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    Overview

    This guide discusses the options for sending data to and from your cellular device. In the table below, Outbound refers to sending data from the device, and Inbound refers to outside data reaching the device.

    MethodOutboundInboundPricingDash SupportNova Support
    Hologram Cloud messagingYesYesDataYesYes
    Direct IPYesReply onlyDataNoYes
    Spacebridge IP TunnelReply onlyYesDataNoYes
    SMS via Hologram CloudYesYesOutbound: Data, Inbound: FreeYesYes
    Circuit-Switched SMSYesYesOutbound: Per message, Inbound: Buy phone numberInbound OnlyYes

    Per-MB data pricing and per-message SMS pricing depends on your zone and billing plan. See Pricing for details.

    Data metering

    Due to networking overhead, the bandwidth used by a device will be higher than the sum of bytes your application sends and receives.

    Each TCP packet (and therefore each Hologram Cloud message) incurs an overhead of about 500-700 bytes. This means that to reduce data charges, you should prefer fewer large messages over more small messages. Messages larger than about 2kB may be split into multiple packets, so this optimization is mainly relevant for very small messages.

    Also, keep in mind that higher-level protocols such as HTTP have their own headers which may inflate the per-message overhead.

    Hologram Cloud Messaging

    The Hologram Cloud is the easiest way to send data between your device and the internet. The mechanisms for inbound and outbound communication are somewhat different, but in both cases, the Hologram cloud forwards messages between the device and the internet using the device’s cellular data.

    Outbound (from Device)

    The Dash provides built-in functions for sending messages to the Hologram Cloud. Any connected device can use its TCP API, even if it’s not on Hologram’s cellular network. Messages take the form of a binary payload (usually UTF8-encoded text), and an optional list of topic strings that you can use for filtering or routing in the cloud.

    By configuring routing rules in the Hologram Dashboard, you can forward messages to other internet services. This means you can update where the Data Router sends your messages without deploying code updates to your cellular devices. See the Data Router Guide for details.

    Inbound (to Device)

    Hologram exposes an API and a Dashboard interface to send TCP or UDP messages to any port on your cellular device. This requires that your device runs embedded Linux or otherwise implements a networking stack. The Hologram Cloud translates the API requests (or Dashboard submissions) into raw TCP/UDP messages and sends them to the desired device.

    See the Cloud Messaging API reference for details.

    Direct IP

    Devices on the Hologram cellular network have access to the full internet, so you may use any IP-based protocol to communicate with any internet-routable host. In fact, Hologram’s Cloud messaging API relies on this standard IP connectivity. Advanced users may wish to bypass the Hologram Cloud and instead communicate directly with servers they control.

    Hologram cellular devices do not have internet-routable IP addresses, so you can’t connect to your device directly from the internet. This means that to send data to your device, the device must first establish a connection to an outside server and request the data. The server can then send data back to the device in a response using the already-established connection. This request-response pattern is the basis for many common protocols such as HTTP.

    If device-originated requests aren’t a good fit for your application, you can connect to your device via a tunnel, covered in the next section.

    SpaceBridge IP Tunnel

    Since devices on the Hologram network do not have internet-routable IP addresses, it’s not possible to directly connect to the devices over the internet. This makes it hard to use the device as a server that can listen for incoming requests. The solution is to establish a connection–or tunnel–through a different server which does have the ability to connect to your device.

    Hologram provides a service called SpaceBridge, which uses secure SSH tunneling to let you connect to any port on your device. The inbound Cloud Messaging feature above relies on a SpaceBridge tunnel behind the scenes. The Cloud Messaging approach is a simpler alternative to SpaceBridge if you don’t require direct access to the network socket.

    See our SpaceBridge guide for details.

    SMS via Hologram Cloud

    Outbound (from Device)

    To avoid the carrier fees associated with sending standard circuit-switched SMS messages, Hologram offers a cloud API for sending SMS via TCP/IP. See Send an SMS via the Hologram Cloud on the Embedded cloud API reference page.

    Outbound SMS messages sent via API will appear as originating from an internal Hologram phone number. This is the case even if you have bought a phone number for receiving inbound SMS messages.

    Inbound (to Device)

    Even if you don’t buy a phone number for your device, you can send SMS messages to it from the Hologram dashboard or API. You may also configure the phone number that the SMS will appear to originate from.

    From the Hologram dashboard page for your device, select the Cloud & messaging view. Use the Send an SMS message form to send an SMS to your device:

    Send SMS dashboard form

    For information on sending SMS to the device via HTTP API, see the Incoming SMS API reference.

    To read inbound SMSes on the device, use AT commands to request them from the modem.

    Circuit-Switched SMS

    Outbound

    To send an SMS via the standard SMS switching network, use AT commands to request them from the cellular modem. Outbound SMS messages will appear as originating from an internal Hologram phone number. This is the case even if you have bought a phone number for receiving inbound SMS messages.

    Inbound

    To receive SMS messages from other SMS-capable devices, you must buy a dedicated phone number for your Hologram device. To buy a phone number, go to a device’s page on the Hologram Dashboard, and select Cloud configuration > Configure phone number.

    To read inbound SMSes on the device, use AT commands to request them from the modem.