Chapter 4 - Network Access Flashcards
1
Q
What is the role of the data link layer?
A
On the sending device it’s role is to prepare data for transmission and control how that data accesses the physical medium. On the receiving end it receives the frame from the physical layer for acceptance and processing.
2
Q
What is the role of the physical layer?
A
On the sending device it’s role is to control how the data is transmitted onto the physical media by encoding the binary digits that represent data into signals. On the receiving end, the physical layer receives signals across the connecting media, and decodes the signal back into data, before passing the frame to the data link layer.
3
Q
What is needed for any network communications to occur?
A
A physical connection to a local network must be established. A physical connection can be a wired connection using a cable or a wireless connection using radio waves.
4
Q
What is an AP?
A
An AP is a wireless access point. In order to offer wireless capability, devices on a wireless network must be connected to a wireless access point.
5
Q
What are ISRs?
A
ISRs are Integrated Service Routers. They offer both wired and wireless connectivity into one device. ISRs offer a switching component with multiple ports allowing multiple devices to be connected to the local area network (LAN) using cables. Many ISRs also include an AP, allowing wireless devices to connect as well.
6
Q
What are NICs?
A
NICs are Network Interface Cards which connect a device to a network. Ethernet NICs are used for a wired connection. WLAN (Wireless local area network) NICs are used for wireless connections. An end-user may include one, or both types of NICs. For example a printer may only have an Ethernet NIC, and a phone or tablet may only have a WLAN NIC.
7
Q
What is the difference in performance level between physical connections?
A
A wireless device will experience degradation in performance based on its distance from a wireless access point. 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.
8
Q
What is the process that data undergoes from a source node to a destination node?
A
The user data is segmented by the transport layer, placed into packets by the network layer, and further encapsulated into frames by the data link layer.
The physical layer encodes the frames and creates the electrical, optical, or radio wave signals that represent the bits in each frame.
These signals are then sent on the media, one at a time.
The destination node physical layer retrieves these individual signals from the media, restores them to their bit representations, and passes the bits up to the data link layer as a complete frame.
9
Q
What are the basic forms of network media?
A
Copper cable - The signals are patterns of electrical pulses
Fiber-optic cable - The signals are patterns of light
Wireless - The signals are patterns of microwave transmissions.
10
Q
Who defines the services and protocols in the TCP/IP suite?
A
The Internet Engineering Task Force (IETF)
11
Q
Who defines and governs the physical layer standards?
A
- International Organization for Standardization (ISO)
- Telecommunications Industry Association/Electronic Industries Association (TIA/EIA)
- International Telecommunication Union (ITU)
- American National Standards Institute (ANSI)
- Institute of Electrical and Electronics Engineers (IEEE)
- National telecommunications regulatory authorities including the Federal Communication Commission (FCC) in the USA and the European Telecommunications Standards Institute (ETSI)
- Regional Cabling standards groups, such as Canadian Standards Association (CSA), European Committee for Electrotechnical Standardization (CENELEC), and JSA/JIS (Japanese Standards Association), which develop local specializations.
12
Q
What do the physical layer standards address?
A
Physical Components - The electronic hardware devices, media, and other connectors that transmit and carry the signals to represent the bits. Hardware components such as NICs, interfaces and connectors, cables materials, and cable designs are all specified in standards associated with the physical layer.
Encoding - A method of converting the stream of data bits into a predefined “code”. Codes are groupings of bits used to provide a predictable pattern that can be recognized by both the sender and the receiver. In networking encoding is a pattern of voltage or current used to represent bits; the 0s and 1s.
Signaling - The physical layer must generate the electrical, optical, or wireless signals that represent the “1” and “0” on the media. The method of representing the bits is called the signaling method. The physical layer standards must define what type of signal represents a “1” and what type of signal represents a “0”. This can be as simple as a change in the level of an electrical signal or optical pulse. For example, a long pulse could represent a 1, a short pulse, a 0.
13
Q
How does Manchester encoding work?
A
Manchester encoding represents a 0 bit by a high to low voltage transition, and a 1 bit as a low to high voltage transition. This type of encoding is used in 10b/s Ethernet, faster data rates require more complex encoding.
14
Q
What is modulation?
A
A common method to send data, it is the process by which the characteristic of one wave (the signal) modifies another wave (the carrier). This is how AM (Amplitude Modulation) and FM (Frequency Modulation) are used to send a signal.
15
Q
What is bandwidth?
