Only $2.99/month
Key Concepts:

Terms in this set (61)

An interface fitted inside a personal computer or network terminal which allows it to communicate with other machines over a network.

Network Interface Cards (NICs) connect a device to the network. Ethernet NICs are used for a wired connection

whereas WLAN (Wireless Local Area Network) NICs are used for wireless. An end-user device may include one or both types of NICs. A network printer, for example, may only have an Ethernet NIC, and therefore, must connect to the network using an Ethernet cable. Other devices, such as tablets and smartphones, might only contain a WLAN NIC and must use a wireless connection.

Not all physical connections are equal, in terms of the performance level, when connecting to a network.

For example, a wireless device will experience degradation in performance based on its distance from a wireless access point. The further the device is from the access point, the weaker the wireless signal it receives. This can mean less bandwidth or no wireless connection at all. Figure 2 shows that a wireless range extender can be used to regenerate the wireless signal to other parts of the house that are too far from the wireless access point. Alternatively, a wired connection will not degrade in performance.

All wireless devices must share access to the airwaves connecting to the wireless access point. This means slower network performance may occur as more wireless devices access the network simultaneously. A wired device does not need to share its access to the network with other devices. Each wired device has a separate communications channel over its Ethernet cable. This is important when considering some applications, such as online gaming, streaming video, and video conferencing, which require more dedicated bandwidth than other applications.
Data is transmitted on copper cables as electrical pulses. A detector in the network interface of a destination device must receive a signal that can be successfully decoded to match the signal sent. However, the longer the signal travels, the more it deteriorates. This is referred to as signal attenuation. For this reason, all copper media must follow strict distance limitations as specified by the guiding standards.

The timing and voltage values of the electrical pulses are also susceptible to interference from two sources:

Electromagnetic interference (EMI) or radio frequency interference (RFI) - EMI and RFI signals can distort and corrupt the data signals being carried by copper media.

Potential sources of EMI and RFI include radio waves and electromagnetic devices, such as fluorescent lights or electric motors.

Crosstalk - Crosstalk is a disturbance caused by the electric or magnetic fields of a signal on one wire to the signal in an adjacent wire. In telephone circuits, crosstalk can result in hearing part of another voice conversation from an adjacent circuit. Specifically, when an electrical current flows through a wire, it creates a small, circular magnetic field around the wire, which can be picked up by an adjacent wire.

To counter the negative effects of EMI and RFI, some types of copper cables are wrapped in metallic shielding and require proper grounding connections.

To counter the negative effects of crosstalk, some types of copper cables have opposing circuit wire pairs twisted together, which effectively cancels the crosstalk.

The susceptibility of copper cables to electronic noise can also be limited by:

Selecting the cable type or category most suited to a given networking environment.
Designing a cable infrastructure to avoid known and potential sources of interference in the building structure.
Using cabling techniques that include the proper handling and termination of the cables.
Networks use copper media because it is inexpensive, easy to install, and has low resistance to electrical current. However, copper media is limited by distance and signal interference.

Data is transmitted on copper cables as electrical pulses. A detector in the network interface of a destination device must receive a signal that can be successfully decoded to match the signal sent. However, the longer the signal travels, the more it deteriorates. This is referred to as signal attenuation. For this reason, all copper media must follow strict distance limitations as specified by the guiding standards.

The timing and voltage values of the electrical pulses are also susceptible to interference from two sources:

Electromagnetic interference (EMI) or radio frequency interference (RFI) - EMI and RFI signals can distort and corrupt the data signals being carried by copper media. Potential sources of EMI and RFI include radio waves and electromagnetic devices, such as fluorescent lights or electric motors.
Crosstalk - Crosstalk is a disturbance caused by the electric or magnetic fields of a signal on one wire to the signal in an adjacent wire. In telephone circuits, crosstalk can result in hearing part of another voice conversation from an adjacent circuit. Specifically, when an electrical current flows through a wire, it creates a small, circular magnetic field around the wire, which can be picked up by an adjacent wire.

To counter the negative effects of EMI and RFI, some types of copper cables are wrapped in metallic shielding and require proper grounding connections.

To counter the negative effects of crosstalk, some types of copper cables have opposing circuit wire pairs twisted together, which effectively cancels the crosstalk.

