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What does OSI stand for?
Open Systems Interconnection, which is a seven-layer network model
What does TCP/IP stand for?
Transmission Control Protocol/Internet Protocol
Name the 7 layers of the OSI model in order (# & name)
Layer 7 Application, Layer 6 Presentation, Layer 5 Session, Layer 4 Transport, Layer 3 Network, Layer 2 Data Link, Layer 1 Physical
Bottom-up mnemonic: "Please Do Not Throw Sausage Pizza Away."
Top-down mnemonic: "All People Seem To Need Data Processing."
What is UTP?
Unshielded Twisted Pair cable. Usually contains four pairs of wires that can transmit and receive data
Define the Physical Layer of OSI
Layer 1 of the OSI model defines the method of moving data between computers, so the cabling and central box are part of the Physical layer (Layer 1). Anything that moves data from one system to another, such as copper cabling, fiber optics, even radio waves, is part of the OSI Physical layer. Layer 1 doesn't care what data goes through; it just moves the data from one system to another system. NOTE: The NIC is NOT considered part of the Physical Layer, but usually is part of Layer 2 (Data Link).
What is a MAC address?
Inside every NIC, burned onto some type of ROM chip, is special firmware containing a unique identifier with a 48-bit value called the media access control address, or MAC address.
No two NICs ever share the same MAC address—ever. Any company that makes NICs must contact the Institute of Electrical and Electronics Engineers (IEEE) and request a block of MAC addresses, which the company then burns into the ROMs on its NICs. Many NIC makers also print the MAC address on the surface of each NIC.
Represented in hex, each hex being 4 bits, hence, 12 hex chars make up the 48-bit MAC address value.
E.g. 00-40-05-60-7D-49
The first six digits, in this example 00-40-05, represent the number of the NIC manufacturer. Once the IEEE issues those six hex digits to a manufacturer—referred to as the Organizationally Unique Identifier (OUI)—no other manufacturer may use them. The last six digits, in this example 60-7D-49, are the manufacturer's unique serial number for that NIC; this portion of the MAC is often referred to as the device ID.
What are 2 other ways to refer to the MAC address?
Most techs just call them MAC addresses, as you should, but you might see MAC-48 or EUI-48 on the CompTIA Network+ exam.
The Institute of Electrical and Electronics Engineers (IEEE) forms MAC addresses from a numbering name space originally called MAC-48, which simply means that the MAC address will be 48 bits, with the first 24 bits defining the OUI, just as described here. The current term for this numbering name space is EUI-48. EUI stands for Extended Unique Identifier.
How do NICs read computer data?
NICs send and receive the computer's binary data as pulses of electricity, light, or radio waves. The NICs that use electricity to send and receive data are the most common, so let's consider that type of NIC.
Just think of a charge on the wire as a one and no charge as a zero.
How does the network get the right data to the right system?
All networks transmit data by breaking whatever is moving across the Physical layer (files, print jobs, Web pages, and so forth) into discrete chunks called frames.
What is a "frame"?
A frame is basically a container for a chunk of data moving across a network. The NIC creates and sends, as well as receives and reads, these frames.
A number of different frame types are used in different networks. All NICs on the same network must use the same frame type, or they will not be able to communicate with other NICs.
Name the parts of a frame & describe each part.
Recipient's MAC | Sender's MAC | Type | Data | FCS
The frame starts with the MAC address of the NIC to which the data is to be sent, followed by the MAC address of the sending NIC.
Type: indicates the specific network technology of the frame.
Data: the payload. Note, NICs don't care at all what the data is; it just passes it along unaware. Special software will take care of what data gets sent and what happens to that data when it arrives. Different types of networks use different sizes of frames, but the frames used in most networks hold at most 1500 bytes of data.
FCS: special bit of checking information called the frame check sequence (FCS).
What is FCS?
Frame Check Sequence. The FCS uses a type of binary math called a cyclic redundancy check (CRC) that the receiving NIC uses to verify that the data arrived intact.
What is MAC addressing?
CompTIA calls the use of the MAC address to get frames to the proper computer or node MAC addressing.
What happens when the data to be sent is larger than the frame size?
The sending system's software must chop the data up into nice, frame-sized chunks, which it then hands to the NIC for sending. As the receiving system begins to accept the incoming frames, the receiving system's software recombines the data chunks as they come in from the network.
What is a hub?
In the early days of networking, the central box was called a hub. A hub was a dumb device, essentially just a repeater. When it received a frame, the hub made an exact copy of that frame, sending a copy of the original frame to every other system on the network.
The interesting part of this process was when the copy of the frame came into all the other systems. Only the NIC to which the frame was addressed would process that frame—the other NICs simply erased it when they saw that it was not addressed to their MAC address. This is important to appreciate: with a hub, every frame sent on a network was received by every NIC, but only the NIC with the matching MAC address would process that particular frame. Wasteful.
