The OSI Reference Model Flashcards
(93 cards)
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