The susceptibility of copper cables to electronic noise can also be limited by:

Selecting the cable type or category most suited to a given networking environment.
Designing a cable infrastructure to avoid known and potential sources of interference in the building structure.
Using cabling techniques that include the proper handling and termination of the cables.
UTP cabling conforms to the standards established jointly by the TIA/EIA. Specifically, TIA/EIA-568 stipulates the commercial cabling standards for LAN installations and is the standard most commonly used in LAN cabling environments. Some of the elements defined are:

Cable types
Cable lengths
Connectors
Cable termination
Methods of testing cable
The electrical characteristics of copper cabling are defined by the Institute of Electrical and Electronics Engineers (IEEE). IEEE rates UTP cabling according to its performance. Cables are placed into categories based on their ability to carry higher bandwidth rates. For example, Category 5 (Cat5) cable is used commonly in 100BASE-TX Fast Ethernet installations. Other categories include Enhanced Category 5 (Cat5e) cable, Category 6 (Cat6), and Category 6a.

Cables in higher categories are designed and constructed to support higher data rates. As new gigabit speed Ethernet technologies are being developed and adopted, Cat5e is now the minimally acceptable cable type, with Cat6 being the recommended type for new building installations.

Click each category of cable in the figure to learn more about their properties.

Some manufacturers are making cables exceeding the TIA/EIA Category 6a specifications and refer to these as Category 7.

category 3 UTP cable
used for voice communication manually phone calls.

Category 5 and 5e (UPT) cable
used for data transmissions
can siupport 100mb/s and can support 1000mb/s but not recommended.

Category 6 (UPT)
used for data transmissions
An added separator is between each pair of wires allowing
Optical fiber cable transmits data over longer distances and at higher bandwidths than any other networking media. Unlike copper wires, fiber-optic cable can transmit signals with less attenuation and is completely immune to EMI and RFI. Optical fiber is commonly used to interconnect network devices.

Optical fiber is a flexible, but extremely thin, transparent strand of very pure glass, not much bigger than a human hair. Bits are encoded on the fiber as light impulses. The fiber-optic cable acts as a waveguide, or "light pipe," to transmit light between the two ends with minimal loss of signal.

As an analogy, consider an empty paper towel roll with the inside coated like a mirror. It is a thousand meters in length, and a small laser pointer is used to send Morse code signals at the speed of light. Essentially that is how a fiber-optic cable operates, except that it is smaller in diameter and uses sophisticated light technologies.

Fiber-optic cabling is now being used in four types of industry:

Enterprise Networks: Used for backbone cabling applications and interconnecting infrastructure devices.

Fiber-to-the-Home (FTTH): Used to provide always-on broadband services to homes and small businesses.

Long-Haul Networks: Used by service providers to connect countries and cities.
Submarine Cable Networks: Used to provide reliable high-speed, high-capacity solutions capable of surviving in harsh undersea environments up to transoceanic distances.

Click here to view a telegeography map that depicts the location of submarine cables.
Wireless media carry electromagnetic signals that represent the binary digits of data communications using radio or microwave frequencies.

Wireless media provides the greatest mobility options of all media, and the number of wireless-enabled devices continues to increase. As network bandwidth options increase, wireless is quickly gaining in popularity in enterprise networks.

Wireless does have some areas of concern, including:

Coverage area: Wireless data communication technologies work well in open environments. However, certain construction materials used in buildings and structures, and the local terrain, will limit the effective coverage.
Interference: Wireless is susceptible to interference and can be disrupted by such common devices as household cordless phones, some types of fluorescent lights, microwave ovens, and other wireless communications.

Security: Wireless communication coverage requires no access to a physical strand of media. Therefore, devices and users, not authorized for access to the network, can gain access to the transmission. Network security is a major component of wireless network administration.

Shared medium: WLANs operate in half-duplex, which means only one device can send or receive at a time. The wireless medium is shared amongst all wireless users. The more users needing to access the WLAN simultaneously, results in less bandwidth for each user.

wireless is increasing in popularity for desktop connectivity, copper and fiber are the most popular physical layer media for network deployments.
WLANs, Ethernet LANs with hubs, and legacy Ethernet bus networks are all examples of contention-based access networks. All of these networks operate in half-duplex mode. This requires a process to govern when a device can send and what happens when multiple devices send at the same time.

The Carrier Sense Multiple Access/Collision Detection (CSMA/CD) process is used in half-duplex Ethernet LANs.

The CSMA process is as follows:

1. PC1 has an Ethernet frame to send to PC3.

2. PC1's NIC needs to determine if anyone is transmitting on the medium. If it does not detect a carrier signal, in other words, it is not receiving transmissions from another device, it will assume the network is available to send.

3. PC1's NIC sends the Ethernet Frame.

4. The Ethernet hub receives the frame. An Ethernet hub is also known as a multiport repeater. Any bits received on an incoming port are regenerated and sent out all other ports.

5. If another device, such as PC2, wants to transmit, but is currently receiving a frame, it must wait until the channel is clear.

6. All devices attached to the hub will receive the frame. Because the frame has a destination data link address for PC3, only that device will accept and copy in the entire frame. All other devices' NICs will ignore the frame.

