OSI Layer

OSI Model

The conceptual framework consisting of seven abstraction layers (Physical, Data Link, Network, Transport, Session, Presentation, Application) used to standardize network device functions and protocol interactions.

Physical Layer (Layer 1)

The OSI layer responsible for the transmission and reception of raw unstructured bit streams over a physical medium, defining voltages, pinouts, cabling, and physical topology.

Data Link Layer (Layer 2)

The OSI layer responsible for node-to-node data transfer, error detection, and framing, operating via MAC addresses and split into two sublayers: LLC and MAC.

Network Layer (Layer 3)

The OSI layer responsible for packet forwarding, routing through internetworks, and logical addressing (e.g., IP addressing) via routers and Layer 3 switches.

Transport Layer (Layer 4)

The OSI layer providing transparent transfer of data between end systems, providing multiplexing, flow control, error recovery, and reliability mechanisms (TCP/UDP).

Session Layer (Layer 5)

The OSI layer that establishes, manages, and terminates communication sessions between applications across a network.

Presentation Layer (Layer 6)

The OSI layer ensuring that data from the application layer is readable by the receiving system, handling data encryption, compression, and translation syntax.

Application Layer (Layer 7)

The OSI layer closest to the end user, providing network services directly to software applications through protocols like HTTP, DNS, and FTP.

Media Access Control (MAC) Address

A unique 48-bit physical identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment (Data Link Layer).

Logical Addressing

A hierarchical network-layer addressing scheme (such as IPv4 or IPv6) assigned independently of hardware, enabling scalable routing across disparate networks.

IPv4 Address

A 32-bit numeric identifier expressed in dotted-decimal notation, split into network and host portions via a subnet mask.

IPv6 Address

A 128-bit hexadecimal identifier utilizing colon-hexadecimal notation and eliminating the need for NAT by drastically expanding address space.

CIDR (Classless Inter-Domain Routing)

A method for allocating IP addresses and IP routing syntax that replaces traditional classful routing, using a slash followed by the number of bits in the routing prefix (e.g., /24).

Subnet Mask

A 32-bit number masking an IP address to divide the address into network and host addresses, determining which subnet a device belongs to.

Default Gateway

The node on a computer network that serves as an access point to another network or the internet, typically a router's IP address.

Cat6 (Category 6)

A standardized twisted-pair cable for Ethernet supporting high-performance data rates up to 10 Gbps over limited distances (55 meters), featuring tighter twists and internal physical separators (splines) to reduce crosstalk.

Crosstalk

The electromagnetic interference induced by adjacent wire pairs within a network cable, which degrades signal integrity at high frequencies.

Single-Mode Fiber (SMF)

An optical fiber with a small core diameter (typically 8-10 microns) that allows only a single mode of light to propagate, enabling extremely high bandwidth and long-distance transmission over kilometers using lasers.

Multi-Mode Fiber (MMF)

An optical fiber with a larger core diameter (typically 50 or 62.5 microns) that permits multiple light rays (modes) to propagate simultaneously, typically used for shorter-distance intra-building links utilizing LEDs or VCSELs.

Modal Dispersion

A distortion phenomenon in multi-mode fiber where different light rays arrive at the receiver at slightly different times due to taking varying paths, limiting bandwidth and distance.

RJ45 (Registered Jack 45)

An 8-position, 8-conductor (8P8C) modular connector interface commonly used for terminating twisted-pair Ethernet cables in local area networks.

LC Connector (Lucent Connector)

A small-form-factor (SFF) fiber optic connector featuring a 1.25 mm ferrule and a latch-clip mechanism, highly popular for high-density patch panels and transceivers (SFP+).

SC Connector (Subscriber Connector / Square Connector)

A snap-in fiber optic connector featuring a 2.5 mm ferrule and a push-pull latching mechanism for quick, secure connections.

Attenuation

The gradual loss of signal strength or amplitude as an electrical or optical signal propagates through a transmission medium over distance.

SFP / SFP+ (Small Form-Factor Pluggable)

Compact, hot-pluggable optical transceiver modules used in switches and routers to interface with various fiber optic or copper cabling standards.