Category: Fedora

There are two major mobile broadband technology families: GSM/UMTS (which three quarters of the world uses) and CDMA/EVDO (used by the rest).  Keep in mind that UMTS uses CDMA as the radio technology, but incompatibly from CDMA/EVDO.

Back to School

GSM is a TDMA (Time Division Multiple Access) technology; communication is divided into a number of slots in which specific devices talk.  Each slot contains voice, data, or signalling information.  When it’s not your turn, you can’t talk.  Pretty simple, but given that it’s a TDMA technology, it’s prone to multipath interference and hard capacity limits.  You also have to carefully plan out your cell layout to ensure that adjacent towers don’t use the same frequency.

CDMA, on the other hand, is an ingenious spread-spectrum technology.  It’s got a great back story with movie stars and a war and stuff.  In contrast to GSM, in a CDMA system every user talks at the same time.  Each user is given a unique sequence of zeros and ones called a “spreading code” which is used to modulate the data stream over a certain frequency range (hence the spread-spectrum part).  On the receive side, when you know a user’s spreading code you apply it to the RF signal and retrieve the original data.  Each user in the cell just sees every other user’s signal as slightly increased background noise.  This is why CDMA is extremely robust against snooping and multipath interference, and why its capacity gracefully degrades as cell utilization increases.

What about Qualcomm?

Qualcomm holds many of the patents on CDMA since they spent a ton of time and money turning CDMA into a viable cellular radio technology 20 years ago.  They are also one of the largest sellers of cellular chipsets in the world.  We as open-source developers have to care, because their stuff shows up in tons of the devices we support.  Users don’t like being told “no”.

Most mobile broadband devices (Qualcomm’s included) appear as USB interfaces providing two or more serial ports.  One port is usually AT-command capable.  If you’re lucky, you get a secondary AT-capable port to use for signal quality and status while the primary port is using PPP for data transmission.  Most GSM/UMTS modems have a second AT port.  Most CDMA modems do not.

So when your device only has one AT-capable port, what language do the other ports speak?

Proprietary Protocol #1: QMI

This protocol is found on newer Qualcomm chipsets like the MSM7k series that show up in Android handsets Qualcomm Gobi data cards.  Google exposed some of the QMI protocol in the Android drivers.  Other details have recently turned up through the Gobi Linux driver sources, though Qualcomm doesn’t distribute sources for the “QCQMI DLKM” that probably contains the protocol mechanics.  It shouldn’t be too hard to reverse-engineer most of the protocol given these sources and a USB sniffer, but nobody has had the time yet.  QMI uses an HDLC-type framing which is quite common in proprietary mobile broadband protocols: a CRC-16 and 0x7E terminates a frame, and the frame is escaped such that 0x7E doesn’t show up in the data.  But since we haven’t reverse-engineered QMI yet, it isn’t the main focus of this post.

Proprietary Protocol #2: DM

Diagnostic Monitor is an older protocol found in most Qualcomm devices.  I’ve been interested in QCDM for a while, since without it, you can’t get signal strength and status from most CDMA devices while connected.  So I’ve been trawling the web for the past couple years looking for anything related to QCDM, and I finally hit the jackpot last fall:  the GPL sources for the Sprint-branded Linksys WRT54G3G-V2 router, which have since disappeared.  They include a GPL-licensed tool called ‘nvtlstatus’ which implements various pieces of the QCDM protocol.  The code is complete junk (as you’d expect from many embedded device manufacturers with schedules to hit) but it worked.

There’s also a sketchy Chinese package called “CDMA_Test.rar” that includes lists of the NVRAM items and some of the DM command numbers.  While not GPL, we can use the command numbering and structure definitions because it falls under the phonebook and interoperability copyright exceptions.  Additionally, there’s the TCL-based (ick) “RTManager” tool that implements some interesting QCDM commands, which, while we can’t use any of the code, is useful for structure field names that I hadn’t already guessed. Third, some guy did some reverse engineering of Novatel devices on Windows and built up a list of commands, subsystems, and NVRAM locations that were useful for confirming what I found in the other sources.

So through a combination of reverse engineering and these sources I wrote libqcdm, which we now use extensively in ModemManager for controlling CDMA devices.