If two devices transmit at the same time, a collision will occur. Both devices will detect the collision on the network, this is the collision detection (CD). This is done by the NIC comparing data transmitted with data received, or by recognizing the signal amplitude is higher than normal on the media. The data sent by both devices will be corrupted and will need to be resent.
Framing breaks the stream into decipherable groupings, with control information inserted in the header and trailer as values in different fields. This format gives the physical signals a structure that can be received by nodes and decoded into packets at the destination.

frame field types include:

Frame start and stop indicator flags - Used to identify the beginning and end limits of the frame.
Addressing - Indicates the source and destination nodes on the media.
Type - Identifies the Layer 3 protocol in the data field.
Control - Identifies special flow control services such as quality of service (QoS). QoS is used to give forwarding priority to certain types of messages. Data link frames carrying voice over IP (VoIP) packets normally receive priority because they are sensitive to delay.
Data - Contains the frame payload (i.e., packet header, segment header, and the data).
Error Detection - These frame fields are used for error detection and are included after the data to form the trailer.
Not all protocols include all of these fields. The standards for a specific data link protocol define the actual frame format.

Data link layer protocols add a trailer to the end of each frame. The trailer is used to determine if the frame arrived without error. This process is called error detection and is accomplished by placing a logical or mathematical summary of the bits that comprise the frame in the trailer. Error detection is added at the data link layer because the signals on the media could be subject to interference, distortion, or loss that would substantially change the bit values that those signals represent.

A transmitting node creates a logical summary of the contents of the frame, known as the cyclic redundancy check (CRC) value. This value is placed in the Frame Check Sequence (FCS) field to represent the contents of the frame. In the Ethernet trailer, the FCS provides a method for the receiving node to determine whether the frame experienced transmission errors.
The data link layer provides addressing that is used in transporting a frame across a shared local media. Device addresses at this layer are referred to as physical addresses. Data link layer addressing is contained within the frame header and specifies the frame destination node on the local network. The frame header may also contain the source address of the frame.

Unlike Layer 3 logical addresses, which are hierarchical, physical addresses do not indicate on what network the device is located. Rather, the physical address is unique to the specific device. If the device is moved to another network or subnet, it will still function with the same Layer 2 physical address.

function of the Layer 2 and Layer 3 addresses. As the IP packet travels from host-to-router, router-to-router, and finally router-to-host, at each point along the way the IP packet is encapsulated in a new data link frame. Each data link frame contains the source data link address of the NIC card sending the frame, and the destination data link address of the NIC card receiving the frame.

An address that is device-specific and non-hierarchical cannot be used to locate a device on large networks or the Internet. This would be like trying to find a single house within the entire world, with nothing more than a house number and street name. The physical address, however, can be used to locate a device within a limited area. For this reason, the data link layer address is only used for local delivery. Addresses at this layer have no meaning beyond the local network. Compare this to Layer 3, where addresses in the packet header are carried from the source host to the destination host, regardless of the number of network hops along the route.

If the data must pass onto another network segment, an intermediate device, such as a router, is necessary. The router must accept the frame based on the physical address and de-encapsulate the frame in order to examine the hierarchical address, or IP address. Using the IP address, the router is able to determine the network location of the destination device and the best path to reach it. When it knows where to forward the packet, the router then creates a new frame for the packet, and the new frame is sent on to the next network segment toward its final destination.
In a TCP/IP network, all OSI Layer 2 protocols work with IP at OSI Layer 3. However, the Layer 2 protocol used depends on the logical topology and the physical media.

Each protocol performs media access control for specified Layer 2 logical topologies. This means that a number of different network devices can act as nodes that operate at the data link layer when implementing these protocols. These devices include the NICs on computers as well as the interfaces on routers and Layer 2 switches.

The Layer 2 protocol used for a particular network topology is determined by the technology used to implement that topology. The technology is, in turn, determined by the size of the network - in terms of the number of hosts and the geographic scope - and the services to be provided over the network.

A LAN typically uses a high bandwidth technology that is capable of supporting large numbers of hosts. A LAN's relatively small geographic area (a single building or a multi-building campus) and its high density of users, make this technology cost-effective.

However, using a high bandwidth technology is usually not cost-effective for WANs that cover large geographic areas (cities or multiple cities, for example). The cost of the long distance physical links and the technology used to carry the signals over those distances typically results in lower bandwidth capacity.

The difference in bandwidth normally results in the use of different protocols for LANs and WANs.

Data link layer protocols include:

Ethernet
802.11 Wireless
Point-to-Point Protocol (PPP)
HDLC
Frame Relay
Click Play to see examples of Layer 2 protocols.