Give a brief description of a switch
Switches filter traffic by MAC address. Rather than sending all incoming frames to all network devices connected to it (like a hub), a switch sends the frame only to the recipient MAC address.
Give a brief overview of CRC
Most FCSs are only 4 bytes long, yet the average frame carries at most 1500 bytes of data. How can 4 bytes tell you if all 1500 bytes in the data are correct? That's the magic of the math of the CRC. Think of the CRC as just the remainder of a division problem. The NIC sending the frame does a little math to make the CRC. Using binary arithmetic, it works a division problem on the data using a divisor called a key. The result of this division is the CRC. When the frame gets to the receiving NIC, it divides the data by the same key. If the receiving NIC's answer is the same as the CRC, it knows the data is good.
What is a 'broadcast address'?
If it doesn't already know the MAC address, a NIC may send a broadcast onto the network to ask for it. The MAC address of FF-FF-FF-FF-FF-FF is the broadcast address—if a NIC sends a frame using the broadcast address, every single NIC on the network will process that frame. That broadcast frame's data will contain a request for a system's MAC address. Without knowing the MAC address to begin with, the requesting computer will use an IP address to pick the target computer out of the crowd. The system with the MAC address your system is seeking will read the request in the broadcast packet and respond with its MAC address.
What are the steps of a complete frame movement?
1. The sending system's operating system hands some data to its NIC. The NIC builds a frame to transport that data to the receiving NIC.
2. After the NIC creates the frame, it adds the FCS, and then dumps it and the data into the frame.
3. The NIC puts both the destination MAC address and its own MAC address onto the frame. It waits until no other NIC is using the cable, and then sends the frame through the cable to the network.
4. The frame propagates down the wire into the central box, which creates copies of the frame and sends it to every other system on the network (if the box is a hub).
5. Every NIC receives the frame and checks the MAC address. If a NIC finds that a frame is addressed to it, it processes the frame; if the frame is not addressed to it, the NIC erases it.
6. The receiving NIC uses the FCS to verify that the data is valid. If it is, the receiving NIC strips off all the framing information and sends the data to the software—the operating system—for processing. The receiving NIC doesn't care what the software does with the data; its job stops the moment it passes on the data to the software.
What device(s) is/are in Layer 1?
NIC, cabling, central box
What device(s) is/are in Layer 2?
NIC
What is LLC?
The many steps a NIC performs to keep data moving—sending and receiving frames over the wire, creating outgoing frames, reading incoming frames, and attaching MAC addresses—are classically broken down into two distinct jobs.
The first job is called the Logical Link Control (LLC). The LLC is the aspect of the NIC that talks to the system's operating system (usually via device drivers). The LLC handles multiple network protocols and provides flow control.
What is the Media Access Controller (MAC)?
The many steps a NIC performs to keep data moving—sending and receiving frames over the wire, creating outgoing frames, reading incoming frames, and attaching MAC addresses—are classically broken down into two distinct jobs.
The second job is called the Media Access Control (MAC), which creates and addresses the frame. It adds the NIC's own MAC address and attaches MAC addresses to the frames. Recall that each frame the LLC creates must include both the sender's and recipient's MAC addresses. The MAC sublayer adds or checks the FCS. The MAC also ensures that the frames, now complete with their MAC addresses, are then sent along the network cabling.
Which layer(s) has/have sub-layers?
Only the Data Link Layer (LLC & MAC).
What happens in Layer 3?
At the Network layer, Layer 3, containers called packets gets created and addressed so they can go from one network to another.
What is IP's main purpose?
The Internet Protocol is the primary logical addressing protocol for TCP/IP. IP makes sure that a piece of data gets to where it needs to go on the network. It does this by giving each device on the network a unique numeric identifier called an IP address. An IP address is known as a logical address to distinguish it from the physical address, the MAC address of the NIC.
What does an IP address contain?
IP uses a rather unique dotted decimal notation (sometimes referred to as a dotted-octet numbering system) based on four 8-bit numbers. Each 8-bit number ranges from 0 to 255, and the four numbers are separated by periods. No two systems on the same network share the same IP address. While MAC addresses are burned into the NICs, IP addresses are set and controlled through system software.
What address do routers use? Why?
Routers use the IP address, not the MAC address, to forward data. This enables networks to connect across data lines that don't use Ethernet, like the telephone network. Each network type (such as Ethernet, SONET, ATM, and others) uses a unique frame.
What are the parts of the IP packet and wrapper?
Each IP packet (contains src IP, dest IP, data) is handed to the NIC, which then encloses the IP packet in a regular frame, creating, in essence, a packet within a frame. Visualize the packet as an envelope, with the envelope in the pneumatic canister frame. The beauty of IP packets is that they can travel unchanged in many frame types, not just those for ethernet transport.