DM Commands

Since DM is a pretty old protocol (2000 and possibly earlier), many of the commands are purely historical and currently unused.  The most interesting ones are:

• DIAG_CMD_VERSION_INFO: grabs firmware build dates and version information
• DIAG_CMD_ESN: grabs the CDMA device’s ESN, which is essentially the IMEI of a CDMA device
• DIAG_CMD_NV_READ and DIAG_CMD_NV_WRITE: NVRAM read/write commands, see below
• DIAG_CMD_SUBSYS: subsystem commands; see below
• DIAG_CMD_STATUS_SNAPSHOT: gives information about the current state and registration of the device on the CDMA 1x network

But given that many aren’t really used anymore, Qualcomm started running out of command IDs a long time ago…

Subsystems

So Qualcomm used command 75 (DIAG_CMD_SUBSYS) to extended the number of available commands; this command takes a subsystem selector and a subsystem command ID, thus getting around the original 8-bit command ID limitation.

There are a number of standard subsystems (Call Manager, HDR Manager, WCDMA, GSM, GPS, etc) but each manufacturer generally implements their own subsystem too.  In this way QCDM isn’t that different from AT commands; while supposedly standardized, each manufacturer inevitably implements a bunch of proprietary commands for their own device because the specs simply don’t cover everything.  This just makes our life harder.

The currently identified subsystems are:

• Call Manager: the most important command here reports the general state of the device, including the registered SID/NID, the terminal state (online/offline), the network mode (2G/3G), and various preferences that control which network the mobile registers with.  This is what we use to determine online/offline mode for CDMA devices since there aren’t any “standard” AT commands we can use to detect both 1x and EVDO registration.  Other commands start and end voice or data calls.
• HDR (High Data Rate, ie EVDO): the most important command here provides EVDO state, which is mostly taken from the state machines specified in the IS-856 standard.  This lets us figure out if the modem is registered on the EVDO network or the CDMA 1x network.
• Novatel: only implemented on Novatel Wireless devices, obviously.  But it provides access to a lot of stuff we want: the Extended Roaming Indicator (ERI) which shows detailed roaming state, the current access-technology the device is using (AMPS, digital, IS-95, CDMA 1x, EVDO r0, EVDO rA, etc), the voice mail and SMS indicators, and more.
• ZTE: for ZTE devices, obviously. I actually did reverse engineer this one using a ZTE AC2726 kindly provided by Huzaifas S. from Red Hat India.  All we’ve got so far is the signal strength, the other fields of the command are unknown.

There are also GSM and WCDMA subsystems used with Qualcomm UMTS chipsets, but since most UMTS devices have multiple AT-capable ports we’re less interested in using QCDM there.

NVRAM Locations

Each device has a number of NVRAM locations in which it stores various parameters like mode preference, roaming, home networks, radio parameters, and a whole bunch of other stuff.  Not all devices implement every location.  I’ve only included the locations that we actually use in libqcdm, but there a couple thousand.  The ones we currently use are:

• DIAG_NV_MODE_PREF: sets the mode preference: analog (ie AMPS), digital (TDMA), CDMA 1x, or EVDO (HDR)
• DIAG_NV_DIR_NUMBER: retrieves your Mobile Directory Number (MDN), aka your phone #
• DIAG_NV_ROAM_PREF: controls whether your device will roam on a partner network or not

The values each contains took a bit of time reverse-engineer using the Sprint connection manager, 3 different Sprint CDMA cards, and some USB traces, but now we’ve got the important parts.

Pulling It All Together

Earlier this year we had a number of bugs from Russian, Indian, and Czech Fedora users where ModemManager simply wouldn’t connect.  MM is pretty clever (a good thing) but the IS-707 AT commands aren’t useful enough to tell us what we need (not good).  The IS-707 standard AT+CAD? and AT+CSS commands really apply to the CDMA 1x network, not the EVDO network, and all these users had EVDO-only plans.  So when ModemManager checked AT+CSS and found that the device wasn’t registered, we sat around polling the registration state for a while.  The modem was already registered on the EVDO network, but not on a CDMA 1x network; of course AT+CSS doesn’t tell us that so MM got it wrong.

The real fix was to utilize QCDM and ask the Call Manager whether the modem was online or not, and if so, whether it had a 1x or an EVDO connection.  Sounds simple, but it took a lot of work to get there.

