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N10-004 Network+ TechNotes
Network components
COLLISION DOMAIN

As you may have read in our Media and Topologies TechNotes, collisions occur on Ethernet networks when multiple nodes on the 'network' put a signal on the wire at exactly the same time and collide with each other. In today's large-fast-growing-bandwidth-eating network environments, this can quickly become a serious problem. When more collisions occur, stations will have to wait longer before they can transmit data, decreasing performance for all nodes in the same collision domain. Networks can be separated in to multiple collisions domains by using the appropriate device. Where exactly the boundaries of a collision domain lie, will be made clear using a network diagram for each of the relevant network components below.



BROADCAST DOMAIN

All devices in the same broadcast domain will receive broadcast frames originating from any other device within the domain. Broadcast frames are frames explicitly directed to all nodes in the same network. Broadcast domains are typically bounded by routers because routers do not forward broadcast frames. Broadcast domains are essentially layer 2 segments, which can be extended or separated by using the appropriate network devices as discussed below.



REPEATERS

A repeaters is a simple device that is used to expand LANs over larger distances by connecting segments. They do not control broadcast or collision domains, they are not aware of upper-layer protocols and frame formats, they merely regenerate/amplify the signal. Repeater operate at the Physical layer of the OSI model. An important rule when using repeaters to expand a network is the 5-4-3 rule, which defines that the maximum distance between two hosts on the same network can be 5 segments, 4 repeaters, and only 3 of the segments can be populated, as illustrated in the following logical network diagram:




HUBS

Hubs, also known as concentrators or multiport repeaters, are used in star/hierarchical networks to connect multiple stations. A hub takes the incoming signal from one port and forwards it to all other ports. There are two main types of hubs: passive and active. A passive hub simply splits the signal and forwards it. An active hub takes the incoming frames, amplifies the signal, and forwards it. Some hubs can be managed allowing individual port configuration and traffic monitoring, these are know as intelligent- or managed hubs.

Hubs operate on the Physical layer of the OSI model and they are protocol transparent. That means they are not aware of the upper-layer protocols such as IP, IPX, nor MAC addressing. Hence they do not control broadcast or collision domains, but they extend them as illustrated below:




The following is a picture of a Fast Ethernet hub.




BRIDGES

Bridges are more intelligent than hubs; they operate on the Data Link layer of the OSI model. They are used to increase network performance by segmenting networks in separate collision domains. Bridges are also protocol transparent, meaning they are not aware of the upper-layer protocols. A bridge maintains a table with MAC addresses of all attached nodes, and on which segment they are located. It takes an incoming frame, reads the destination MAC address and consults the table to decide what should be done with the frame. If the location of the destination MAC address is listed in the table, the frame is forwarded to the corresponding port. The frame will be discarded if the destination port is the same as the port from which the frame arrived. If the location is not known yet, the frame will be flooded through all outgoing ports/segments. This is also true for broadcast frames.

As illustrated below, bridges control collision domains, they do not control broadcast domains:




SWITCHES

Switches were developed to improve network performance even more. Switches are very similar to bridges as they also maintain a table with MAC addresses per port to make forwarding decisions, operate at the Data Link layer (layer 2) of the OSI model, and are protocol transparent. Some of the main differences between switches and bridges are:

- Switches have more ports than bridges. Switches are meant to replace hubs and improve network performance by creating a separate collision domain per port.
- Bridges switch in software whereas switches switch in hardware (integrated circuits).
- Switches offer more variance in speed; an individual port can be assigned 10 Mb/s, 100 Mb/s, 1 Gb/s or even more.

As illustrated below, switches control collision domains, they do not control broadcast domains by default:



However, switches can control broadcast domains when Virtual Local Area Networks (VLANs) are configured. Most modern switches support VLANs, which are logical groups of network devices in which the members can be located on different physical segments.

