How the Coordinated Set Identification Profile Lets BLE Devices Work as a Team — TWS Earbuds, Hearing Aids and Beyond
What This Tutorial Series Covers
This is a three-part, hands-on walkthrough of the Bluetooth Coordinated Set Identification Profile (CSIP). By the end of the series you will know exactly how a smartphone recognises that two earbuds belong together, how it discovers every member of the group, and how it prevents two phones from stepping on each other while sending commands to the same devices.
This first part covers the why and the what — the problem CSIP solves, the core concepts, and where the profile sits in the Bluetooth stack. No prior CSIP knowledge is assumed.
Keywords Covered in This Article
🔍 The Problem: BLE Devices Don’t Know They Belong Together
Imagine you put on a pair of truly wireless earbuds. To you, they are one product — left and right working in perfect sync. But the Bluetooth radio in your phone sees two completely separate devices. Each earbud has its own independent Bluetooth address, its own GATT server, and its own advertising packets.
So how does your phone know to group them together and show one entry in Settings? How does it know to route left-channel audio to one bud and right-channel audio to the other? How does it prevent a second phone nearby from interfering with the volume command your phone is sending?
Before CSIP was standardised, every manufacturer solved this with their own proprietary firmware tricks. Those solutions were not interoperable. CSIP is the Bluetooth SIG’s universal answer — a specification that any vendor can implement to achieve true plug-and-play multi-device coordination over BLE.
A sports team has individual players, but they all wear the same jersey number that identifies their club. CSIP gives every BLE device in a group the equivalent of a shared team jersey — a secret key called the SIRK. Any scanner that knows the team’s key can scan the room and immediately recognise every player that belongs to the same club, without each player having to announce their membership out loud.
📋 What Is CSIP?
CSIP (Coordinated Set Identification Profile) is a Bluetooth SIG profile specification that defines how a central device discovers a group of BLE peripherals and manages them as a unit. It was first adopted in March 2021 and is used extensively in the Bluetooth LE Audio ecosystem.
CSIP is deliberately agnostic about what the grouped devices actually do. It does not care whether they are earbuds, hearing aids, speaker arrays, medical sensors, or tyre pressure monitors. Its only responsibility is to answer three questions:
Identify what Coordinated Set a given device belongs to, and read the shared identity key that proves membership.
Scan the area and use the identity key to recognise every other member of the same group, even when they use privacy-enabled Bluetooth addresses.
Provide a locking mechanism so only one controller at a time can run a coordinated operation, preventing conflicts when multiple devices are connected.
📦 What Is a Coordinated Set?
A Coordinated Set is a named group of one or more BLE devices that share a common purpose. The devices in the group all hold the same secret key (the SIRK), which is what binds them together. A set can have as few as one member or many, and the actual application running on those devices is defined by separate higher-layer profiles.
CSIP supports a wide variety of product categories. Here are some concrete examples:
An important point: CSIP does not define what the Coordinated Set does. That is the job of other profiles (like the Hearing Access Profile or LE Audio profiles). CSIP only defines how you discover the group and manage access to it.
🗺️ Where Does CSIP Fit in the BLE Stack?
CSIP is a profile — it sits at the application layer and uses the Generic Attribute Profile (GATT) as its communication foundation. CSIP itself defines roles and procedures. The actual data structures that carry set information live inside a GATT service called CSIS (Coordinated Set Identification Service).
Because CSIP is built entirely on GATT, every operation in CSIP — reading device identity, writing lock states, receiving notifications — uses standard GATT read, write, and notify procedures that you already know from working with other BLE services. There is nothing proprietary under the hood.
CSIP requires the Bluetooth Core Specification version 4.2 or later. It works over both LE and BR/EDR transports, though most modern deployments use LE.
📖 Key Terminology Reference
These six terms will appear throughout all three parts of this series. Make sure you understand each one before continuing.
Coordinated Set
A group of one or more BLE devices configured to work together for a specific scenario. All members share the same secret identity key (SIRK). The group size and purpose are defined by higher-layer profiles.
Set Coordinator
The device (typically a phone or tablet) that discovers the Coordinated Set and its members. It acts as a GATT client and takes on the GAP Central role over LE. It can also acquire exclusive access (lock) to the set.
Set Member
Each device belonging to the Coordinated Set. It acts as a GATT server and takes on the GAP Peripheral role over LE. It must expose the CSIS service. A single device can simultaneously belong to more than one Coordinated Set.
CSIS
Coordinated Set Identification Service. The GATT service that every Set Member must host. It contains four characteristics that expose the group’s identity and membership details. One CSIS instance per set membership.
SIRK
Set Identity Resolving Key. A 16-byte shared secret that is the unique fingerprint of the entire Coordinated Set. Every member holds the same SIRK. The Set Coordinator reads this key and uses it to recognise other members during scanning.
RSI
Resolvable Set Identifier. A compact 6-byte token that each Set Member includes in its advertising packets. It is derived from the SIRK using a hash function. Only a device that already knows the SIRK can verify whether an RSI belongs to its Coordinated Set.
Both earbuds expose the same SIRK → the coordinator recognises them as the same Coordinated Set
📚 Series Roadmap
The problem CSIP solves, what a Coordinated Set is, where CSIP fits in the Bluetooth stack, and the six essential terms you need to know before going deeper.
Deep dive into Set Coordinator and Set Member roles. How the four CSIS characteristics work — SIRK (encrypted vs plaintext), Set Size, Member Lock, and Member Rank. How RSI is advertised and verified. What happens when a device belongs to multiple sets.
Step-by-step walkthroughs of all five CSIP procedures with sequence diagrams: Coordinated Set Discovery, Set Members Discovery, Lock Request, Lock Release, and Ordered Access. Security requirements, bonding, CTKD, and RSI privacy.
You Have the Foundation — Now Go Deeper
Part 2 breaks down the two CSIP roles in detail, explains every characteristic inside the CSIS service, and shows you exactly how SIRK and RSI work together to identify group members during a BLE scan.