Next, since most CDMA devices only expose one AT-capable port, we need a way to get signal strength from the device while it’s connected and the primary port is talking PPP.  I’ll cover that in another blog post; stay tuned.  We still don’t have a good way to figure out which EVDO revision (either 0 or A) we’re using, nor can we get a reliable roaming indicator yet.

All of this is built in Fedora 12, 13, and rawhide if you’d like to take it for a spin.

The Kernel Side

Many devices provide the AT port via the standard CDC-ACM serial mechanism, which is picked up automatically by the kernel drivers.  But their QCDM-capable ports are only exposed via vendor-specific USB interfaces, so I created the qcaux driver to handle these ports; it’s in the 2.6.34 kernel.  With qcaux.ko and a recent version of ModemManager stuff will Just Work.

Why You Care

First a big shout to Qualcomm for keeping this shit secret.  NOT.  Double-plus-shout-out for keeping QMI secret; it’s a pretty simple protocol and there’s not much there worth keeping under wraps.  It might be nice to let open-source developers actually talk to your hardware.

With that out of the way, you care because we now have better support for a whole bunch of mobile broadband devices.  We even have support for CDMA signal strength while connected for the vast majority of CDMA devices that only expose one AT port.  I’ll talk about that later, since it’s quite an interesting story.

Why Sierra Wireless Rocks and Qualcomm Doesn’t

Buy Sierra stuff.  It’s top quality and they actually care about open-source, unlike Qualcomm’s mobile broadband division.  Last year I initiated a dialogue with Sierra about releasing some details of their proprietary Command and Status (CnS) protocol.  Being able to talk CnS to their modems gets us a lot that AT commands and even QCDM don’t provide, like roaming indicator, access technology, and RSSI.

And guess what?  They actually listened, did the work, and put the documentation under a Creative Commons license too.  I hear it’ll show up soon on their support site if it’s not there already (document #2131024, “CDMA 1xEV-DO CnS Reference”).

Sierra rocks.  Now if only Qualcomm would do it too…

Thomas Johnson (High School Janitor)

“Oh yeah, I’ve seen that code.  It’s worse than what I clean up in the bathrooms after Prom or Homecoming.  The kids get high and drunk and party too hard and puke all over the place.  I deal with enough vomit from 7:30 to 6; I wouldn’t touch the staging drivers with a mop twice as long as the one I have at work.”

Just Say No

Thomas just found out that none of the “staging” wifi drivers will work with hidden access points because they don’t set the IW_SCAN_CAPA_ESSID capability bit.  Furthermore, the most popular “staging” drivers (for the Ralink hardware used in many netbooks) don’t even have specific SSID scanning capability at all.

Why do you care?  Hidden APs don’t broadcast their network ID, which misinformed people think is more secure (hint: it’s not).  Before a driver can associate to the network, it needs to discover available APs and capabilities, which requires a probe-request, which exposes the network ID to everyone anyway.  But that requires driver support which none of the staging drivers have.

I fixed this issue upstream two years ago by adding IW_SCAN_CAPA_ESSID to Wireless Extensions.  Of course the staging WiFi drivers that many distros enable never got fixed because the vendor it came from didn’t bother to work with the community in the first place.  And people wonder why they don’t work.

Broadly speaking, staging WiFi drivers come in two flavors: (a) old dried gum from under the cafeteria table (drivers with a future), and (b) fresh vomit from the hung-over kid in your math class (those without a future).

The drivers with a future (winbond, rtl81xx) are or will based on the kernel-standard mac80211 wireless stack, which implements the 802.11 WiFi specification in the kernel.  Since they use the standard mac80211 stack, they get all these nice features like probe-scanning and the correct capability bits for free.  All you have to do is work on supporting the hardware itself.

The drivers without a future (rt2860, rt2870, rt3070, rt3090, wlan-ng, vt665x) are based on forks of the ancient ieee80211 stack that Intel’s ipw2x00 drivers forked from the hostap driver.  Each of these drivers includes their own copy of the core ieee80211 stack forked at different times and with different hacks.  When a bug shows up, that means 4x the work, and 4x the chance for the fix to slip through the cracks.  Which is why these drivers have no future.  They are a maintenance nightmare.  Besides, they have crap like this:

pAdapter->StaCfg.bScanReqIsFromWebUI = TRUE;

It just blows my mind why people think staging wifi drivers are a great idea.  There’s a reason staging drivers set the TAINT_CRAP flag in your kernel; because that’s what they literally are.