Virtual Local Area Networks (VLANs) offer the following main benefits:
- Scalability – members of a VLAN can be miles apart and still act as a single LAN.
- Manageability – members can be easily relocated to a different VLAN without having to change the physical connection.
- Security – traffic to and from VLANs can be filtered or simply not implemented.

A VLAN can be based on Port IDs, MAC addresses, protocols or applications even. For example, port 1 to 12 on a switch could be assigned to VLAN 1, and port 13 to 24 to VLAN 2, resulting in two different broadcast domains. An example of a large network with VLANs is an office building with a switch on each of the three floors and a main switch connecting them all together. An administrator would be able to maintain a list of MAC addresses, assign stations from different floors to a single VLAN, and for example create a VLAN (hence separate broadcast domain) for each department in the company. Switches can share their MAC address table information with other switches so the path to a destination can be quickly found.

The following diagram represents a switch configured with two VLANs. As in the previous diagram, each port forms a collision domain, and as you can see in this diagram, the network is separated in two broadcast domains using VLANs. If the network protocol used in this network would be TCP/IP, the VLANs would each have its own (sub-)network address, for example VLAN 1 could be assigned the class C 192.168.110.x and VLAN 2 192.168.220.x. A router would have to be attached to the switch to allow actual communication between the VLANs configured on one or multiple switches.





ROUTERS

Routers are used to interconnect multiple (sub-)networks and route information between these networks by choosing an optimal path ("route") to the destination. They operate on the Network layer (Layer 3) of the OSI model and in contradiction to hubs, bridges, and switches, routers are protocol-aware. Examples of these layer 3 routed protocols are IP, IPX, and AppleTalk. Routers make forwarding decisions based on a table with network addresses and there corresponding ports, this table is known as the route table. Common use of routers is to connect different type of networks (for example 100BaseTX and ATM, or 100BaseFX and Frame Relay) and to interconnect LANs into a WAN. The concept of routing will be covered in more detail in another TechNote covering the most popular routed protocol: TCP/IP.

As illustrated below, routers control collision domains and broadcast domains:





The network components described above are often used in conjuction. The following network diagram shows a simple network using three of them:


GATEWAYS

A gateway is a hardware device or a computer running software that allows communication between networks with dissimilar network protocols or architectures. Gateways are very intelligent devices, generally they operate on the Transport layer and higher (Session, Presentation, Application). A gateway could be used to allow IPX/SPX clients access to the Internet through a TCP/IP uplink. The gateway would convert IPX/SPX traffic to TCP/IP and vice versa. Another common use of a gateway is to connect an Ethernet network to an IBM SNA mainframe environment.


CSU/DSU

A CSU/DSU (Channel Service Unit/Data Service Unit) is a hardware device about the size of an external modem, which converts digital data frames from the communication technology used on a local area network (LAN) into frames appropriate to a wide-area network (WAN) and vice versa. A CSU/DSU is primarily used on both ends of a T-1 or T-3 connection. A T1 or T3 is a fast digital leased line, often used for high-speed internet connections (will be covered in more detail in our WAN Technologies TechNotes).


NICs

A Network Interface Card (NIC), typically an expansion card in a computer, is used to connect a system to the physical network media. Some mainboards and most portable computers are equipped with a built-in (onboard) NIC. NICs are available for different types of network media, the most common today being Ethernet NICs with a RJ-45 socket for UTP/STP cabling and wireless network adapters with an antenna. To install a network interface card you need a free ISA, PCI, PCMCIA, USB, or other expansion slot or port and an appropriate driver, which the computer's operating system will use to communicate with the NIC. Some older ISA NICs can be manually configured to use a particular IRQ. This is done by setting jumpers or dip switches. Some other NICs allow the IRQ and other settings to be configured by using configuration software.

