Brandon Ogilvie Introduction number mobile computing small-office

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Clock Solutions WiFi (IEEE 802.11)
Brandon Ogilvie Introduction
number mobile computing small-office homeoffice (SOHO) networks with broadband data service continues increase, requirement provide high-bandwidth hassle-free connections between broadband gateway user-client local area network (LAN) also increasing. Although copper fibre optic wires deliver such bandwidth LAN, neither relatively hassle-free mobile SOHO environments. Wires limit mobility require installation within subscriber's premise, which costly. alternate solution wireless quite hassle-free, however historically been limited bandwidth, thus resulting unacceptably data rates. address wireless bandwidth issue, Institute Electronic Electrical Engineers (IEEE) developed standards govern wireless networking protocols equipment performance. IEEE 802.11 standard establishes protocols using multiple radio frequency (RF) channels band network devices together over tens, hundred, even thousands meters. Such networks termed wireless local area networks (WLAN), commonly nicknamed WiFi. WiFi equipment market expected reach CY2003, with strong growth expected continue. ensure reliable, reasonably-secure wireless network connections optimum data rates while supporting maximum number simultaneous user-clients, highperformance quartz crystal (xtal) crystal clock oscillator (XO) stabilizes radio baseband/MAC processors each WiFi device. today's WiFi marketplace, equipment vendors offer variety products, including: Client NICs (PCMCIA, USB, PCI, etc); Access Points with 10/100 Ethernet; Access Points with Routing (multiple 10/100/1000 Ethernet up/downlinks); Access Points with embedded broadband uplinks (DSL, etc); Access Points serving point-to-point Bridges; Inter-Access Points (IAP) moving (roaming) users. result technology advancements that enable more data throughput wireless client-AP connection, several versions IEEE 802.11 standard exist. Table summarizes three primary versions standard.
Data RefClk Range Rate (typ) short Mbps Mbps Mbps IEEE 802.11 standards
Background
Wireless local area networks (WLAN) consist several elements, namely client access point. client represents group devices within WLAN that connected single point aggregation access point that connects Internet other network infrastructure. Clients notebook desktop computer, workstation, printer, other point-of-use device. Access points (AP) location-fixed, serving bridge between wireless user wired LAN. such, analogous cellular base station. Figure represents typical WLAN network configuration.
Standard 802.11a 802.11b 802.11g Year 1999 1999 2003 Table
With presence multiple standards, client NICs served given access point operate with different radio frequencies maximum data rates (bandwidth). Moreover, data encoding schemes differ between standards. Consequently, access points often backwards compatible with legacy standards, each standard introduced, public
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"hotspots" (access points made generally available public) must upgraded deliver comprehensive support various clients that roam into coverage area hotspot. This, addition hotspot installations, client upgrades, clients seeking more flexible network connectivity, contributes explosive growth WiFi marketplace. Market research firms estimate that million chipsets will shipped CY2003 with CAGR through CY2006 Insights 2003). these, approximately incorporated into access points incorporated into client NICs (iSuppli 2003). Although WiFi clients consumed significantly higher volumes than access points, estimated US$2 billion WLAN equipment market CY2003 stems from access points, with client NICs accounting remaining (Infometrics 2003). added functionality access points compared with client NICs accounts this price differential. Depending particular application, following frequency control devices incorporated into design: baseband/MAC reference clock; Sleep-mode reference clock; 10/100/1000 Ethernet reference clock; VCXO; Filter.
Application: Overview
Within each client, 802.11-enabled network interface card (NIC) with radio transceiver communicates with similar radio located access point. addition radio, each client access point also contains baseband/MAC controller. This controller regulates flow data between radio host device. Numerous semiconductor manufacturers have introduced chipsets that perform radio baseband/MAC function (although previous generations chipsets consisted five devices each, today's chipsets typically consist chips radio baseband/MAC). WiFi chipset vendors include Aetheros, AMD, Broadcom, Marvell, Micro Devices, Overall, more then fifty (50) vendors were offering 802.11-ready chipsets CY2002 Insights 2003). Figure typical functional block diagram 802.11 application.