So what’s the right thing to do?

There’s one huge reason why dead-end staging drivers are a bad idea: there aren’t enough developers.  So do you spend that effort  on maintaining unmaintainable shit code?  Or do you spend it on fixing the code that has a future?  Most of the time you can’t do both.

If you choose to maintain the staging drivers, then things become worse over time since the staging code is simply less tested and less maintainable.   So you continue to drop hacks and fixes onto an ever-growing steaming pile of manure.  Nobody cares much about the driver (because it doesn’t use the standard kernel interfaces and thus doesn’t have a future), so your staging driver never benefits from all the great feature work and bug fixing that the mac80211 and wireless developers are doing.

But if you choose to help fix the upstream drivers that do use mac80211 (like rt2x00), and thus have a future, maybe for a few months some users won’t have great wireless.  But they didn’t before either.  But then 6 months later, all the users get great wireless with features like power saving, background scanning, WiFi Direct, Bluetooth 3, access-point mode, etc.  Those things will never be done to the staging drivers, because those drivers are a dead-end maintenance nightmare, because their code is awful, and because they don’t use the standard kernel wireless stack.

I know I’d invest the effort where it helps users the most, even if it means a few more months of subpar driver support while the official upstream drivers get fixed and the staging drivers go untouched.  That’s how things actually get better when you can’t fix everything at once.

Lately ofono and ConnMan have been in the news, and that’s sparked some discussion about how these two projects relate to NetworkManager.  I’ve mostly just been ignoring that discussion and focusing on making NetworkManager better.  But at some point the discussion needs to become informed and the facts need to be straightened out.

So what makes NetworkManager great?

• Flexibility: NM’s D-Bus interface provides a ton of control and information about the network connections of your machine.  Developers and applications simply don’t take enough advantage of this.  Imagine mail automatically pulled whenever the corporate VPN is up.  Or more restrictive firewalls when connected to public networks.  Yeah, you can do that today with NetworkManager.
• Works everywhere: from the mainframe to the power desktop to the netbook and lower.  There’s nothing stopping you from running NetworkManager on an s390 or a Palm Pre.
• Integration: most users like NetworkManager’s distro integration, so it’s on by default (but can be turned off for running bare-metal).  NM will read your distro’s network config files: ifcfg on Fedora, /etc/network/interfaces on Debian, etc.  It doesn’t pretend the rest of the world doesn’t exist, but it can if you tell it to.
• Connection Sharing: you can share your 3G connection to the wired or the wifi interface, or the other way around.  How you share is completely up to you.
• VPN: it’s got plugins for Cisco (vpnc), openvpn, openconnect, and pptp.  An ipsec/openswan plugin is being written.  It’s just easy to use the VPN of your choice.
• Makes Linux better: by not working around stupid vendor drivers or other broken components, NetworkManager drives many improvements in drivers, kernel APIs, the supplicant, and desktop applications.    Five years ago I posted a list of wifi problems, many of which got fixed because NetworkManager users complained about them.  Stuff like WPA capability fixes, hidden SSID fixes, suspend/resume improvements, Ad-Hoc mode fixes, and lots of improvements to wpa_supplicant to name just a few.  By encouraging drivers to be open, by fixing bugs in the open drivers and the stack instead of hacking around them, and by encouraging vendors to work upstream, NetworkManager makes Linux better for you.

What great stuff is coming next?

• Bluetooth
  
• IPv6
• Bridging
• Getting rid of HAL
• ModemManager and better mobile broadband

All in all, a lot of great stuff is on the plate.  NetworkManager already works well for a ton of people, but we’d like to make it work better for a lot more people.  And it will.

So what about ConnMan?

I recently came across a slide deck about ConnMan which makes both disappointing and inaccurate claims about NetworkManager.  It’s also worth emphasizing the philosophical differences between the two projects.

First, ConnMan primarily targets embedded devices, netbooks, and MIDs (slide #1).  When ConnMan was first released in early 2008, NetworkManager 0.7 was under heavy development, and NetworkManager 0.6 clearly did not meet the requirements.  But 0.7, released in November 2008, works well for a wide range of use-cases and hardware platforms.

