Friday, October 24, 2008
Thursday, October 23, 2008
OSL MODEL
What is the OSI model?
The OSI (Open System Interconnection) model is a standard developed by the ISO (International Standards Organization) to describe the flow of data on a computer network. The model takes into account the flow of data from the physical connections up to the end user application.
The OSI model defines a "layered" architecture in the form of a protocol stack. There are specific, discrete functions that take place at each layer of the protocol stack with lower level layers providing services to upper layers. When two systems communicate on the network, information is sent down through the protocol stack of one system, over the cable and then up through the protocol stack to the appropriate layer on the other system.
Data flow under the OSI model is organized into the following seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
What is the purpose of the OSI model?
The OSI model (as well as IEEE 802 and NDIS) were established to standardize the design and construction of computer networks for developers and hardware manufacturers. Standards allow hardware and software components from a variety of different vendors to operate together. Without standards, everything would be vendor-specific and interoperability would suffer.
Almost all vendors implement standards in slightly different ways. This is normally done to take advantage of unique functionality in their product that just won't fit neatly into the model. However, if a vendor strays too far from the standards, they risk creating a 'closed' or proprietary product that is not supported by the rest of the industry.
IBM mainframes use Systems Network Architecture (SNA) that is a set of layered protocols like the OSI model. However, the SNA layers are not directly comparable to the OSI model layers. This makes interoperability between PC-based networks and IBM mainframes more difficult. Exam Tip. Certification candidates need to understand the OSI and IEEE models because they provide a framework from which many of the technical concepts are taught. This understanding can be a big help when troubleshooting network problems in the real world.
The OSI (Open System Interconnection) model is a standard developed by the ISO (International Standards Organization) to describe the flow of data on a computer network. The model takes into account the flow of data from the physical connections up to the end user application.
The OSI model defines a "layered" architecture in the form of a protocol stack. There are specific, discrete functions that take place at each layer of the protocol stack with lower level layers providing services to upper layers. When two systems communicate on the network, information is sent down through the protocol stack of one system, over the cable and then up through the protocol stack to the appropriate layer on the other system.
Data flow under the OSI model is organized into the following seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
What is the purpose of the OSI model?
The OSI model (as well as IEEE 802 and NDIS) were established to standardize the design and construction of computer networks for developers and hardware manufacturers. Standards allow hardware and software components from a variety of different vendors to operate together. Without standards, everything would be vendor-specific and interoperability would suffer.
Almost all vendors implement standards in slightly different ways. This is normally done to take advantage of unique functionality in their product that just won't fit neatly into the model. However, if a vendor strays too far from the standards, they risk creating a 'closed' or proprietary product that is not supported by the rest of the industry.
IBM mainframes use Systems Network Architecture (SNA) that is a set of layered protocols like the OSI model. However, the SNA layers are not directly comparable to the OSI model layers. This makes interoperability between PC-based networks and IBM mainframes more difficult. Exam Tip. Certification candidates need to understand the OSI and IEEE models because they provide a framework from which many of the technical concepts are taught. This understanding can be a big help when troubleshooting network problems in the real world.
Wednesday, October 22, 2008
LAN Architectures
What are the 3 most common LAN architectures?
The 3 most common types of LAN architectures* are:
Ethernet
Token Ring
ArcNet
* These architectures are sometimes referred to as "lower-level protocols" because they represent the specifications for the IEE802 model which encompasses the physical (1st) and data link (2nd) layers of the OSI model.
What is Ethernet? What IEEE standard does it conform to?
Ethernet is a popular, relatively inexpensive, easy-to-install LAN architecture with the following characteristics:
Uses the CSMA/CD media access control.
Data transmission normally occurs at 10Mbps.
Typically implemented in a bus or star topology.
Ethernet LANs are normally distinguished by the type of cable they use (Thinnet, Thicknet, or Twisted Pair).
The Ethernet architecture conforms to most but not all of the IEEE 802.3 specification (the physical layers are identical but the MAC layers are somewhat different).
How does the nomenclature "10base2" describe Ethernet cable?
An Ethernet LAN is often described in terms of three parameters: transmission rate, transmission type, and segment distance.