Although SaRonix does currently offer filters, SaRonix does offer crystal, clock oscillator, VCXO products with optimal performance WiFi equipment. following sections present primary factors involved selecting frequency control device each type reference clock.
Application: Baseband/MAC REFCLK
Both client designs incorporate transceiver baseband/MAC that operate with common reference clock (REFCLK) input. Typical reference clock frequencies (802.11b), (802.11a 802.11g). baseband/MAC uses reference clock control access wireless network regulating timing, encryption, encoding decoding, movement data between radio (WLAN) host device. transceiver uses reference clock generate high frequency reference that stabilizes radio's (2.4 GHz), which either embedded specified external component. order ensure frequency lock between radios access point client tight-stability reference clock necessary. typical performance requirement ±25ppM all-inclusive frequency stability (includes initial calibration tolerance 25°C frequency changes over operating temperature, power load fluctuations, aging). Moreover, access points with dense channel spacing (capable serving several hundred clients simultaneously) require particularly phase noise.
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Client often designed incorporate power "sleep" mode conserve battery power. During sleep mode, continuous reference clock signal baseband/MAC necessary; crystal clock oscillator with low-power stand-by function recommended. Oscillators with low-power stand-by function consume 99.9% less power when disabled than oscillators with traditional tri-state output disable function. Oscillators low-power stand-by mode require time reactivate, however. typical oscillation output delay Advances crystal oscillator performance have enabled reductions supply voltage, resulting overall power reductions. Today, operating with 2.5V, 2.84V, 3.3V supply that meet stability phase-noise requirements WiFi readily available. When selecting between discrete oscillator design based quartz crystal (xtal) versus pre-packaged crystal clock oscillator (XO), 802.11 WiFi designs benefit from performance overall cost-savings using crystal clock oscillator (XO) instead quartz crystal. Although direct cost quartz crystal less than that crystal clock oscillator, all-inclusive frequency stability ±25ppM, higher frequency requirements, phase noise considerations tight channel spacing, real-estate negate perceived cost-savings using quartz crystal resonator. During manufacturing process, calibrated finetuned achieve ±25ppM all-inclusive performance. application, quartz crystal inside shielded from load variations that would otherwise pull frequency quartz crystal higher lower, crystals susceptible such when used directly circuit. Since ±25ppM overall tolerance temperature load power aging stability sits threshold manufacturability (barring costly temperature compensation schemes), slight load variations quartz crystal resonator would pull frequency outside ±25ppM allowance, thus resulting network connection failures. less susceptible these external load variations, such using reduces risk failure. Moreover, most quartz crystal resonators above 30~35 must used overtone mode. Attempting overtone crystal directly application successfully minimizing phase noise tighter channel spacing formidable challenge even most experienced designer, consumes significant time effort debug external oscillator circuit. Using quartz crystal instead allinclusive, pre-packaged increases risk consumes significant engineering resources. Overall, specifying rather than xtal better design alternative. Finally, packaged crystal oscillator (XO) occupies less real estate than conventional quartz crystal discrete oscillator
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design. Consequently, using saves overall cost timeto-market system.
Application: Sleep Mode REFCLK
Some designs client NICs industry-standard 32.768 crystal low-power "sleep" mode conserve battery power (preferred most portable applications such notebook computers). particular chipset selected baseband/MAC determines whether sleep mode reference clock utilized. Tuning fork crystals (32.768 kHz) feature maximum drive level mere such used commonly sleep mode reference clocks. While sleep mode, most functions baseband/MAC shutdown. Meanwhile, 32.768 crystal remains active. baseband/MAC will continue receive process 32.768 signal this establish wake-up intervals. These regular wake-up intervals necessary notify that client still within WLAN coverage area, check queued data. During this wake-up period, baseband/MAC reference clock oscillator activated re-establish connection with Without these regular wake-ups, will assume that client left coverage area, rerouting queued data.