A
The capacity of a medium to carry data. Digital bandwidth measure the amount of data that can flow from one place to another in a given amount of time. Bandwidth is sometimes thought of as the speed that bits travel, however that is not accurate. In both 10Mb/s and 100Mb/s Ethernet, the bits are sent at the speed of electricity. The difference is the number of bits that are transmitted per second.
16
Q
How is bandwidth measured?
A
Bandwidth is typically measured in kilobits per seconds (kb/s), megabits per second (Mb/s), or gigabits per second (Gb/s).
17
Q
What factors determine the practical bandwidth of a network?
A
- The properties of the physical media
- The technologies chosen for signaling and detecting network signals.
- Physical media properties, current technologies, and the laws of physics all play a role in determining the available bandwidth.
18
Q
What is throughput?
A
The measure of the transfer of bits across the media over a given period of time. Due to a number of factors, throughput usually does not match the specified bandwidth in physical layer implementations. On a network with multiple segments throughput cannot be faster than the slowest link in the path from source to destination. Even if all or most of the segments have high bandwidth, it will only take one segment in the path with low throughput to create a bottleneck to the throughput of the entire network.
19
Q
What might influence throughput?
A
- The amount of traffic
- The type of traffic
- The latency created by the number of network devices encountered between source and destination
20
Q
What is latency?
A
The amount of time, including delays, for data to travel from one given point to another.
21
Q
What is goodput?
A
Goodput is the measure of usable data transferred over a given period of time. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgments, and encapsulation.
22
Q
What are standards for copper media defined for?
A
- Type of copper cabling used
- Bandwidth of the communication
- Type of connectors used
- Pinout and color codes of connections to the media
- Maximum distance of the media
23
Q
What are the benefits and disadvantages of copper media?
A
Copper media is inexpensive, easy to install, and has low resistance to electrical current. However, copper media is limited by distance and signal interference.
24
Q
What is signal attenuation?
A
The longer a signal travels, the more it deteriorates. Copper media is prone to this, and must follow strict distance limitations as specified by the guiding standards.
25
What causes signal interference?
```
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.
Crosstalk - Crosstalk is a disturbance casused 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.
```
26
How does copper wire counteract the effects of signal interference and crosstalk?
To counteract EMI and RFI, some types of copper are wrapped in metallic shielding and require proper grounding connections.
To counteract the negative effects of crosstalk, some types of copper cables have opposing circuit wire pairs twisted together, which effectively cancels crosstalk.
It 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.
27
What are the main types of copper media used in networking?
Unshielded twisted-pair (UTP), shielded twisted-pair (STP), coaxial.
28
Describe a UTP.
Unshielded twisted-pair cabling is the most common network media. It's terminated with RJ-45 connectors. It's used for interconnecting network hosts with intermediate networking devices, such as switches and routers. Consists of four pairs of color-coded wires that have been twisted together and encased in a flexible plastic sheath that protects from minor physical damage. The twisting of wires helps protect against signal interference from other wires. The color codes identify the individual pairs of wires and aid in cable termination.
29
Describe an STP.
Shielded twisted-pair provides better noise protection than UTP cabling, however it is significantly more expensive and difficult to install. It uses an RJ-45 connector. It combines the techniques of shielding to counter EMI and RFI, and wire twisting to counter crosstalk. STP cables are terminated with special shielded STP data connectors. If improperly grounded the shield may act as an antenna and pick up unwanted signals. Each pair of wires is wrapped in a foil shield, and then they are wrapped in an overall metallic braid or foil.
30
Describe a coaxial cable.
Coax for short, they get their name from the fact that there are two conductors that share the same axis. Coaxial cable consists of:
-A copper conductor used to transmit electronic signals
-A layer of flexible plastic insulation surrounding a copper conductor
-The insulating material is surrounded in a woven copper braid, or metallic foil, that acts as the second wire in the circuit, and as a shield for the inner conductor. The second layer, or shield, reduces amount of outside electromagnetic interference.
-The entire cable is covered with a cable jacket to prevent minor physical damage.
Uses BNC, N type and F type connectors.
31
Where are coaxial cables used?
Primarily used in wireless installations, where coax cables attach antennas to wireless devices to carry radio frequency (RF) energy between the antennas and the radio equipment. Also used in cable internet installations, cable service providers provide internet connectivity to their customers by replacing portions of the coax cable and supporting amplification elements with fiber-optic cable. However, the wiring inside the customer's premises is still coax cable.