"10base2" means:
10 - transmission rate or through put of 10Mbps
base - transmission type is baseband rather than broadband network (i.e., the signal is placed directly on the cable, one signal at a time)
2 - the maximum segment distance in meters times 100; in this case 200 meters (actually only 185 meters)
(In "10baseT," the T stands for twisted pair. Segment distance is 100 meters.)
What are the key characteristics of 10Base2 Ethernet.
A 10Base2 Ethernet LAN conforms generally to the IEEE 802.3 standard. Also known as Thinnet Ethernet, it has the following key characteristics:
Transmits at 10 Mbps
Uses Thinnet coaxial cable
Supports a maximum of 30 nodes per segment
Uses local bus topology
Minimum distance between computers is 0.5m (not including drop cables)
Maximum length of segment is 185m
Up to 5 segments can be connected (but only 3 can accommodate nodes)
Connected with BNC connectors (T-connectors)
Used primarily for smaller workgroups or departments
What is 100VG-AnyLAN?
· 100VG-AnyLAN. The VG stands for "Voice Grade" in recognition of the fact that it transmits at 100 Mbps over voice grade twisted-pair cable (Category 3).* Supports cable segments of only 100m. Uses a new media access control method called demand priority rather than the older CSMA/CD. IEEE standard 802.12.
· *100VG-AnyLAN can transmit on Category 3 cable because it uses all 4-wire pairs (quartet signaling). 10baseT uses only 2 of the wire pairs in Category 3 UTP.
What are the key characteristics of 10baseT Ethernet.
A 10baseT Ethernet LAN conforms generally to the IEEE 802.3 standard. Also known as Twisted Pair Ethernet, it has the following key characteristics:
Transmits at 10 Mbps
Normally uses UTP cable
Minimum grade of UTP cable is Category 3
Typically supports about 12 nodes per hub*
Uses star or bus topology
Minimum distance between nodes is 2.5m
Maximum length of segment is 100m
Most common type of Ethernet LAN; used in both small and large networks
*There is no hard and fast rule here. Hubs can be connected together so in total a 10BaseT LAN can support up to 1024 nodes without a bridge.
What is Token Ring? What IEEE standard does it conform to?
Token ring is a relatively expensive LAN architecture that is strongly influenced by IBM. It is very stable and can be expanded without a significant degradation in network performance.
Token ring uses the token passing media access control. Data transmission normally occurs at 4 or 16 Mbps depending on the cable.
Token ring is normally implemented in a logical ring/physical star topology with a MAU (Multistation Access Unit) as the hub. The maximum number of stations on one ring is 260 for shielded twisted pair and 72 for unshielded twisted pair (UTP). There can be up to 33 MAUs per ring.
Token Ring LANs normally use shielded twisted pair (STP) but may also use unshielded twisted pair (UTP) or fiber-optic cable. The maximum distance to the MAU from the workstation depends on the cable and varies from 45 meters for UTP to 100 meters for STP.
The Token Ring architecture conforms generally to the IEEE’s 802.5 specification
What is ArcNet? What IEEE standard does it conform to?
ArcNet (Attached Resource Computing Network) is a relatively inexpensive, reliable, and easy-to-install LAN architecture with the following characteristics:.
Additional workstations are easily added.
ArcNet is a baseband, token passing media access control architecture
ArcNet is relatively slow. Data transmission occurs at 2.5 Mbps. (20 Mbps for ArcNet Plus)
ArcNet can be implemented in a bus or star topology.
ArcNet LANs normally use coaxial cable but can also use twisted pair or fiber-optic cable. Maximum cable segment depends on the type of cable and hub connection (120m -600m)
The ArcNet architecture conforms very loosely to the IEEE’s 802.4 specification. ArcNet is a baseband star or bus and 802.4 defines a broadband bus.
What factors should be considered when selecting a LAN architecture?
The following factors should be considered when selecting a LAN architecture:
Cost - How much is available in the budget and what quantifiable benefits will the LAN provide?
Reliability - How dependent is the organization’s operations on the LAN?
Speed - How many concurrent users and how critical is response time?
Size - How many computers will be connected and what is the overall distance between them?
Security - How important is secure communications to the network?
Growth - What are the organization’s plans for growth?
Administration - How will the LAN be administered? Who will assume what responsibilities?