What happens to the framed IP packet when it reaches the router?
When you send data from one computer to another on a TCP/IP network such as the Internet, that data can go through many routers before it reaches its destination.
Each router:
- strips off the incoming frame,
- determines where to send the data according to the IP address in the packet,
- creates a new frame, and
- sends the packet within a frame on its merry way.
The new frame type will be the appropriate technology for whatever connection technology connects to the next router. That could be a cable or DSL network connection, for example. The IP packet, on the other hand, remains unchanged.
What happens when the packet reaches the destination router?
Once the packet reaches the destination subnet's router, that router will strip off the incoming frame (no matter what type), look at the destination IP address, and then add a frame with the appropriate destination MAC address that matches the destination IP address.
What happens when the packet reaches the destination NIC?
The receiving NIC strips away the Ethernet frame and passes the remaining packet off to the software. The networking software built into your operating system handles all the rest of the work. The NIC's driver software is the interconnection between the hardware and the software. The NIC driver knows how to communicate with the NIC to send and receive frames, but it can't do anything with the packet. Instead, the NIC driver hands the packet off to other programs that know how to deal with all the separate packets and turn them into Web pages, e-mail messages, files, and so forth.
Basic overview of how the transport protocol breaks up the data on sending.
The transport protocol breaks up the data into chunks called segments or datagrams (depending on the specific transport protocol used) and gives each segment some type of sequence number. (Datagrams are simpler and don't get sequence numbers.). Embedded into the data of each packet containing a segment is a sequencing number. By reading the sequencing numbers, the receiving system knows both the total number of segments and how to put them back together.
What is the primary purpose of the Transport Layer?
It's the assembler/disassembler software. As part of its job, the Transport layer also initializes requests for packets that weren't received in good order.
What's the primary purpose of the Session Layer?
Layer 5, the Session layer, handles all the sessions for a system. The Session layer initiates sessions, accepts incoming sessions, and opens and closes existing sessions.
What's the primary purpose of the Presentation Layer?
The Presentation layer (6) translates data from lower layers into a format usable by the Application layer, and vice versa.
What is the primary purpose of the Application Layer?
Layer 7 encompasses the APIs that allow applications to use the network functions of a system.
What is encapsulation in network modeling?
The term encapsulation encompasses the entire process of preparing data to go onto a network. This includes all the steps from the application to the Application, Presentation, Session, Transport, Network, and Data Link layers. Each layer adds more information so that the data gets to the correct recipient and the recipient knows what to do with the data.
The receiving computer reverses the process, stripping all the extra header information out as the data goes up the stack. This reverse process is called de-encapsulation.
Summarize the TCP/IP layers' functions
The Application layer programs create the data. The Transport layer breaks the data into chunks, putting those chunks into TCP segments or UDP datagrams. The Internet layer adds the IP addressing and creates the IP packets. The Link layer wraps the IP packet into a frame, with the MAC address information and a frame check sequence (FCS).
Name the 4 layers of the TCP/IP model
Application, Transport, Internet, Link/Network Interface. Note that these layers are not numbered like the OSI model.
What is the Link/Network Interface layer in TCP/IP and what OSI layer(s) do(es) it encompass?
Any part of the network that deals with complete frames is in the Link layer. The moment the frame information is stripped away from an IP packet, we move out of the Link layer and into the Internet layer. The TCP/IP model lumps together the OSI model's Layer 1 and Layer 2 into a single layer. All of the pieces in the OSI model (cabling, hubs, physical addresses, and NICs) sit squarely in the Link layer.
What is the Internet layer in TCP/IP and what OSI layer(s) do(es) it encompass?
Any device or protocol that deals with pure IP packets—getting an IP packet to its destination—sits in the Internet layer. IP addressing itself is also part of the Internet layer, as are routers and the magic they perform to get IP packets to the next router. IP packets are created at this layer. It maps to the Network Layer in OSI.
What is the Transport Layer in TCP/IP and what OSI layer(s) do(es) it encompass?
The Transport layer combines features of the OSI Transport and Session layers with a dash of Application layer. It involves assembling and disassembling of data. The Transport layer breaks that data into chunks, adding port numbers and sequence numbers, creating the TCP segment. The Transport layer then hands the TCP segment to the Internet layer, which, in turn, creates the IP packet.
What is connection-oriented protocol and what type of packet does it use?
Client and server do a pre-check to make sure there's a good connection before sending data. This uses Transmission Control Protocol (TCP).
What is connectionless protocol and what type of packet does it use?
Client will send data to server without knowing if there is a connection. It uses User Datagram Protocol (UDP).
What is a TCP segment, basically?