32
What hazards are copper media susceptible to?
Fire hazards, which exist because cable insulation and sheaths may be flammable, or produce toxic fumes when heated or burned.
Electrical hazards, which occur when copper wires conduct electricity in undesirable ways. For example, a defective network device could conduct currents to the chassis of other network devices. Networking cabling could also present undesirable voltage levels when used to connect devices that have power sources with different ground potentials. Finally, copper cabling may conduct voltages caused by lightning strikes to network devices.
33
What are some safety precautions to avoid copper media hazards?
- The separation of data and electrical power cabling must comply with safety codes.
- Cables must be connected correctly
- Installations must be inspected for damage
- Equipment must be grounded correctly
34
How do UTP cables counter the effects of crosstalk?
Cancellation - Designers now pair wires in a circuit, when two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. The two magnetic fields cancel each other and also cancel out any outside EMI and RFI signals. Varying the number of twists per wire pair further enhances the cancellation effect.
35
Define UTP cabling standards.
```
The cabling standards are established by the TIA/EIA. 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
```
36
Define UTP categories.
The electrical characteristics of copper cabling are defined by the Institute of Electrical and Electronic Engineers (IEEE). Cables are placed into categories based on bandwidth rates. Category 5 (Cat5) cable is used commonly in 100BASE-TX Fast Ethernet installations. Other categories include Enhanced Category 5 (Cat5e), Category 6 (Cat6), and Category 6a. Cables in higher categories are designed and constructed to support higher data rates. Cat5e is now the minimally acceptable cable type.
Cat3: Used for voice communication, mostly phone lines
Cat5 & Cat5e: Used for data transmission. Cat5 supports 100Mb/s, and can support 1000Mb/s, but it is not recommended. Cat5e supports 1000Mb/s.
Cat6: Used for data transmission, an added separator is between each pair of wires allowing it to function at higher speeds. Supports 1000Mb/s - 10 Gb/s, though 10 Gb/s is not recommended.
37
What happens when copper cabling is terminated?
There is the possibility of signal loss, and the introduction of noise into the communication circuit. When terminated improperly, each cable is a potential source of physical layer performance degradation. It is essential that all copper media terminations be of high quality to ensure optimum performance with current and future network technologies.
38
What are the main cable types obtained by using specific wiring conventions?
Ethernet straight-through: The most common type of networking cable, commonly used to interconnect a host to a switch and a switch to a router.
Ethernet crossover: A cable used to interconnect similar devices. For example a switch to switch, host to host, or router to router.
Rollover: A Cisco proprietary cable used to connect a workstation to a router or switch console port.
39
What are the standard wire pairs of UTP Cable?
```
T568A:
Pair 1 (Blue): 4&5
Pair 2 (Orange): 3&6
Pair 3 (Green): 1&2
Pair 4 (Brown): 7&8
```
```
T568B:
Pair 1 (Blue): 4&5
Pair 2 (Orange): 1&2
Pair 3 (Green): 3&6
Pair 4 (Brown): 7&8
```
For both wiring standards, the left most wire in the pair is the striped wire, and the right most wire in the pair is the solid wire, except for pair 1, the blue pair, where this is swapped.
Ethernet Straight Through: Both ends are T568A, or T568B
Ethernet Crossover: One end T568A, the other end T568B
40
What are the parameters that should be tested on a UTP cable?
- Wire Map
- Cable length
- Signal loss due to attenuation
- Crosstalk
41
What are fiber-optic cables?
Optical fiber is a flexible, but extremely this, 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. Fiber-optic cable can transmit signals with less attenuation and is completely immune to EMI and RFI. It can transmit data over longer distances and at higher bandwidths than any other networking media.
42
What kind of industries use fiber-optic cabling?
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.
43
What are the components of a fiber-optic cable?
Jacket: Typically PVC, protects the fiber against abrasion, moisture, and other contaminants.
Strengthening Material: Surrounds the buffer, prevents the fiber cable from being stretched when it is being pulled. The material used is often the same material used to produce bulletproof vests.
Buffer: Used to help shield the core and cladding from damage.
Cladding: Made from slightly different chemicals than those used to create the core. It acts like a mirror by reflecting light back into the core of the fiber, keeping the light in the core as it travels down the fiber.
Core: The light transmission element at the center of the optical fiber. Typically silica or glass. Light pulses travel through the core.
44
What generates the light pulses read by fiber-optic cables?