What is the most common bus topology normally used with the following architectures: 10base2, 10base5, 10baseT, Token Ring, and ArcNet?
Common LAN architectures and their topologies include:
Ethernet 10base2 - local bus
Ethernet 10base5 - regular bus
Ethernet 10baseT - bus or star
Token Ring - ring (physical star)
ArcNet - star or distributed star (interconnected)
The 3 most common types of LAN architectures* are:
Ethernet
Token Ring
ArcNet
* These architectures are sometimes referred to as "lower-level protocols" because they represent the specifications for the IEE802 model which encompasses the physical (1st) and data link (2nd) layers of the OSI model.
What is Ethernet? What IEEE standard does it conform to?
Ethernet is a popular, relatively inexpensive, easy-to-install LAN architecture with the following characteristics:
Uses the CSMA/CD media access control.
Data transmission normally occurs at 10Mbps.
Typically implemented in a bus or star topology.
Ethernet LANs are normally distinguished by the type of cable they use (Thinnet, Thicknet, or Twisted Pair).
The Ethernet architecture conforms to most but not all of the IEEE 802.3 specification (the physical layers are identical but the MAC layers are somewhat different).
How does the nomenclature "10base2" describe Ethernet cable?
An Ethernet LAN is often described in terms of three parameters: transmission rate, transmission type, and segment distance.
"10base2" means:
10 - transmission rate or through put of 10Mbps
base - transmission type is baseband rather than broadband network (i.e., the signal is placed directly on the cable, one signal at a time)
2 - the maximum segment distance in meters times 100; in this case 200 meters (actually only 185 meters)
(In "10baseT," the T stands for twisted pair. Segment distance is 100 meters.)
What are the key characteristics of 10Base2 Ethernet.
A 10Base2 Ethernet LAN conforms generally to the IEEE 802.3 standard. Also known as Thinnet Ethernet, it has the following key characteristics:
Transmits at 10 Mbps
Uses Thinnet coaxial cable
Supports a maximum of 30 nodes per segment
Uses local bus topology
Minimum distance between computers is 0.5m (not including drop cables)
Maximum length of segment is 185m
Up to 5 segments can be connected (but only 3 can accommodate nodes)
Connected with BNC connectors (T-connectors)
Used primarily for smaller workgroups or departments
What is 100VG-AnyLAN?
· 100VG-AnyLAN. The VG stands for "Voice Grade" in recognition of the fact that it transmits at 100 Mbps over voice grade twisted-pair cable (Category 3).* Supports cable segments of only 100m. Uses a new media access control method called demand priority rather than the older CSMA/CD. IEEE standard 802.12.
· *100VG-AnyLAN can transmit on Category 3 cable because it uses all 4-wire pairs (quartet signaling). 10baseT uses only 2 of the wire pairs in Category 3 UTP.
What are the key characteristics of 10baseT Ethernet.
A 10baseT Ethernet LAN conforms generally to the IEEE 802.3 standard. Also known as Twisted Pair Ethernet, it has the following key characteristics:
Transmits at 10 Mbps
Normally uses UTP cable
Minimum grade of UTP cable is Category 3
Typically supports about 12 nodes per hub*
Uses star or bus topology
Minimum distance between nodes is 2.5m
Maximum length of segment is 100m
Most common type of Ethernet LAN; used in both small and large networks
*There is no hard and fast rule here. Hubs can be connected together so in total a 10BaseT LAN can support up to 1024 nodes without a bridge.
What is Token Ring? What IEEE standard does it conform to?
Token ring is a relatively expensive LAN architecture that is strongly influenced by IBM. It is very stable and can be expanded without a significant degradation in network performance.
Token ring uses the token passing media access control. Data transmission normally occurs at 4 or 16 Mbps depending on the cable.
Token ring is normally implemented in a logical ring/physical star topology with a MAU (Multistation Access Unit) as the hub. The maximum number of stations on one ring is 260 for shielded twisted pair and 72 for unshielded twisted pair (UTP). There can be up to 33 MAUs per ring.
Token Ring LANs normally use shielded twisted pair (STP) but may also use unshielded twisted pair (UTP) or fiber-optic cable. The maximum distance to the MAU from the workstation depends on the cable and varies from 45 meters for UTP to 100 meters for STP.