Application: Ethernet REFCLK
Unlike client NICs that connect directly host device's PCMCIA, PCI, slot, access points must connect signals communicated with clients other pieces networking hardware. most common protocol used wired network connectivity 10/100 Ethernet, while highly integrated access points begin offer Gigabit Ethernet speed (1000 Base-T). Mbps optimal bandwidth client connections case with 802.11a WiFi access points using 10/100/1000 network connections have sufficient bandwidth incur data-throughput bottleneck. 10/100/1000 Ethernet require local reference clock; typical reference clock frequencies 10/100 Ethernet while typical reference clock frequencies 1000 Base-T Ethernet 62.5 MHz. All-inclusive frequency stability 10/100/1000 Ethernet clock typically specified ±50ppM. given Ethernet chipset usually specifies whether quartz crystal crystal clock oscillator (XO) used. Common 10/100 applications crystal while 1000 Base-T applications utilize enhanced economical performance available with single 10/100/1000 Ethernet reference clock used time single 10/100/1000 uplink multiple 10/100 ports AP-Router configuration.
Application: VCXO
Many WiFi access points deployed SOHO environment create WLAN. SOHO, digital subscriber line (DSL) leading protocol used worldwide connect local network Internet service provider's (ISP's) wide area network (WAN); modem (located SOHO) connects SOHO WAN. effort value reducing number pieces networking equipment SOHO, some WiFi equipment vendors offer access points with embedded modem port. Such device requires VCXO track lock data stream from ISP's central office (CO). typical frequencies VCXO 17.664 35.328 MHz. Discrete crystal VCXO well prepackaged allinclusive VCXOs readily available marketplace design alternatives. discrete crystal solution costs less than prepackaged VCXO, increases component count, real-estate, engineering resources required design debug VCXO circuit, overall risk. Furthermore, crystal selection long-term reliability quality essential building successful VCXO circuit. Specifying crystal with sufficiently high pullability compulsory overcome variations crystal frequency stability, track lock data stream from Ensuring such characteristics usually requires custom crystal specification; off-the-shelf crystals guaranteed meet pullability requirements applications.
Solutions: 10/100/1000 Ethernet REFCLK
Common frequencies: Typical Stability: S1613 S1614 S1612 (10/100 Base-T) 62.5 (1000 Base-T) ±50ppM (all-inclusive)
xtal, 5x11mm, cost xtal, 5x12mm SMD, cost 5x7mm SMD, 3.3V 5x7mm SMD, 2.5V 5x7mm SMD, 1.8V
Solutions: VCXO
Common frequencies: PI6X035 PI6CX100-35 ST1307 ST1317 HC-49/U 17.664 35.328 Pericom xtal+IC chipset, xtal Pericom xtal+IC chipset, VCXO, 5x7mm SMD, 3.3V VCXO, 5x7mm SMD, 3.3V xtal, 5x11mm, profile xtal, 5x11mm, cost
About Author
Brandon Ogilvie product manager SaRonix product division Pericom Semiconductor Corporation, ISO9001 certified quartz crystal oscillator designer manufacturer that supplied frequency control products since 1975. SaRonix Pericom support their global customer base with engineering, sales manufacturing facilities worldwide. more information about this topic frequency control products general, local sales contacts, applications engineering support, other product corporate information, please visit SaRonix site www.SaRonix.com Pericom website www.Pericom.com.
Solutions
Although SaRonix Pericom offer wide range crystal, clock, timing products, following recommendations offer best solutions WiFi equipment:
Solutions: Baseband/MAC REFLCK
Common frequencies: Typical stability: NKS6 NKS7 S1633 S1634 S1613 S2560 S2580 S1614 ±25ppM (all-inclusive)
xtal, 3.5x6mm SMD, seam-seal tight stability xtal, 5x7mm SMD, seam-seal tight stability 3.2x5mm SMD, ±25ppM, 3.3V 3.2x5mm SMD, ±25ppM, 2.5V 5x7mm SMD, ±25ppM, 3.3V 5x7mm SMD, ±25ppM, 2.84V 40~50MHz 5x7mm SMD, ±25ppM, 2.84V 5x7mm SMD, ±25ppM, 2.5V
Solutions: Sleep Mode REFCLK
Common frequency: 32S12 32.768 xtal, d3x8mm d2x6mm, 32.768 xtal, d2x8mm d1.5x7mm SMD, 32.768 xtal, 4x10mm 3x9mm SMD, 32.768
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