Lasers, or light emitting diodes. Semiconductor devices called photodiodes detect the light pulses and convert them to voltages. The laser light transmitted over fiber-optic cabling can damage the human eye.
45
What are the types of fiber-optic cable?
```
Single-mode fiber (SMF): a very small core, uses expensive laser technology to send a single ray of light. Suited for long distance applications, commonly used for distances of several thousand meters.
Multimode fiber (MMF): larget core than single mode, uses LED emitters for light pulses. Light from an LED enters the multimode fiber at different angle. Suited for long distances, but not as long as single mode. Commonly used with LANs, or distances of a couple hundred meters. Popular because they can be powered by low-cost LEDs.
```
46
What are the fiber-optic connectors?
```
Straight-Tip (ST) connectors: The connector locks securely with a "twist-on/twist-off" bayonet style mechanism.
Subscriber Connector (SC) connectors: Sometimes called square, or standard connector. It is a widely adopted LAN and WAN connector that uses a push-pull mechanism to ensure positive insertion. Used with multimode, and single-mode.
Lucent Connector (LC) Simplex connectors: Smaller version of the SC connector. Sometimes called little or local connector, quickly growing in popularity due to its small size.
Duplex Multimode LC Connectors: Similar to a LC simplex connector, but using a duplex connector.
```
47
How do fiber optic cables handle a duplex operation?
Light can only travel in one direction over optical fiber, so two fibers are required. They are terminated with a pair of standard single fiber connectors. Some connectors accept both the transmitting and receiving fiber in a single connector, called a duplex connector.
48
What a re the common fiber patch cords?
SC-SC multimode patch cord (orange)
LC-LC Single-mode patch cord (yellow)
ST-LC Multimode Patch cord (orange)
SC-ST Single-mode patch cord (yellow)
Single-mode fiber patch cords are yellow, and multimode cords are orange (or aqua)
49
How do you test fiber optic cabling?
Three common types of errors are:
Misalignment - The fiber optic media are not precisely aligned to one another when joined.
End Gap - The media does not completely touch at the splice or connection
End Finish - The media ends are not well polished, or dirt is present at the termination.
A quick and easy test can be done by shining a bright flashlight into one end, and seeing if the light appears on the other end. This does not ensure performance, however.
An optical time domain reflectometer (OTDR) can test fiber optic cable segments. It injects a test pulse of light and measures backscatter and reflection of light detected as a function of time. It will calculate the approximate distance at which these faults are detected along the length of the cable.
50
What is wireless media?
It carries electromagnetic signals representing binary digits of data using radio or microwave frequencies. It provides the greatest mobility options of all media, and the number of wireless-enabled devices is constantly increasing.
51
What are some areas of concern for wireless media?
Coverage Area: Wireless data communication technologies work well in open environments. However certain construction materials, both in structures and terrain, will limit effective coverage.
Interference: Susceptible to interference and can be disrupted by devices like household cordless phone, fluorescent light, microwaves, and other wireless communications.
Security: Requires no access to a physical strand of media, therefore devices not authorized for access to the network can gain access to the transmission. Security is a big 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 uses, meaning the more users needing to access the WLAN simultaneously, the less bandwidth each user receives.
52
What are the wireless media standards?
Wi-Fi: Standard IEEE 802.11 - Wireless LAN (WLAN) technology, commonly referred to as Wi-Fi. WLAN uses a contention based protocol known as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). The wireless NIC must first listen before tranmitting to determine if the radio channel is clear. If another device is transmitting, the NIC must wait until the channel is clear. A trademark of the Wi-Fi Alliance, used with certified products that belong to WLAN devices based on the IEEE 802.11 standard.
Standard IEEE 802.15: Bluetooth - Wireless Personal Area Network (WPAN) standard, commonly know as "Bluetooth" uses a device pairing process to communicate 1-100m
Standard IEEE 802.16: WiMAX - Commonly know as Worldwide Interoperability for Microwave Access (WiMAX), uses a point-to-multipoint topology to provide wireless broadband access.
Other wireless technologies, like cellular and satellite can provide data network connectivity.
53
What areas of the wireless media standards are physical layer specifications applied to?
- Data to radio signal encode
- Frequency and power of transmission
- Signal reception and decoding requirements
- Antenna design and construction.
54
What is a wireless LAN?
Common wireless data implementation, which enable devices to connect wirelessly via a LAN.
