The OSI Reference Model Flashcards
The OSI Model
Layer 1: Physical Layer Layer 2: Data Link Layer Layer 3: The Network Layer Layer 4: The Transport Layer Layer 5: The Session Layer Layer 6: The Presentation Layer Layer 7: The Application Layer
Acrostic:
All People Seem To Need Data Processing.
Protocol Data Unit (PDU) Names
Layer 4: Transport Layer = Segments
Layer 3: Network Layer = Packets
Layer 2: Data Link Layer = Frames
Layer 1: Physical Layer = Bits
Layer 1: Physical Layer
The concern of the physical layer is the transmission of bits on the network along with the physical and electrical characteristics of the network. The Physical Layer defines the following:
- How bits are represented on the medium.
- Wiring standards for connectors and jacks
- Physical topology
- Synchronizing bits
- Bandwidth usage
- Multiplexing strategy
Examples of devices defined by the physical layer standards include hubs, wireless access points, and network cabling.
Physical Layer: How to represent bits on the medium
Data on a computer network is represented as a binary expression. Electrical voltage (on copper wiring) or light (carried via fiber-optic cabling) can represent these 1s and 0s. The presence or absence of voltage on a wire portrays binary 1 or 0. Similarily, the presence or absence of light on a fiber-optic cable renders a 1 or 0 in binary. An alternate approach to portraying binary data is State Transition Modulation, where the transition between voltages or the presence of light shows a binary value.
Physical Layer: Wiring standerds for connectors and jacks
TIA/EIA-568-B standard, how to wire an RJ-45
Physical Layer: Physical topology
Layer 1 devices view a network as a physical topology (as opposed to a logical topology).
Examples:
- Bus
- Ring
- Star
Physical Layer: Synchronizing bits
For two networked devices to successfully communicate at the physical layer, they must agree on when one bit stops and another bit starts. Specifically, the devices need a method to synchronize the bits.
Synchronizing bits: Asynchronous
With this approach, the sender states that it is about to start transmitting by sending a start bit to the receiver. When the receiver sees this, it starts its own internal clock to measure the next bits. After the sender transmits its data, it sends a stop bits to say that it has finished its tranmission.
Synchronizing bits: Synchronous
This approach synchronizes the internal clocks of both the sender and the receiver to ensure that they agree on when bits begin and end. A common approach to make this synchronization happen is to use an external clock (for example, a clock given by the service provider). The sender and receiver reference this clock.
Bandwidth Usage:
The two fundamental approaches to bandwidth usage on a network are Broadband and Baseband.
Bandwidth Usage: Broadband
Broadband technologies divide the bandwidth available on a medium (for example, a copper or fiber-optic cabling) into different channels. A sender can then transmit different communication streams over the various channels. For example, consider frequency-division multiplexing (FDM) used by a cable modem. Specifically, a cable modem uses certain ranges of frequencies on the cable to carry incoming data, another range of frequencies for outgoing data, and several other frequency ranges for various TV stations.
Bandwidth Usage: Baseband
Baseband technologies, in contrast, use all the available frequencies on a medium to send data. Ethernet is an example of a networking technology that uses baseband.
Physical Layer: Multiplexing Strategy
Multiplexing allows multiple communication sessions to share the same physical medium. Cable TV, as previously mentioned allows you to receive multiple channels over a single physical medium (for example, a coaxial cable plugged in the back of your television).
Multiplexing Strategy: Time-division multiplexing (TDM)
TDM supports different communication sessions (for example, different telephoneconversations in a telephony network) on the sane physical medium by causing the sessions to take turns. For a brief period, defined as a time slot, data from the first session is sent, followed by data from the second session. This continues until all sessions have had a turn, and the process repeats itself.
Multiplexing Strategy: Statistical time-division multiplexing (StatTDM)
A downside to TDM is that each communication session receives its own time slot, even if one of the sessions dow not have any data to send at the moment. To make a more efficient use of available bandwidth, StatTDM dynamically assigns time slots to communications sessions on as as-needed basis.
Multiplexing Strategy: Frequency-division multiplexing (FDM)
FDM divides a medium’s frequency range in channels, and different communication sessions send their data over different channels. As previously described, this approach to bandwidth usage is called broadband.
Layer 2: The Data Link Layer
The data link layer is concerned with the following:
- Packaging data into frames and transmitting those frames on the network.
- Performing error detection/correction.
- Uniquely finding network devices with an address.
- Handling flow control.
In fact, the data link layer is unique from the other layers in that it has two sublayers of its own: MAC and LLC. Examples of devices defined by the data link layer standards include switches, bridges, and NICs.
The Data Link Layer: Media Access Control (MAC) Physical Addressing
A MAC address is a 48-bit address assigned to a device’s network interface card (NIC). MAC addresses are written in hexadecimal notation (for example, 58:55:ca:eb:27:83). The first 24 bits of the 48-bit address is the vendor code. The IEEE Registration Authrority assigns a manufacturer one or more unique vendor codes. The last 24 bits of a MAC address are assigned by the manufacturer, and the act as a serial number for the device. No two MAC addresses in the work should have the same value.
The Data Link Layer: Media Access Control (MAC) Logical Topology
Layer 2 devices view a netowrk as a logical topology.
Example:
- Bus
- Ring
The Data Link Layer: Media Access Control (MAC) Method of Transmitting on The Media
With several devices connected to a network, there needs to be some strategy for deciding when a device sends on the media. Otherwise, multiple devices might send at the same time and thus interfere with one another’s transmissions.
