Glossary

Takes the next fully established connection from a listening socket's queue and returns a brand-new socket for it. The listening socket keeps listening.

The device that bridges wireless clients onto the wired network and coordinates its basic service set. It's not the same thing as a router, even when they share a box.

Acknowledgement. In TCP, it's a flag plus a number that tells the sender which byte the receiver expects next. 802.11 has its own, separate link-layer ACK for every unicast frame.

How strongly an antenna focuses energy in its best direction compared with a reference, measured in dBi or dBd. Gain redistributes power. It doesn't amplify it.

Any loss of signal strength along the path, caused by distance, walls, cables or connectors. Measured in dB.

A management frame that an access point broadcasts roughly every 102 ms, announcing its SSID, supported rates, security settings and capabilities.

Adjusting the phase of the signal on several antennas so the waves add up at the client's position. It steers energy without moving anything.

Assigns a local address and port to a socket. Servers bind to a well-known port, and clients usually let the kernel pick an ephemeral one.

The MAC address that identifies one access point radio on one band. A single SSID can be served by many BSSIDs.

A slice of spectrum identified by its centre frequency and width. Channel 36, for example, is centred on 5180 MHz and can be 20 to 160 MHz wide.

How much unacknowledged data a TCP sender lets itself have in flight. It grows while ACKs arrive smoothly and shrinks when packets are lost.

Starts the TCP three-way handshake with a remote address. It returns once the handshake completes or fails, or right away for non-blocking sockets.

Carrier Sense Multiple Access with Collision Avoidance. Wi-Fi stations listen before talking and wait a random backoff, because a radio can't detect a collision while it's transmitting.

Antenna gain in decibels relative to an isotropic radiator, a theoretical antenna that radiates equally in every direction.

Power in decibels relative to one milliwatt. 0 dBm is 1 mW, +30 dBm is 1 W, and −90 dBm is one picowatt.

Dynamic Frequency Selection. On 5 GHz channels shared with radar, access points must check for radar and leave the channel if they detect it.

Dynamic Host Configuration Protocol. It gives a joining device its IP address, subnet, gateway and DNS servers in four messages: Discover, Offer, Request and Ack.

The simplest practical antenna: two conductors, each about a quarter wavelength long, fed in the middle. It has about 2.15 dBi of gain and a doughnut-shaped pattern.

Effective Isotropic Radiated Power: transmitter power plus antenna gain minus cable losses. This is the number regulators limit.

A temporary local port the OS assigns to the client side of a connection. IANA suggests 49152–65535, and Linux uses 32768–60999 by default.

The TCP flag that says “I'm done sending.” Each direction closes separately, so a graceful close usually involves two FINs and two ACKs.

The football-shaped region around the direct path between two antennas. Keep about 60% of the first zone clear, or the link loses signal even when you have line of sight.

Free-space path loss: the loss from a signal spreading out over distance, with no obstacles. It rises 6 dB every time the distance doubles.

SYN, SYN-ACK, ACK. Both sides exchange initial sequence numbers and options, such as MSS and window scaling, before any data flows.

The network-layer address of an interface: 32 bits for IPv4 and 128 bits for IPv6. Routers use it to forward packets hop by hop.

The time it takes data to get from A to B. Every layer adds some, from serialization on the radio to queues in routers to scheduling in your app.

Marks a bound socket as passive, so the kernel accepts incoming connections into a backlog queue for accept() to collect.

A 48-bit hardware address used on the local link. Wi-Fi frames carry up to four of them, and modern phones randomize theirs per network for privacy.

Multiple-Input Multiple-Output. Several antennas send independent spatial streams on the same channel, using multipath to tell them apart.

Maximum Segment Size: the largest payload TCP will put in one segment. It's 1460 bytes on a standard 1500-byte Ethernet MTU with IPv4.

Maximum Transmission Unit: the largest packet a link can carry without fragmenting it. 1500 bytes is the Ethernet standard.

Copies of the same signal arriving by different routes at slightly different times. They can cancel each other out (fading) or, with MIMO, add capacity.

Holds back small writes while earlier data is still unacknowledged and combines them into fewer packets. Disable it with TCP_NODELAY for latency-sensitive protocols.

The background level of RF energy on a channel, often around −90 to −95 dBm. Signals must rise well above it to be decoded.

Orthogonal Frequency-Division Multiplexing splits a channel into many narrow subcarriers. OFDMA, introduced in Wi-Fi 6, assigns groups of subcarriers to different clients at the same time.

The orientation of a wave's electric field: vertical, horizontal or circular. The antennas at both ends of a link should match.

A 16-bit number that identifies an application endpoint on a host. Combined with the IP addresses and the protocol, ports identify a connection.

Resending data the sender believes was lost. It's triggered by a timeout (RTO) or by three duplicate ACKs (fast retransmit).

Received Signal Strength Indicator, usually reported in dBm. A less negative number means a stronger signal, so −55 beats −75.

The TCP reset flag, which aborts a connection immediately. You'll see it as “connection refused” or “connection reset by peer.”

Round-trip time: how long a segment takes to reach the peer and its ACK to come back. TCP uses it to set its retransmission timers.

Signal-to-noise ratio: signal strength minus the noise floor, in dB. It decides which modulation, and therefore which data rate, a link can sustain.

The operating system's handle for one end of a network conversation. For TCP, a connection is identified by protocol, source IP and port, and destination IP and port.

Service Set Identifier: the human-readable network name, up to 32 bytes long. Hiding it doesn't make a network secure.

The TCP synchronize flag. It opens a connection and carries the initial sequence number.

Transmission Control Protocol: a reliable, ordered, connection-oriented byte stream with flow control and congestion control, built on top of IP.

The state that the side which closes first stays in for twice the maximum segment lifetime (60 seconds on Linux). It stops stray packets from an old connection from corrupting a new one.

User Datagram Protocol: connectionless datagrams with no ordering or delivery guarantees. It's simple and low-latency, and DNS, games, VoIP and QUIC all run on it.

The distance one cycle of a wave travels: λ = c / f. It's 12.5 cm at 2.4 GHz and about 6 cm at 5 GHz.

How many bytes the receiver will accept beyond what it has already acknowledged. Window scaling lets it grow beyond 64 KB.

The current Wi-Fi security standard. WPA3-Personal replaces the pre-shared key exchange with SAE, which resists offline password guessing.

A directional antenna with one driven element, a reflector behind it and several directors in front. Typical gain is 10–17 dBi.