Generally a wireless LAN requires:
A wireless access point (AP) which concentrates the wireless signals and connects to the existing copper-based network infrastructure (ex: Ethernet). Home and small business wireless routers integrate the functions of a router, switch, and AP into one device.
Wireless NIC adapters, which provide wireless communication capability to each network host.
55
What are the upsides and downsides of wireless media?
The savings on costly premises wiring, and the convenience of host mobility are very beneficial. Administrators need to develop and apply stringent security policies and processes to protect wireless LANs from unauthorized access and damage. A number of WLAN Ethernet-based standards have emerged, and care must be taken in purchasing wireless devices to ensure compatibility.
56
What is the data link layer responsible for?
- Allowing the upper layers to access the media.
- Accepting Layer 3 packets and packaging them into frames.
- Preparing network data for the physical network
- Controlling how data is placed and received on the media
- Exchanging frames between nodes over a physical network media, such as UTP or fiber-optic
- Receiving and directing packets to an upper layer protocol
- Performing error detection.
57
What are nodes?
The layer 2 notation for devices connected to a common media. Nodes build and forward frames. The OSI data link layer is responsible for the exchange of Ethernet frames between source and destination nodes over a physical network media.
58
What are the sublayers of the data link layer?
- Logical Link Control (LLC) - The upper sublayer communicates with the network layer. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to utilize the same network interface and media.
- Media Access Control (MAC) - The lower sublayer, defines media access processes performed by the hardware. Provides data link layer addressing and access to various network technologies. ex: Communicates with Ethernet LAN technology to send and receive frames over copper or fiber-optic cable. Communicates with wireless technologies such as Wi-Fi and Bluetooth to send and receive frames wirelessly.
59
What is the media access control method?
The technique the data link layer uses for getting the frame on and off the media. Without the data link layer, network layer protocols such as IP, would have to make provisions for connecting to every type of media that could exist along a delivery path. IP would also have to adapt every time a new network technology or medium was developed. This process would hamper protocol and network media innovation and development. Data link layer protocols govern how to format a frame for use on different media, and the protocols may differ from media to media. At each hop along the path an intermediary device accepts frames from one medium, de-encapsulates the frame and the forwards the packets in a new frame. The headers of each frame are formatted for the specific medium it will cross.
60
Who defines the open standards and protocols that apply to the network access layer?
- Institute of Electrical and Electronics Engineers (IEEE)
- International Telecommunication Union (ITU)
- International Organization for Standardization (ISO)
- American National Standards Institute (ANSI)
61
What does the media access control method depend on?
Topology - How the connection between the nodes appears to the data link layer
Media sharing - How the nodes share the media. The media sharing can be point-to-point, such as in WAN connections, or shared such as in LAN networks.
62
What are network topologies?
The topology of a network is the arrangement or relationship of the network devices and the interconnections between them. LAN and WAN topologies can be viewed in two ways:
Physical topology: Refers to the physical connections and identifies how end devices in infrastructure devices (routers, switches, etc.) are interconnected. Will show the layout of the network in terms of a physical building, describing which room each device is in.
Logical topology: Refers to the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network, defined by data link layer protocols. Will show the layout of the network in terms of smaller networks, describing what network each device is a part of, and the IP of that network. The logical topology is what influences the type of network framing and media access control.
63
What kind of topologies are WANs commonly connected by?
Point-to-Point - The simplest topology that consists of a permanent link between two endpoints. Very popular WAN topology
Hub and Spoke - A WAN version of the star topology in which a central site interconnects branch sites using point-to-point links.
Mesh - This topology provides high availability, but requires every end system be interconnected to every other system. The administrative and physical costs can be large. Each link is essentially a point-to-point link to the other node.
Hybrid - A variation or combination of the above, such as a partial mesh in which some, but not all, end devices are interconnected
64
What is the difference between physical and logical point-to-point topology?
Physical point-to-point topologies directly connect two nodes. They do not have to share the media with other hosts, and a node does not have to make any determination about whether an incoming frame is destined for it or another node. This means all frames on the media can only travel to or from the two nodes.
In logical point-to-point topologies, the end nodes communicating can be physically connected via a number of intermediate devices. The source and destination nodes may be indirectly connected to each other over some geographical distance. However the use of physical devices in the network does not affect the logical topology.
65
What is a virtual circuit?
A logical connection created within a network between two network devices. The two nodes on either end of the virtual circuit exchange the frames with each other, even if the frames are directed through intermediary devices. They are impontant logical communication constructs used by some Layer 2 technologies.