The Token Ring architecture conforms generally to the IEEE’s 802.5 specification
What is ArcNet? What IEEE standard does it conform to?
ArcNet (Attached Resource Computing Network) is a relatively inexpensive, reliable, and easy-to-install LAN architecture with the following characteristics:.
Additional workstations are easily added.
ArcNet is a baseband, token passing media access control architecture
ArcNet is relatively slow. Data transmission occurs at 2.5 Mbps. (20 Mbps for ArcNet Plus)
ArcNet can be implemented in a bus or star topology.
ArcNet LANs normally use coaxial cable but can also use twisted pair or fiber-optic cable. Maximum cable segment depends on the type of cable and hub connection (120m -600m)
The ArcNet architecture conforms very loosely to the IEEE’s 802.4 specification. ArcNet is a baseband star or bus and 802.4 defines a broadband bus.
What factors should be considered when selecting a LAN architecture?
The following factors should be considered when selecting a LAN architecture:
Cost - How much is available in the budget and what quantifiable benefits will the LAN provide?
Reliability - How dependent is the organization’s operations on the LAN?
Speed - How many concurrent users and how critical is response time?
Size - How many computers will be connected and what is the overall distance between them?
Security - How important is secure communications to the network?
Growth - What are the organization’s plans for growth?
Administration - How will the LAN be administered? Who will assume what responsibilities?
What is the most common bus topology normally used with the following architectures: 10base2, 10base5, 10baseT, Token Ring, and ArcNet?
Common LAN architectures and their topologies include:
Ethernet 10base2 - local bus
Ethernet 10base5 - regular bus
Ethernet 10baseT - bus or star
Token Ring - ring (physical star)
ArcNet - star or distributed star (interconnected)
NETWORK CONCEPTS
Basic Cabling :
The three most common types of cabling media used in LANs are:
Coaxial Cable. Coaxial cable comes in two versions: Thinnet and Thicknet. Thinnet looks like regular TV cable.* It is about 1/4 inch in diameter and is very flexible and easy to work with. In contrast, Thicknet is about 1/2 inch in diameter and not very flexible. Thicknet is older and not very common anymore except as a backbone within and between buildings. Coax transmits at 10 Mbps..
Twisted Pair. Twisted pair looks like telephone wire and consists of insulated strands of copper wire twisted together. There are two versions of twisted pair cable: Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP). STP is commonly used in Token Ring networks and UTP in Ethernet networks where it is referred to as "10baseT." Transmission rates vary between 10-100 Mbps..
Fiber-Optic Cable. Fiber-optic cable consists of a thin cylinder of glass surrounded by glass cladding, encased in protective outer sheath. Fiber-optic cable is very fast (100 Mbps). It can transmit over long distances (2 km +) but is expensive.
*Don’t confuse Thinnet cable (RG 58) with cable TV cable (RG 59). They look alike but they are not interchangeable.
The three most common types of cabling media used in LANs are:
Coaxial Cable. Coaxial cable comes in two versions: Thinnet and Thicknet. Thinnet looks like regular TV cable.* It is about 1/4 inch in diameter and is very flexible and easy to work with. In contrast, Thicknet is about 1/2 inch in diameter and not very flexible. Thicknet is older and not very common anymore except as a backbone within and between buildings. Coax transmits at 10 Mbps..
Twisted Pair. Twisted pair looks like telephone wire and consists of insulated strands of copper wire twisted together. There are two versions of twisted pair cable: Shielded Twisted Pair (STP) and Unshielded Twisted Pair (UTP). STP is commonly used in Token Ring networks and UTP in Ethernet networks where it is referred to as "10baseT." Transmission rates vary between 10-100 Mbps..
Fiber-Optic Cable. Fiber-optic cable consists of a thin cylinder of glass surrounded by glass cladding, encased in protective outer sheath. Fiber-optic cable is very fast (100 Mbps). It can transmit over long distances (2 km +) but is expensive.
*Don’t confuse Thinnet cable (RG 58) with cable TV cable (RG 59). They look alike but they are not interchangeable.
What are the recommended maximum segment lengths for each type of cable?