NetworkManager scales from netbooks, MIDs, and embedded devices with custom-written UIs to desktops to large systems like IBM’s s390.  You get the best of both worlds: from phenomenal cosmic power down to itty-bitty living space.

ConnMan explicitly doesn’t try to integrate with existing distributions (slide #5), partly due to it’s requirements to be as light-weight as possible.  But NetworkManager will use your distro’s normal network config and startup scripts if you tell it to do (but you don’t have to).  Early in NetworkManager days we tried to ignore the rest of the world too.  Turns out that doesn’t work so well; users demand integration with their distribution.  But ConnMan doesn’t pretend to be general purpose, and due to its embedded focus, it can wave this issue away.

Both ConnMan and ofono reject well-established technologies like GObject (but still uses glib) in favor of re-implementing much of GObject internally anyway. This is a curious decision as GObject is not a memory hog and not a performance drag for these cases.  The NIH syndrome continues with libgppp, libgdhcp, and libgdbus, where instead of improving existing, widely-used tools like dhclient/dhcpcd, pppd, and dbus-glib, ConnMan opts to re-implement them in the name of being more “lightweight”.  With embedded projects that ConnMan targets (like Maemo and Moblin) already using GObject and dhcpcd, I don’t understand why this tradeoff was made.  Perhaps this visceral dislike of GObject and dbus-glib was one reason the project’s creators decided to write their own connection manager instead of helping to improve existing ones.

NetworkManager in contrast re-uses and helps improve components all over the Linux stack.  Because of that, more people benefit from the fixes and improvements that NetworkManager drives in projects like avahi, wpa_supplicant, the kernel, pppd, glib, dbus-glib, ModemManager, libnl, PolicyKit, udev, etc.

Taking a look at the deck

I have things to say about most of the slides, but I’ll concentrate on the most interesting and misinformed ones instead.

• Very Complex Design: a complete strawman, because it doesn’t say anything.  NetworkManager 0.7 is a mature project with many useful features.  NM is based around a core of objects, each one performing actions based on signals and events from other objects.  It’s modular and flexible.  It’s just not a ConnMan-style box of lego blocks with a rigid plugin API and all the problems that causes.
• Large Dependency List: NM requires things like wpa_supplicant, udev, dbus, glib, libuuid, libnl, and a crypto library.  pppd and avahi are optional. This list is certainly not large.  When you take ConnMan and its optional dependencies (most of which are needed in a useful system) the list is just about the same.
• Too Much Decision-making in the UI: Completely bogus and frankly incomprehensible.  The core NM daemon provides a default policy which is in no way connected to the UI, and the rest is up to the user.  nm-applet contains no policy whatsoever.  If the objection is to nm-applet’s desktop-centric interaction model, then it’s important to know there is no lack of applets for different use-cases.
• Tries to work around distro problems: this is completely a matter of perspective.  Since Intel was creating its own Linux distribution (moblin), they didn’t have to work around any existing issues; these were simply waved away.  Unfortunately NetworkManager lives in the world of reality and not some universe full of ponies.  For users that expect it, NetworkManager integrates with your distros existing network config, init scripts, and DNS resolution.  For users that don’t care, NetworkManager can run bare-metal.
• Too much GNOME-like source code: seriously, what the hell?  I’m not sure where to begin with this one.  The NetworkManager core does not depend on GNOME.  At all. Yeah, the source-code is in the Gnome style, but is that seriously an issue?

(Misinformation shaded blue for your protection)

The User Settings service is contained in the applet, and it’s completely optional.  The System Settings service has been merged back into the NetworkManager core daemon and is no longer a separate process.  That same commit ported NM from HAL to udev; thus HAL is no longer required.  NetworkManager always used HAL/udev for device detection instead of RTNL (ie, netlink).  NetworkManager also hasn’t used WEXT for a long time; wpa_supplicant handles kernel wireless configuration.  NetworkManager uses distro networking scripts only for service control, as does ConnMan.  The rest of the slide is quite petty and just splits hairs.

Where to?

It’s unlikely that either NetworkManager or ConnMan will disappear in the near future.  That means we’ll all have to live with two mutually exclusive connection managers and two completely different network configuration systems.  I think that’s pretty pointless, but I don’t get the last word anyway, since that’s not how Open Source works.  The users will decide which solution works best for them.  And that means NetworkManager will keep getting better, keep getting more useful, and will continue to be the easiest network management solution around.