Ch. 2-OSI Model

Transmission Control Protocol(TCP)

A connection-oriented transport pro- tocol. Connection-oriented transport protocols provide reliable transport, in that if a segment is dropped, the sender can detect that drop and retransmit that dropped segment. Specifically, a receiver acknowledges segments that it receives. Based on those acknowledgments, a sender can determine which segments were successfully received.

Cyclic Redundancy Check(CRC)

A mathematical algorithm that is executed on a data string by both the sender and receiver of the data string. If the calculated CRC values match, the receiver can conclude that the data string was not corrupted during transmission.

Physical Layer

Layer 1 of the OSI model, it is concerned with the transmission of bits on a network.

Protocol Data Unit(PDU)

The name given to data at different layers of the OSI model. Specifically, the PDU for Layer 4 is segment. The Layer 3 PDU is packet, the Layer 2 PDU is frame, and the Layer 1 PDU is bit.

Transport Layer

As Layer 4 of the OSI model, it acts as a dividing line between the upper layers and lower layers. Specifically, messages are taken from the upper layers (Layers 5-7) and encapsulated into segments for transmission to the lower layers (Layers 1-3). Similarly, data streams coming from lower layers are decapsulated and sent to Layer 5 (the session layer) or some other upper layer, depending on the protocol.

Session Layer

As Layer 5 of the OSI model, it's responsible for setting up, maintaining, and tearing down sessions.

Network Layer

Layer 3 of the OSI model, it is primarily concerned with forwarding data based on logical addresses.

Presentation Layer

Layer 6 of the OSI model, it is responsible for the formatting of data being exchanged and securing the data with encryption.

Application Layer

Layer 7 of the OSI model, it provides application services to a network. An important, and an often-misunderstood concept, is that end-user applications do not reside at the application layer. Instead, the application layer supports services used by end-user applications. Another function of the application layer is advertising available services.

Current State Modulation

One way to electrically or optically represent a binary 1 or 0 is to use current state modulation, which represents a binary 1 with the presence of voltage (on a copper cable) or the presence of light (on a fiber-optic cable). Similarly, the absence of light or voltage represents a binary 0.

State Transition Modulation

One way to electrically or optically represent a binary 1 or 0 is to use the transition between a voltage level (for example, going from a state of no voltage to a state of voltage, or vice versa, on a copper cable) or the transition of having light or no light on a fiber optic cable to represent a binary 1. Similarly, a binary 0 is represented by having no transition in a voltage level or light level from one time period to the next. This approach of representing binary digits is called state transition modulation.

Time-Division Multiplexing(TDM)

Supports different communication sessions (for example, different telephone conversations in a telephony network) on the same physical medium, by allowing sessions to take turns. For a brief period of time, defined as a time slot, data from the first session is sent, followed by data from the second sessions. This continues until all sessions have had a turn, and the process repeats itself.

Network Interface Layer(TCP/IP Stack)

The Network Interface Layer of the TCP/IP stack (also known as the network access layer) encompasses the technologies addressed by Layers 1 and 2 (that is, the physical and data link layers) of the OSI model.

Transport Layer(TCP/IP Stack)

The transport layer of the TCP/IP stack maps to Layer 4 (transport layer) of the OSI model. The two primary protocols found at the TCP/IP stack's transport layer are TCP and UDP.

Internet Layer(TCP/IP Stack)

This layer of the TCP/IP stack maps to Layer 3 (network layer) of the OSI model. Although multiple routed protocols (for example, IPv4 and IPv6) may reside at the OSI model's network layer, the Internet layer of the TCP/IP stack focuses on IP as the protocol to be routed through a network.

Data Link Layer

As Layer 2 of the OSI model, this layer is concerned with the packaging of data into frames and transmitting those frames on a network, performing error detection/correction, uniquely identifying network devices with an ad- dress, and handling flow control.

User Datagram Protocol(UDP)

A connectionless transport protocol. Connectionless transport protocols provide unreliable transport, in that if a segment is dropped, the sender is unaware of the drop, and no retransmission occurs.

Application Layer(TCP/IP Stack)

Addresses concepts described by Layers 5, 6, and 7 (that is, the session, presentation, and application layers) of the OSI model.

TCP/IP Stack

Also known as the DoD model, this four-layer model (as opposed tonthe seven-layer OSI model) targets the suite of TCP/IP protocols.