TCP segments have fields that ensure the connection-oriented communication works properly. Strip away the IP addresses from an IP packet. What's left is a chunk of data in yet another container called a TCP segment. TCP segments have many other fields that ensure the data gets to its destination in good order.
What is a UDP datagram, basically?
UDP gets data from the Application layer and adds port and length numbers plus a checksum to create a container called a UDP datagram. A UDP datagram lacks most of the extra fields found in TCP segments, simply because UDP doesn't care if the receiving computer gets its data.
What is the Application Layer in TCP/IP, and what OSI layer(s) do(es) it map to?
The TCP/IP Application layer combines features of the application, presentation, and session layers of the OSI model. Every application, especially connection-oriented applications, must know how to initiate, control, and disconnect from a remote system. No single method exists for doing this. Each TCP/IP application uses its own method.
What is the range of TCP/IP port numbering?
1 to 65535
List the TCP/IP Model Layers and Corresponding Data Structures for each
Link Layer==Frame, Internet Layer==IP Packet, Transport Layer==TCP segment/UDP datagram, Application Layer==the data/payload initializing or receiving.
Bus topology vs Ring topology - describe each?
Bus topology involves a single cable (bus) that connects all computers in a line. A ring topology connects all computers on the network with a ring of cable.
How does a bus topology network push data around?
In bus topology networks, data from each computer simply goes out on the whole bus. A network using a bus topology needs termination at each end of the cable to prevent a signal sent from one computer from reflecting at the ends of the cable, quickly bringing the network down.
How does a ring topology network push data around?
In a ring topology network, in contrast to a bus, data traffic moves in a circle from one computer to the next in the same direction. With no end to the cable, ring networks require no termination.
Basic concept of Star network topology
The star topology uses a central connection box for all the computers on the network.
What's the main advantage of a Star topology over ring and bus?
Star topologies have a huge benefit over ring and bus topologies by offering fault tolerance—if one of the cables breaks, all of the other computers can still communicate.
What did bus and ring topology hybrids do to make them act more like star topologies?
The ring topology network designers struck first by taking the entire ring and shrinking it into a small box and connecting computers to that box (like a star). Bus designers shrunk their segment (bus) into a single box that had terminators built in, and spanned out like a star.
What was IBM's implementation of the Ring topology called?
Token Ring.
Describe more modern topology nomenclature.
When we talk about topology today, we separate how the cables physically look (the physical topology) from how the signals travel electronically (the signaling topology or logical topology).
What does a Mesh topology network look like?
In a mesh topology network, every computer connects to every other computer via two or more routes. Some of the routes between two computers may require traversing through another member of the mesh network.
Describe the 2 types of meshed topology networks
In a "partially meshed topology network", at least two machines have redundant connections. Every machine doesn't have to connect to every other machine. In a "fully meshed topology network", every computer connects directly to every other computer.
What does a point-to-multipoint topology look like?
In a point-to-multipoint topology, a single system acts as a common source through which all members of the point-to-multipoint network converse.
What does a point-to-point topology look like?
Point-to-Point
In a point-to-point topology network, two computers connect directly together with no need for a central device of any kind.
The 2 main types of cable groups
Copper and Fiber-optic
The 2 primary types of copper cabling
Coaxial and twisted pair.
Describe the parts of a coaxial cable
Coaxial cable contains a central conductor wire (usually copper) surrounded by an insulating material, which, in turn, is surrounded by a braided metal shield.
Why is it called a coaxial cable?
The cable is referred to as coaxial (coax for short) because the center wire and the braided metal shield share a common axis or centerline.
Describe EMI's effect on data transmission
Many devices in the typical office environment generate magnetic fields, including lights, fans, copy machines, and refrigerators. When a metal wire encounters these magnetic fields, electrical current is generated along the wire. This extra current—Electromagnetic Interference (EMI)—can shut down a network because it is easily misinterpreted as a signal by devices like NICs.
How does a coax cable prevent EMI?
To prevent EMI from affecting the network, the outer mesh layer of a coaxial cable shields the center wire (on which the data is transmitted) from interference
What type of connector does a coax have at each end?
These cables use an F-connector.
What do cable modems usually use to connect?
RG-6, rather than the thinner RG-59
What is a RG Rating
Radio Grade - The U.S. military developed these ratings to provide a quick reference for the different types of coax. The only important measure of coax cabling is its Ohm rating, which is usually printed on the cable.
What does the Ohm rating indicate?
The Ohm rating of a particular piece of cable describes the impedance of that cable.
Define Impedance
Impedance describes a set of characteristics that define how much a cable resists the flow of electricity. This isn't simple resistance, though. Impedance is also a factor in such things as how long it takes the wire to get a full charge—the wire's capacitance—and more.
START AT TWISTED PAIR
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