Thursday, 13 November 2014

Basic Internet Connectivity Setup Using HWIC-3G-GSM Card

In addition to working on some spanking new 4G Cisco 819 devices, I've occasionally had to slum it by providing Internet access with a normal 1800 / 2800 series router and an HWIC-3G-GSM card. Once you know what's involved the config is remarkably simple but it can be difficult to understand what's what at first.

The video below takes a very quick walk through setting this up, some further explanation is given beneath that for those interested.

Building Blocks

While there are a few mobile-specific pieces of configuration, anyone who has previously worked on ISDN, async modems or ADSL on Cisco routers will probably find a lot of familiar concepts. Here are the main elements of a 3G / 4G configuration:

Cellular Profile 

This is where the APN address and authentication mode are configured. These are saved to the modem's NVRAM as soon as they are applied. Here's an example of how to set a cellular profile on the two different platforms:

Router#cell 0/0/0 gsm profile create 1
Profile 1 will be created with the following values:                            
PDP type = IPv4                                                                 
APN =                                                               
Are you sure? [confirm]                                                         

Profile 1 written to modem                                                      

The number "1" here indicates which profile slot on the modem will be used to store the details. This is significant later on because there may be multiple APNs configured and the router needs to know which to use when connecting.

This example is for three, a UK mobile carrier which is interesting because the APN uses no authentication. If your APN requires authentication, simply follow the APN with either a pap or chap keyword, the username and finally the password.

Note that this is applied at the exec prompt rather than in config mode.

Cellular Interface

The physical radio interfaces are referred to using the "Cellular" prefix, in this case Cellular0/0/0. The Cellular interface is where the dialer, authentication and IP details are normally configured - I say normally as there are many different ways to configure dialers depending on what kind of load balancing and resilience are required. For a typical 3G deployment, though, you will only have one physical interface and so the simplest way is to forget about pools and put the config straight onto that.

Here is an example configuration showing the key elements:

interface Cellular0/0/0
 ip address negotiated
 encapsulation ppp
 dialer in-band
 dialer string "*98*1#"
 dialer-group 1
 ppp chap refuse
dialer-list 1 protocol ip permit
ip route Cell0/0/0

The key thing to notice here is the "*98*1#" dialer string. The "*98*" and "#" are fixed, the "1" refers to the profile slot number used earlier. If you used a different slot, refer to it here.

The rest is fairly standard dialer stuff, in this example I've made the dialer-list so that any IP traffic will cause it to connect.

Sundry Config

At this point the router should be able to connect to the cellular network. For most purposes, though, you will need to either set up NAT or some sort of VPN tunnel for the connection to be of any use. These are set up the same way as for any other setup.

Testing and Diagnostics

How can you tell whether the cellular connection is coming up? The first clue is that log entries similar to the following should appear:

%LINK-3-UPDOWN: Interface Cellular0/0/0, changed state to 

To check whether the modem is attached to the radio network, use the following commands:

Router#show cell 0/0/0 network                                                  
Current Service Status = Normal, Service Error = None                           
Current Service = Combined                                                      
Packet Service = HSDPA (Attached)                                               
Packet Session Status = Active                                                  
Current Roaming Status = Home                                                   
Network Selection Mode = Automatic                                              
Country = GBR, Network = 3 UK                                                   
Mobile Country Code (MCC) = 234                                                 
Mobile Network Code (MNC) = 20                                                  
Location Area Code (LAC) = 24                                                   
Routing Area Code (RAC) = 24                                                    
Cell ID = 14827                                                                 
Primary Scrambling Code = 81                                                    
PLMN Selection = Automatic                                                      
Registered PLMN = 3 , Abbreviated =                                             
Service Provider =                                                              
Router#show cell 0/0/0 radio                                                    
Radio power mode = ON                                                           
Current Band = WCDMA 2100, Channel Number = 10564                               
Current RSSI = -76 dBm                                                          
Band Selected = Auto                                                            
Number of nearby cells = 1                                                      
Cell 1

        Primary Scrambling Code = 0x51
        RSCP = -77 dBm, ECIO = -0 dBm           

Note that the band and channel need to be populated, the network should display the expected carrier name and the packet service should show as attached. The actual band and service type will vary depending on carrier, coverage, area and equipment used.

If the network status remains in "Emergency Only" and you get no MSISDN showing in your show cell 0/0/0 hardware command, particularly if it is accompanied by messages saying "%CELLWAN-2-SIM_LOCKED: [Cellular0/0/0]: SIM is locked", then you have probably locked the SIM (i.e. by setting up a startup PIN on a phone handset) and will need to unlock it as follows:

Router #cell 0/1/0 gsm sim unlock 1234
!!!WARNING: SIM will be unlocked with pin=1234(4).
Do not enter new PIN to unlock SIM. Enter PIN that the SIM is configured with.
Call will be disconnected!!!
Are you sure you want to proceed?[confirm]

*Dec  7 22:15:38.035: %LINK-3-UPDOWN: Interface Cellular0/0/0, changed state to up


If the radio interface is up but a data connection cannot be established then all the usual debugs may be used:

debug dialer (to verify it is trying to dial)
debug chat (sometimes useful to deduce whether APN is configured correctly)
debug ppp negotiation (shows the PPP negotiation process from agreeing basic link properties and authentication type, through the authenticating stage and up to IP being allocated)

A full deep-dive into these debugs wouldn't really be appropriate for this post, in any case it's usually fairly evident where the problem lies. UPDATE: The promised dialer / PPP debugging guide is available here - it's written for PPPoE but the vast majority of it is applicable to cellular interfaces as well.


Video accompanying this blog post

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