66
What kind of topologies are LANs commonly connected by?
Star - End devices are connected to a central intermediate device. Early star topologies interconnected end devices using Ethernet hubs. However star topologies now use Ethernet switches. Easy to install, scalable, and easy to troubleshoot.
Extended Star - In an extended star topology, additional Ethernet switches interconnect other star topologies. A hybrid technology.
Bus - All end systems are chained to each other and terminated in some form on each end. Infrastructure devices such as switches are not required to interconnect the end devices. Bus topologies using coax cables were used in legacy Ethernet networks due to it's inexpensive nature.
Ring - End systems are connected to their respective neighbor forming a ring. The ring does not need to be terminated. Used in legacy Fiber Distributed Data Interface (FDDI) and Token Ring networks.
67
What is duplex communication?
Refers to the direction of data transmission between two devices.
Half-duplex communication means that both devices can transmit and receive on the media, but not simultaneously. Used with legacy bus topologies and Ethernet hubs.
Full-duplex communication means both devices can transmit and receive on the media at the same time. The data link layer assumes that the media is available for transmission for both node at any time. Ethernet switches operate in full-duplex mode by default.
68
What are multi-access networks?
Network topologies that share a common medium with multiple nodes. Ethernet LANs and WLANs are examples. At any one time there may be a number of devices attempting to send and receive data on the same network media.
69
What are the basic access control methods for shared media?
Contention-based access - All nodes operating in half-duplex compete for the use of the medium, but only one device can send at a time. Ethernet LANs using hubs and WLANs are examples. Theses systems do not scale well under heavy media use. Ethernet LANs using switches do not use this, instead they operate in full-duplex mode.
Controlled Access - Each node has its own time to use the medium. These deterministic types of networks are inefficient because a device must wait its turn to access the medium. Legacy Token Ring LANs are an example.
70
What is the CSMA/CD?
The carrier sense multiple access/collision detection (CSMA/CD) process. It is used in half-duplex Ethernet LANs.
The 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 it will assume the network is free.
3. PC1's NIC sends the Ethernet Frame.
4. The Ethernet hub receives the frame. The 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, but since the frame has a destination data link addressed for PC3, only PC3 will accept and copy the entire frame. All other devices will ignore it.
If two devices transmit at the same time a collision will occur. Both devices will detect the collision on the network. This is done by the NIC comparing data transmitted with data received, or by recognizing the signal amplitude as higher than normal. The data sent by both devices will need to be resent.
71
What is CSMA/CA?
Uses IEEE 802.11 WLANs, Carrier Sense Multiple Access/Collision Avoidance. Uses a similar method to CSMA/CD to detect if the media is clear. Does not detect collisions but attempts to avoid them by waiting before transmitting. Each device that transmits includes the time duration that it needs for the transmission. All other wireless devices receive this information and know how long the medium will be unavailable. After a wireless device sends and 802.11 frame, the receiver returns an acknowledgement so that the sender knows that frame arrived.
72
Describe a frame.
The data link layer prepares a packet for transport across the local media by encapsulating it with a header and a trailer to create a frame.
Each frame has 3 basic parts:
-Header
-Data
-Trailer
All data link layer protocols encapsulate the Layer 3 PDU within the data field of the frame. However the structure of the frame and the fields in the header and trailer may vary based on protocol.
73
T/F There isn't a frame structure that meets the needs of all data transportation?
True, depending on the environment the amount of control information needed in the frame varies to match the access control requirements of the media and logical topology.
74
What are some generic frame field types?
Header:
Frame Start Indicator Flag - Used to identify the beginning 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 like Quality of service (QoS). QoS give forwarding priority to certain types of messages. Data link frames carrying voice over IP (VoIP) normally receive priority because they are sensitive to delay.
Packet (Data):
Data - Contains the frame payload (i.e. packet header, segment header, and the data)
Trailer:
Error Detection - These frame fields are used for error detection and are included after the data to form the trailer. Used to determine if the frame arrived without error. Accomplished by placing a logical or mathematical summary of the bits that comprise the frame in the trailer. Needed: the signals on the media could be subject to interference, distortion, or loss that would change the bit values.
Frame Stop Indication Flag - Used to identify the end of the frame
75
What are the different kinds of data link layer protocols?
- Ethernet
- 802.11 Wireless
- Point-to-Point Protocol (PPP)
- HDLC
- Frame Relay
