The AirPort Express is a simplified and compact AirPort Extreme base station. It allows up to 10 networked users, and includes a feature called AirTunes (predecessor to AirPlay

AirPlay

AirPlay is a proprietary protocol stack/suite developed by Apple Inc. that allows wireless streaming of audio, video, and photos, together with related metadata between devices...

). The original version (M9470LL/A, model A1084) was introduced by Apple on June 7, 2004, and includes an analog–optical audio mini-jack output, a USB

Universal Serial Bus

USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....

 port for remote printing or charging the iPod (iPod Shuffle only), and a single Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 port.

The main processor of the 802.11g AirPort Express is a Broadcom

Broadcom

Broadcom Corporation is a fabless semiconductor company in the wireless and broadband communication business. The company is headquartered in Irvine, California, USA. Broadcom was founded by a professor-student pair Henry Samueli and Henry T. Nicholas III from the University of California, Los...

 BCM4712KFB wireless networking chipset, which has a 200 MHz MIPS

MIPS architecture

MIPS is a reduced instruction set computer instruction set architecture developed by MIPS Technologies . The early MIPS architectures were 32-bit, and later versions were 64-bit...

 processor built in. The audio is handled by a Texas Instruments

Texas Instruments

Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...

 Burr-Brown PCM2705 16-bit digital-to-analog converter

Digital-to-analog converter

In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

.

An updated version (MB321LL/A, model A1264) supporting the faster 802.11 Draft-N draft specification and operation in either of the 2.4 GHz and 5 GHz

ISM band

The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....

 bands, with almost all other features identical, was introduced by Apple on March 17, 2008. The revised unit includes an 802.11a/n (5 GHz) mode, which allows adding Draft-N to an existing 802.11b/g network without disrupting existing connections, while preserving the increased throughput that Draft-N can provide. Up to 10 wireless units can connect to this AirPort Express.

The Airport Express functions as a wireless access point

Wireless access point

In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...

 when connected to an Ethernet network. It can be used as an Ethernet-to-wireless bridge under certain wireless configurations.
It can be used to extend the range of a network, or as a printer and audio server.

An often overlooked feature of the AirPort Express is that it uses connectors which combine a 3.5 mm jack socket and a mini-TOSLINK optical digital connnector, allowing connection to an external digital-to-analog converter

Digital-to-analog converter

In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

 (DAC) or amplifier with internal DAC. This allows CDs ripped in iTunes

ITunes

iTunes is a media player computer program, used for playing, downloading, and organizing digital music and video files on desktop computers. It can also manage contents on iPod, iPhone, iPod Touch and iPad....

 in Apple Lossless

Apple Lossless

Apple Lossless Apple Lossless Apple Lossless (also known as ALAC (Apple Lossless Audio Codec), or ALE (Apple Lossless Encoder) is an audio codec developed by Apple Inc. for lossless data compression of digital music. After initially being proprietary for many years, in late 2011 Apple open sourced...

 format to stream to the AirPort Express

Remote Audio Output Protocol

Remote Audio Output Protocol , is an RTSP/RTP based protocol made by Apple Inc. It powers the AirPlay technology built into AirPort Express wireless access point as well as the Apple TV...

 which will in turn output a bitstream which is bit-for-bit identical to the original CD (provided volume adjustments, Sound Check and Sound Enhancer are switched off for playback in iTunes). DTS-encoded CDs ripped to Apple Lossless audio files which decode as digital noise in iTunes will play back correctly when the AirPort Express is connected via TOSLINK to a DTS-compatible amplifier–decoder.

On a MacOS system, the audio output of the airport express can only be used to wirelessly stream audio files from within iTunes to an attached stereo system. However, it can not be used to output the soundtrack of iTunes video content to an attached stereo.

AirPort Express	Model	Date	Wi-Fi standard	Wireless-to-Ethernet Bridge mode
M9470LL/A	A108x	June 2004 - March 2008	802.11b/g	No
MB321LL/A	A1264	March 2008 – Present	802.11a/b/g/Draft N	Yes

The suffix LL/A in the part number designates the US model.