Dual SIM vs. Dual Modem

The latest LTE modem offerings come with a lot of technology and features built into them.  Some of the benefits include better throughput speeds, automatic SIM detection, automatic carrier switching and network flexibility for your deployments.  The goal for this article is to clarify the difference between dual SIM and dual modem cellular deployments. 

Things to remember:

• In a single modem, only one SIM can be active at a time
• In North American cellular market, a modem must have a specific carrier firmware image (modem firmware) loaded to work reliably (ex: VZW, ATT, Generic (T-Mobile, Canada), Sprint) on the respective carrier network.

Dual SIM (Single Modem):

A typical cellular gateway (aka cellular router) usually has a single modem with

dual SIM slots.  An important point to remember is that only ONE SIM can be active/connected in a modem at a time.  This deployment is beneficial in the following scenarios:

• You have a data plan cap on the SIM.  If one SIM reaches it's cap, the gateway should failover to the second SIM. 
• You have an “Unlimited Plan” on the SIM and would like to utilize multiple SIMs so that when gateway hits a certain usage data threshold, it will switch to the other SIM (ex: when wireless carrier offers "unlimited data" SIMs but throttles speeds (to 2G/3G service equivalent) after XGB data usage in a billing cycle)
• You want to deploy the cellular gateway but don’t know which carrier has better coverage at the target location/site

Carrier redundancy/failover is not recommended for single modem setups since the modem must “reflash” itself (software switching of modem firmware which can be 3-7 minute process) to the carrier network of the SIM card in order to check signal strength and availability.  Additionally, when swapping to second SIM, the modem will require a different APN for the new carrier.  Because of the changes/steps involved, a dual modem solution is the way to go vs. a dual SIM single modem for applications involving carrier redundancy/failover.

The ideal use case for Dual SIM (single modem) is single carrier use with data plan caps or if you don't know which carrier is "choice" at the time of procurement.  For data plan caps, if one SIM reaches its cap, the gateway can be configured to failover to second SIM.  Because the gateway is a dual SIM (single modem) and not dual modems, it cannot provide two simultaneous connections or be load-balanced.

Dual Modem:

Some cellular gateways can be purchased/configured with dual or even quad modems.  In this setup, the cellular gateway will have two or four physical cellular modems available internally (they can be embedded, modular or a combination of both).  Each modem has one/two SIM slots depending on the hardware, providing four or up to eight SIM slots available in total.  An important point to remember is that since this is a DUAL/QUAD modem setup, multiple SIMs can be active simultaneously (one SIM from each modem).  This deployment is useful for the following scenarios: 

• All scenarios described above for Single Modems PLUS
• You want carrier redundancy for added resilience in your networking application (ex: automatic failover or handoff between Verizon and AT&T depending on coverage/signal). Failover to the alternate carrier(s) is instantaneous and the gateway can evaluate signal conditions in real time and initiate failover
• You want to utilize multiple SIMs simultaneously for a faster connection/higher bandwidth (ex: Load Balancing or Bonding)

The ideal use case for Dual Modem is carrier redundancy and/or load-balancing/bonding applications. Because the gateway is dual/multi modem, it is better suited to handle the intricate dynamics involved with multiple carriers. 

Cellular gateways and their related technology are getting sophisticated and offers us greater flexibility, agility, and performance for network deployments/applications.  Give us a call to help you with your cellular gateway needs and determine the best solution for you.

Links of Interest

• Cellular Routers and Gateways