What Is SMPP?

The two sides: ESME and SMSC

An ESME (external short message entity) is any system outside the mobile network that sends or receives SMS: a bank's notification platform, a CPaaS provider, an aggregator. An SMSC (short message service centre) is the network element that stores and forwards SMS to handsets. SMPP defines how the two talk; the same protocol is also used between aggregators, with one acting as the server.

Binds

A session starts with a bind request carrying a system ID and password. There are three bind types:

Bind	The ESME can
bind_transmitter	Send messages only
bind_receiver	Receive messages and delivery receipts only
bind_transceiver	Both, on one session

The session stays open. Both sides send enquire_link periodically to confirm the other is alive, and close it with unbind.

Sending a message: submit_sm

Each message is a submit_sm PDU containing, among other fields, the source address (sender ID), destination address, data coding and the message text. The SMSC answers with submit_sm_resp, which carries a status code and a message ID. A zero status means the message was accepted, not that it was delivered.

Delivery receipts: deliver_sm

If the sender set registered_delivery, the SMSC later sends a delivery receipt as a deliver_sm on a receiver or transceiver bind. It references the original message ID and reports a final state such as DELIVRD (delivered), UNDELIV (undeliverable), EXPIRED or REJECTD.

Throughput and windowing

Throughput is usually expressed in messages per second (often called TPS) and limited per account. The window is how many requests a client may send before it must wait for responses. Sending faster than agreed returns a throttling error (ESME_RTHROTTLED); a well-behaved client slows down instead of reconnecting.

Common error codes

Code	Meaning
ESME_RINVPASWD	Invalid password at bind
ESME_RALYBND	Already bound
ESME_RINVSRCADR	Invalid source address (sender ID)
ESME_RINVDSTADR	Invalid destination address
ESME_RTHROTTLED	Sending faster than allowed
ESME_RMSGQFUL	Message queue full

SMPP and security

SMPP was designed for trusted links. Credentials travel in the bind, and the protocol has no built-in checks on sender ID or content. That is why providers restrict binds by IP address, set per-account limits and add policy layers such as an SMPP firewall, and why connections over the public internet should be encrypted with TLS or a VPN.

Connecting to Wise Network over SMPP

See SMPP connectivity for our supported binds, throughput and onboarding steps.