The type of cable plays a role in how fast a signal will degrade as it is transmitted. The following are the recommended maximum cable lengths:
Thinnet- maximum length of segment (terminator to terminator) is 185 meters (607 feet)
Thicknet -maximum length of segment (single run) is 500 meters (1,640 feet)
Unshielded Twisted Pair (UTP) - maximum length of cable between hub and computer is 100 meters (328 feet)
Fiber-Optic - maximum length of cable is 2 kilometers. (6,562 feet)
What is a backbone?
A backbone is a generic term used to describe media that interconnects a number of computers, segments or subnets.
In its most common form, a backbone is used to connect hubs. Each hub represents a segment on which individual workstations are connected via UTP cable. The hubs from different segments are then connected to each other with thinnet cable. In this case, the thinnet cable functions as the backbone that links the hubs.
In another example, a backbone may be a length of cable that serves as a trunk. Drop cables are attached from the backbone to connect individual workstations.
A backbone is often used to connect networks in separate buildings. Organizations typically use fiber-optic cable for this type of backbone. Thicknet is also used as a backbone.
Characteristics of coaxial cable?
Coaxial cable (coax) has the following characteristics:
Transmission rate of about 10 Mbps
Maximum cable length of 185 meters for Thinnet, 500 meters for Thicknet
Good resistance to electrical interference
Less expensive than fiber-optics but more expensive than twisted pair.
Flexible and easy to work with (Thinnet)
Wire type is 20 AWG for Thinnet (R-58) and 12 AWG for Thicknet.
Ethernet designation is 10base2 (Thinnet) or 10base5 (Thicknet, also referred to as 'standard Ethernet').
What are the different types of coaxial cable?
The following designations are used to distinguish the different types of coaxial cable:
RG-58 A/U – Thinnet, stranded wire core, 50 ohms
RG-58 /U – Thinnet, solid wire core, 50 ohms
RG-59 – Thicknet, cable television, broadband
RG-62 – ArcNet, 75 ohms
A cable's designation is typically printed on the its outer sheathing. As a general rule, you cannot mix coax cable types on the network.
What connection hardware is used with thinnet coaxial cable?
Thinnet utilizes the following connection hardware referred to as ‘BNC’ components:
Terminator – a resister used to absorb the signal once it reaches the end of the bus; connects to a T- or barrel connector; RG-58 requires a 50-ohm terminator; RG-62 requires a 75-ohm terminator.
Cable Connector – the interface at the end of the cable that is used to connect to a barrel or T-connector
T-Connector – used to connect to a NIC and another cable connector or a terminator (you cannot connect directly to a NIC with a cable connector; you must use a T-connector)
Barrel Connector – used to splice to segments of cable together or attach a terminator at the end of the cable
What connection hardware is used with thicknet coaxial cable?
· Thinnet utilizes the following connection hardware referred to as ‘BNC’ components:
· Thicknet utilizes an AUI (Attachment Unit Interface) connector to attach to a Network Interface Card (NIC). The AUI connector has 15 pins and is alternatively referred to as a DB-15 or DIX connector.
· One of the unique characteristics of Thicknet is the use of an external transceiver. (The transceiver is used to convert signals from parallel to serial for transmission on the network.) The Thicknet cable attaches to the transceiver via a clamp or vampire tap. The NIC connects to the transceiver using a drop cable with AUI connectors.
What is Plenum cable?
· Thinnet utilizes the following connection hardware referred to as ‘BNC’ components:
· Plenum refers to the space in buildings between the ceiling and the next floor above it. Because of the potential fire hazard, building codes are very specific about what type of wiring can be placed in this area. refers to the space in buildings between the ceiling and the next floor above it. Because of the potential fire hazard, building codes are very specific about what type of wiring can be placed in this area.
· Plenum cable refers to coaxial cabling that meets the minimum standards to allow it to be strung in the plenum area without having to use special conduit. The insulation and jacket on plenum cabling must be fire resistant and not give off toxic fumes when burned. refers to coaxial cabling that meets the minimum standards to allow it to be strung in the plenum area without having to use special conduit. The insulation and jacket on plenum cabling must be fire resistant and not give off toxic fumes when burned.
· Not all coaxial cable is plenum cable. The most common type of coaxial cable is PVC (polyvinyl chloride) which is more flexible and easier to work with than plenum but does not have the same fire resistance features. PVC cable can give off toxic fumes when burned.
