussed with additional detail in Chapter 5.

C:Documents and Settings>ping 10.0.0.1

Pinging 10.0.0.1 with 32 bytes of data:

Reply from 10.1.1.1: bytes=32 time<1ms TTL=255
Reply from 10.1.1.1: bytes=32 time<1ms TTL=255
Reply from 10.1.1.1: bytes=32 time<1ms TTL=255
Reply from 10.1.1.1: bytes=32 time<1ms TTL=255

Ping statistics for 10.0.0.1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

C:Documents and Settings>ping 10.0.0.2

Pinging 10.0.0.2 with 32 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 10.0.0.2:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss)

This layer corresponds with the Data Link and Physical layers of the OSI model. As mentioned earlier in the chapter, this layer manages hardware addressing and physical data transfer.

End users are connected at the Access layer; therefore, this layer may also be referred to as the desktop layer. These end users may also be combined to form a workgroup. Virtual LAN (VLAN) workgroups are defined by virtual access lists or filter lists at the Access layer to allow for a continuation of the policies implemented at the Distribution layer. This functionality further controls internetwork resource access granted to each end user or workgroup. Users may access locally available resources at this level or they may be directed to the Distribution layer to access remotely available resources.

In the hierarchy, the Distribution layer is the middle man between the access and core layers. You may also hear this layer called the workgroup layer. Although the Distribution layer acts as a gathering point for the Access layer devices, it also uses a router or Layer 3 switch whenever necessary to determine how to traverse packets to the core layer.

You achieve traffic control at this layer by using various policies that ultimately provide network management and security.

Primary functions of the Distribution layer include the following:

The Core layer is the foundation or backbone of the network. Much like a building would falter without its foundation, a network would fall apart without the structure provided by the core layer. As mentioned before, the Distribution layer manages access to the core. This enables the core to focus on speed and reliability. The goal is to provide high-speed switching as quickly and efficiently as possible. Any latency or delay can affect everyone on that network. Because speed is of the essence, the policies implemented at the Distribution layer (for example, filtering, access lists, and so on) should not occur at the Core layer.

Redundancy and fault tolerance are also important to the successful design of a core. If a network is set up with full redundancy, any failures should be transparent to the end user, which is the definition of a fault tolerant network.

If you were tasked with the design of a new network, would you select a Layer 2 core or a Layer 3 core? Although the Layer 2 core consists of a switched hierarchical setup, a Layer 3 core consists of a routed hierarchical setup.

The answer depends upon the individual requirements of the company in question. If the primary requirement is speed, perhaps a Layer 2 core is appropriate. If a stated desire is the additional network control that is available with a routed solution, then a Layer 3 core would fit the bill.

Although the hierarchical model is typically mentioned in published reviews for the CCENT examination, Cisco also created the Enterprise Composite Network Model (ECNM) for larger-scale network implementations.

The hierarchical model can assist in the implementation of a small- to moderate-sized network. The composite model goes a step further and provides a guide for creating a larger network. Because Cisco has a group of topics listed as “Planning and Designing,” it is important to have a general knowledge of their models for network design.

The following are some recommended readings on the subject of standard internetworking models: