# SMPP SMSC trunk

An **SMPP SMSC** (Short Message Service Center) **trunk** lets DIDWW connect
to your own SMSC as an ESME (External Short Messaging Entity). DIDWW delivers
SMS sent to the DIDs assigned to the trunk to your SMSC, and your SMSC can
send outbound SMS to DIDWW over the same connection.

Use this trunk type when your SMSC or SMPP server should accept the connection
from DIDWW. If your application should connect and bind to DIDWW instead, use
an [SMPP ESME trunk](../smpp-esme/smpp-esme-trunk.html). To receive inbound
SMS on an HTTP endpoint or by email, use an [HTTP IN trunk](../http-in/http-in-trunk.html) or an [SMS to Email
trunk](../sms-to-email/sms-to-email-trunk.html).

## How DIDWW connects to your SMSC

In SMPP, an ESME is an external application that connects to an SMSC to send
and receive SMS. The SMSC stores, forwards, converts, and delivers the
messages. With an SMPP SMSC trunk, your system is the SMSC and DIDWW is the
ESME. DIDWW opens the connection and sends the bind request, and your SMSC
accepts it.

DIDWW connects from the DIDWW SMPP server of the trunk's **Server location**:
**US, NY** or **NL, AMS**. It uses the **Host**, ports, **System ID**, and
**Password** that you enter in the trunk settings, and your SMSC validates
these credentials. Your firewall must accept connections from the IP address
of the trunk's **Server location**. See [Server locations and IP
addresses](../technical-data/smpp-specification.html#smpp-endpoints).

After you create the trunk, DIDWW starts the bind process automatically. If a
connection attempt fails, DIDWW retries it after the trunk's **Connection
timeout**. The DIDWW User Panel shows whether DIDWW is bound to your SMSC and
the reason for the last failed attempt.

## Bind direction and message exchange

The **Transceiver mode** setting defines how DIDWW binds to your SMSC:

- **Off** (default): DIDWW opens two sessions. The transmitter session on the
  **TX port** sends messages to your SMSC. The receiver session on the **RX
  port** receives messages from your SMSC.
- **On**: DIDWW opens one transceiver session on the **TX port** to both send
  and receive messages. The **RX port** is ignored.

When **Use SSL** is on, DIDWW connects to your SMSC over TLS and establishes
TLS before it sends the bind request. **Use SSL** does not change the ports.
Your SMSC must accept TLS connections on the ports you enter.

Over the bind, DIDWW and your SMSC exchange messages as SMPP Protocol Data
Units (PDUs):

- **Inbound SMS:** DIDWW submits messages sent to the DIDs assigned to the
  trunk with `submit_sm`.
- **Outbound SMS:** Your SMSC sends messages to DIDWW with `deliver_sm`.
- **Delivery receipts:** Your SMSC can return delivery receipts (DLRs) with
  `deliver_sm`.

The receiving side acknowledges `submit_sm` with `submit_sm_resp` and
`deliver_sm` with `deliver_sm_resp`. For the bind requests that your SMSC
must accept, see [Mandatory SMPP SMSC bind
parameters](how-to-guides/accept-smpp-smsc-bind-requests.html#smpp-smsc-bind-parameters). For the PDU details, see [SMPP
message flow](../technical-data/smpp-specification.html#smpp-pdu-flow).

```
        %%{init: {
  "theme": "base",
  "themeVariables": {
    "fontSize": "14px",
    "primaryColor": "#e0f2fe",
    "primaryBorderColor": "#38bdf8",
    "primaryTextColor": "#1f2d3d",
    "secondaryColor": "#ccfbf1",
    "secondaryBorderColor": "#2dd4bf",
    "secondaryTextColor": "#1f2d3d",
    "lineColor": "#0066cc",
    "textColor": "#1f2d3d",
    "nodeTextColor": "#1f2d3d",
    "titleColor": "#1f2d3d",
    "clusterBkg": "#f8fafc",
    "clusterBorder": "#cbd5e1",
    "edgeLabelBackground": "#ffffff"
  }
}}%%

flowchart TB

    subgraph inbound["Inbound SMS"]
        direction LR
        sender["Sender"] --> |"SMS to DID"| did["Assigned DID number"]
        did --> trunk_in["SMPP SMSC trunk (DIDWW as ESME)"]
        trunk_in --> |"submit_sm"| smsc_in["Your SMSC"]
    end

    subgraph outbound["Outbound SMS"]
        direction LR
        smsc_out["Your SMSC"] --> |"deliver_sm"| trunk_out["SMPP SMSC trunk (DIDWW as ESME)"]
        trunk_out --> |"SMS"| recipient["Recipient"]
    end

    inbound ~~~ outbound

    classDef endpoint fill:#e0f2fe,stroke:#0066cc,color:#1f2d3d,stroke-width:1.5px
    classDef process fill:#ccfbf1,stroke:#2dd4bf,color:#1f2d3d,stroke-width:1.5px

    class sender,smsc_in,smsc_out,recipient endpoint
    class did,trunk_in,trunk_out process

    style inbound fill:#f8fafc,stroke:#cbd5e1,color:#1f2d3d
    style outbound fill:#f8fafc,stroke:#cbd5e1,color:#1f2d3d

    linkStyle 0,1,2,3,4 stroke:#0066cc,stroke-width:1.5px
```

Long SMS are sent as concatenated (multipart) messages. DIDWW submits each
part of an inbound multipart SMS to your SMSC in a separate `submit_sm` that
carries a User Data Header (UDH) in the `short_message` field. Your SMSC must
process the UDH and reassemble the parts. When your SMSC sends a long SMS, it
sends each part with its own UDH. For encoding limits, concatenation, and how
message parts are billed, see [SMS encoding
reference](../technical-data/sms-encoding-reference.html#sms-encoding-reference).