A NIC provides operations up to layer 2 of the OSI model. The NIC's interface itself is a Physical layer (layer 1) device, the physical address (also known as MAC address) of the adapter as well as the drivers to control the NIC are located at the Data Link layer's MAC sub-layer. In an Ethernet network for example, every NIC attached to the same segment receive every ‘frame’ to discover the MAC address. Frames that do not match the local NIC’s MAC address are discarded; frames that do match the local NIC’s address are forwarded up the OSI model to the next layer to be processed by the network layer protocol. Obviously, a NIC must be able to interpret the MAC address, hence operate up to the MAC sub-layer of layer 2 of the OSI model.



An image of a Fast Ethernet network interface card.

Most of today's NICs are equipped with status indicators in the form of LEDs. These LEDs can be used to troubleshoot network problems. A green led indicates the NIC is physically connected to the network and flashes when activity occurs. I.e. the port is transmitting or receiving data; this is also known as the heartbeat. When the NIC supports multiple speeds, for example 10 and 100 Mbps, there can be a green led for each speed, of which one is lit, indicating the current speed. Some NICs, as well as other network devices such as hubs, include an orange or red LED that flashes when collisions occur. If the collision LED flashes repeatedly or continuously there may be other devices utilizing the network heavily, or the NIC maybe be configured incorrectly or may be malfunctioning.

As described earlier, network interfaces are physically configured with an address known as the MAC address (MAC is short for Media Access Layer), layer 2 address, Burned In Address (BIA), or physical address. The following is an example of a MAC address: 00-10-E3-42-A8-BC. The first six hexadecimal digits specify the vendor/manufacturer of the NIC; the other six define the host. MAC addresses are supposedly unique across the planet.


MODEMS


Modems are used for low-speed long-distance connections over telephone lines. They convert parallel digital data into serial analog data and vice versa. This allows digital devices such as computers to communicate over an analog medium.

There are two main types of modems:
- Internal expansion cards (e.g. ISA, PCI) or 'On-board' (integrated in mainboard)
- External modems that connect to the serial RS-232 or USB port and often have their own power supply.

A telephone line is connected to the modem using a RJ-11 connector displayed below:




TRANSCEIVERS (MEDIA CONVERTERS)

Replacing the network interface when a different media type is being implemented can be expensive or even impossible if it is integrated into the network device. For example, when 10BaseT twisted-pair Ethernet started to replace 10Base2 and 10Base5 coaxial Ethernet, most of the network equipment in use, such as routers, didn’t have a RJ-45 socket but an 10Base5 AUI port. Transceivers, also referred to as media converters, were developed to overcome this problem and allow for a more affordable transition to newer network technologies. The following picture shows an Ethernet transceiver with an AUI Ethernet port on one side and an RJ-45 socket on the other.

More advanced media converters are available to connect copper media connection to fiber optic media, for example, transceivers that convert 10BaseT to 10BaseFL or 100BaseT to 100BaseFX. Or those that allow fiber optic media to connect to a IEEE 1394 interface and hence drastically increase the maximum distance.



 
Current related exam objectives for the Network+ N10-004 exam.
NOTE: The original exam objectives mention also ISDN adapters (will be covered in WAN Technologies TechNotes) and Wireless Access Points (will be covered in Wireless Networking TechNotes)

1.6 Identify the purposes, features and functions of the following network components:
- Hubs
- Switches
- Bridges
- Routers
- Gateways
- CSU / DSU (Channel Service Unit / Data Service Unit)
- NICs (Network Interface Card)
- Modems
- Transceivers (media converters)

2.1 Identify a MAC (Media Access Control) address and its parts.

2.4 Identify the OSI layers at which the following network components operate:
- Hubs
- Switches
- Bridges
- Routers
- Network Interface Cards

3.8 Identify the main characteristics of VLANs (Virtual Local Area Networks).

4.3 Given a network scenario, interpret visual indicators (For example: link LEDs (Light Emitting Diode) and collision LEDs (Light Emitting Diode)) to determine the nature of a stated problem.



Click here for the complete list of exam objectives.

Discuss this TechNote here Author: Johan Hiemstra




 

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