What are the key characteristics of UTP?
Unshielded Twisted Pair (UTP) has the following key characteristics:
Transmission rate of 10-100 Mbps
Maximum cable segment of 100 meters
Most susceptible to electrical interference or ‘crosstalk’ (although shielding may lessen the impact)
Less expensive than coax or fiber-optic. In some cases, preinstalled telephone wire may be used in the network (if it is of sufficient grade).
Very flexible and easy to work with
Wire type is 22-26AWG
Uses an RJ-45 connector
Ethernet designation is 10baseT
What are the characteristics of shielded twisted pair (STP)?
Shielded twisted pair (STP) is similar to UTP except it contains a copper braid jacket to ‘shield’ the wires from electrical interference. It can support transmissions over greater distances than UTP.
What are the 5 categories of UTP and what is the minimum acceptable category for 10baseT?
The following categories of unshielded twisted pair (UTP) cable were established by the EIA/TIA* to support the networks indicated:
Category 1 - Traditional telephone cable; supports voice only, not data
Category 2 - Data transmissions up to 4 Mbps (but not token ring)
Category 3 - 10 Mbps Ethernet
Category 4 - 16 Mbps token-ring
Category 5 - 100 Mbps; supports ATM
The minimum acceptable cable for 10baseT Ethernet is Category 3.
*EIA/TIA 568 is the standard developed by the Electronic Industries Association/Telecommunications Industry Association applies to all UTP that works with networks.
Are there other cable specifications?
Yes. There are several different specifications used to classify cable. One of the oldest is the AWG (American Wire Gauge) rating. This rating measures the thickness or gauge of the wire with the size being inverse to the rating. For example, a 22 AWG cable is thicker than a 24 AWG cable. 22 AWG wire is typically used in telephone wire and UTP.
IBM uses its own system of cable classification whereby cables are categorized as ‘types.’ For example, Type 3 wire is basically equivalent to the Category 3 wire discussed earlier. However, not all of the types used by IBM coincide exactly with a particular category. In the non-IBM world, UTP cable is typically referred to by its category classification and coax by its RG designation.
What connectors are used with UTP?
UTP uses a connection called the RJ-45 connector. It looks similar to a common telephone connector (RJ-11) except it is slightly larger. The RJ-45 has 8 pins while the RJ-11 has only 4.
UTP cable typically runs from a computer’s NIC and plugs directly into a wall plate much like a regular telephone. It is not uncommon to find wall plates in newer buildings labeled as ‘Voice’ (for telephones) or ‘Data’ (for UTP). The cable running from the wall jack is rarely attached directly into a hub. Instead the individual cables are collected and organized with patch panels located in a ‘wiring closet.’
Do I have to have a hub to use UTP cable?
No. If you have two computers and their NICs have RJ-45 connections, then you can simply connect them with UTP cable. However, you will need a special type of UTP cable called a ‘crossover cable.’ This cable is a normal UTP cable with the pin connections switched on one end so that the NICs can talk to each other. You can make a crossover cable by removing the RJ-45 connector at one end and switching (or ‘crossing’) the wire pairs. The downside to using a crossover cable is that it only allows you to connect two computers. If you need to connect more than two computers, you have to buy a hub or use coax cable.
What is fiber-optic cable and how does it work?
Fiber-optic cable uses optical rather than electrical pulses to transmit signals.
Fiber-optic cable consists of pure silicon glass cylinders or strands surrounded by cladding. Each strand can pass a signal in only one direction so fiber-optic cable on a network typically consists of at least two strands: one for sending and one for receiving. Electronic signals generated by the computer are converted to optical signals in the form of photons which are transmitted (flashed) down the cable by a laser or light-emitting diode. A photo-detector on the other end collects the optical signals and they are converted back to electrical signals. Unlike copper cable, the signals on fiber-optic cable are not subject to the problems of attenuation, capacitance, or crosstalk. This greatly increases the potential transmission distance. In addition fiber-optic cable is more secure than copper cable. It does not generate electromagnetic signals and any external tap is easily detected by a reduction in signal strength. Fiber-optic cable is generally more expense than copper cable
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