The Data Link Layer: Logical Link Control (LLC) Connection Services
When a device on a network recieves a message from another device on the network, that recipient device can give feedback to the sender in the for of and acknowledgement message.
Logical Link Control (LLC) Connection Services:
Flow Control
Limits the amount of data a sender can send at one time; this prevents the sender from overwhelming the receiver with too much information.
Logical Link Control (LLC) Connection Services:
Error Control
Allows the recipient of data to let the sen know whether the expected data frame was received or whether it was received but is corrupted. The recipient figures out whether the data frame is corrupt by mathematically calculating a checksum of the data received. IF the calculated checksum does not match the checksum received with the data frame, the recipient of the data draws the conclusion that the data frame is corrupted and can then notify the sender via an acknowledgement message.
The Data Link Layer: Logical Link Control (LLC) Synchronizing Transmissions
Senders and recievers of data frames need to coordinate when a data frame is being transmitted and should be recieved
Synchronizing Transmissions Synchronizing Transmissions: Isochronous
With isochronous transmission, network devices look to a common device in the network as a clock source, which creates fixed length time slots. Network devices can determine how much free space, if any, is available within a time slot and then insert data in an available time slot. A time slot can accommodate more than one data frame. Isochronous transmission does not need to provide clocking at the beginning of a data string (as does synchronous transmission) or for every data frame (as does asynchronous transmission). As a result, isochronous transmission uses little overhead when compared to asynchronous or synchronous transmission methods.
Synchronizing Transmissions Synchronizing Transmissions: Asynchronous
With asynchronous transmission, network devices reference their own internal clocks, and network devices do not need to synchronize their clocks. Instead, the sender places a start bit at the beginning of each frame and a stop bit at the end of each data frame. These start and stop bits tell the receiver when to monitor the medium for the presence of bits.
Asynchronous: Parity Bit
The Parity but, might also be added to the end of each byte in a frame to detect an error in the frame. For example, if even parity error detection (as opposed to odd parity error detection) is used, the parity bit (with value of their 0 or 1) would be added to the end of a byte, causing the total number of 1s in the data frame to be an even number. If the receiver of a byte is configured for even parity error detection and receives a byte where the total number of bits (including the parity bit) is even, the receiver can conclude that the byte was not corrupted during transmission. NOTE: The parity bit might not be effective if a byte has more than one error.
Synchronizing Transmissions Synchronizing Transmissions: Synchronous
With synchronous transmission, two network devices that want to communicate between themselves must agree on a clocking method to show the beginning and ending of data frames. One approach to providing this clocking is to use a separate communications channel over which a clock signal is sent. Another approach relies on specific bit combinations or control characters to indicate the beginning of a frame or a byte of data. Like asynchronous, synchronous can perform error detection. However, rather than using parity bits, synchronous communications runs a mathematical algorithm on the data to create a cyclic redundancy check (CRC). If both the sender and receiver calculate the same CRC value for the same chunck of data, the receiver can conclude that the data was not corrupted during transmission.
Layer 3: The Network Layer
The network layer is primarily concerned with forwarding data based on logical addresses. The Network Layer concerns:
- Logical Addressing
- Switching
- Route Discovery and Selection
- Connection Services
- Bandwidth Usage
- Multiplexing Strategy
Devices found at the network layer include touters and multilayer switches. Most common Layer 3 protocol on which the internet is based is, IPv4. However, IPv6 is becoming more common.
The Network Layer: Logical Addressing:
The network layer uses logical addressing to make forwarding decisions. A variety of routed protocols (for example, AppleTalk, and IPX) have their own logical addressing schemes, but by far, the most widely deployed routed protocol is Internet Protocol (IP).
The Network Layer: Switching:
Engineers often associate the term switching with Layer 2 technologies; however, the concept of switching also exists at Layer 3. Switching, at its essence, is making decisions about how data should be forwarded.
Layer 3 Switching: Packet Switching
With packet switching, a data stream is divided into packets. Each packet has a Layer 3 header that includes a source and destination Layer 3 address. Another term for packet switching is routing.
Layer 3 Switching: Circut Switching
Circut switching dynamically brings up a dedicated communication link between two parties for those parties to communicate.
Example:
Making a phone call on a landline - the telephone company’s switching equipment interconnects your home phone with the phone system of the business you are calling. This interconnection (that is, circuit) only exists for the duration of the call.
Layer 3 Switching: Message Switching
Message switching is usually not well suited for real-time application because of the delay involved. Specifically with message switching, a data stream is divided into messages. Each message is tagged with a destination address, and the messages travel from one network device to another network device on the way to their destination. Because these devices might briefly store the messages before forwarding them, a network using message switching is sometimes called a store-and-forward network. Metaphorically, you could visualize message switching like routing an email message, where the email message might be briefly stored on an email server before forwarding to the recipient.
The Network Layer: Route Dicscovery and Selection
Because Layer 3 devices make forwarding decisions based on logical network address, a Layer 3 device might ned to know how to reach various network addresses. For example, a common Layer 3 device is a router. A router can maintain a routing table indicating how to forward a packet based on the packet’s destination network address. A router can have its routing table populated via manual configuration (that is by entering static routes), via a dynamic routing protocol (for example, RIP, OSPF, or EIGRP), or simply by the fact that the router is directly connected to certain networks.
The Network Layer: Connection Services
Just as the data link layer offers connection services for flow control and error control, connection services also exist at the network layer. Connection services at the network layer can improve the communication reliability, if the data link’s LLC sublayer is not performing connection services.
Layer 3 Connection Services: Flow Control (AKA congestion control)
Helps prevent a sender form sending data more rapidly than the receiver is capable of receiving it.