## Sender IDs and DID numbers

DID numbers have a different role in each direction:

- **Inbound SMS:** The trunk, or an SMS trunk group that contains it, must be
  [assigned](../../../phone-numbers/my-numbers/how-to-guides/assign-sms-trunk.html#assigning-sms-trunk) to one or more DIDs that support
  inbound SMS. Messages sent to those DIDs are then delivered to your SMSC.
- **Outbound SMS:** DID assignment is not required. The sender ID, also called
  the source address, depends on the type of message:

  - **P2P messages** use one of your DID numbers as the sender ID. By default,
    all DIDs with Outbound P2P SMS enabled in your account can be used. In
    **Source Address Settings**, you can limit the trunk to specific DID
    numbers. See [P2P Outbound SMS access](../../how-sms-service-works.html#how-sms-service-works-p2p-access).
  - **A2P messages** use a verified Sender ID, such as an Alphanumeric Sender
    ID. Associate an active [Sender ID Verification](../../sender-id-verifications/index.html) with the SMPP SMSC trunk.

## Failover with SMS trunk groups

An SMPP SMSC trunk can be added to an [SMS trunk group](../trunk-groups/sms-trunk-groups.html) with other SMS trunks.
When
the group is assigned to a DID, DIDWW attempts the member trunks for inbound
SMS in priority order, starting with the highest-numbered priority, and tries
the next trunk if one is unavailable.

---

## Get started

Create a trunk, prepare your SMSC to accept the bind from DIDWW, and assign
the trunk to your DID numbers.

**Create an SMPP SMSC trunk**

Select the Server location, enter the connection details and
credentials of your SMSC, and create the trunk.

[Create an SMPP SMSC trunk](how-to-guides/create-smpp-smsc-trunk.html)

**Accept SMPP SMSC bind requests**

Prepare your SMSC and firewall to accept the bind requests that DIDWW
sends, and confirm the bind.

[Accept SMPP SMSC bind requests](how-to-guides/accept-smpp-smsc-bind-requests.html)

**Assign an SMS trunk**

Assign the trunk or its SMS trunk group to DID numbers that support
inbound SMS.

[Assign an SMS trunk](../../../phone-numbers/my-numbers/how-to-guides/assign-sms-trunk.html#assigning-sms-trunk)

## Connection status

Confirm that DIDWW is bound to your SMSC.

**Check SMPP SMSC trunk connection status**

Confirm in the DIDWW User Panel whether DIDWW is bound to your SMSC.

[Check SMPP SMSC trunk connection status](how-to-guides/check-smpp-smsc-trunk-connection-status.html)

## Manage trunks and failover

Change existing trunks and combine them with other SMS trunks for failover.

**Manage SMPP SMSC trunks**

View credentials, edit trunk settings, move a trunk to another Server
location, or delete trunks.

[Manage SMPP SMSC trunks](how-to-guides/manage-smpp-smsc-trunk.html)

**Create an SMS trunk group**

Combine SMS trunks into one group for failover.

[Create an SMS trunk group](../trunk-groups/how-to-guides/create-sms-trunk-group.html)

**Set priorities in an SMS trunk group**

Control the order in which the member trunks of a group are attempted.

[Set priorities in an SMS trunk group](../trunk-groups/how-to-guides/set-sms-trunk-group-priorities.html)

## Technical references

**SMPP SMSC trunk settings reference**

Find field definitions, defaults, and constraints for SMPP SMSC trunk
settings.

[SMPP SMSC trunk settings reference](smpp-smsc-trunk-settings-reference.html)

**SMPP specification**

Review the SMPP protocol details, message flow, encoding, and
concatenation.

[SMPP Specifications](../technical-data/smpp-specification.html#service-incoming-sms)

**Server locations and IP addresses**

Look up the IP address of each DIDWW SMPP server location for your
firewall.

[Server Locations and IP Addresses](../technical-data/smpp-specification.html#smpp-endpoints)

**SMS encoding reference**

Check encoding limits, concatenation, and fragment billing.

[SMS encoding reference](../technical-data/sms-encoding-reference.html#sms-encoding-reference)

## Related resources

Prepare Sender IDs, review message activity, and compare with SMPP ESME trunks.

**Sender ID Verifications**

Register sender IDs for application-to-person (A2P) messaging.

[Sender ID Verifications](../../sender-id-verifications/index.html)

**Inbound SMS logs**

Review inbound SMS delivered through your SMS trunks.

[Inbound SMS Logs](../../../logs-analytics/sms-logs/inbound-logs.html#inbound-logs)

**Outbound SMS logs**

Review outbound SMS sent from your account.

[Outbound SMS Logs](../../../logs-analytics/sms-logs/outbound-logs.html#outbound-logs)

**SMPP ESME trunk**

Connect your own application to DIDWW over SMPP instead.

[SMPP ESME trunk](../smpp-esme/smpp-esme-trunk.html)