diff --git a/src/adn_server/application/ports.py b/src/adn_server/application/ports.py
index 919d2ad..e457a3e 100644
--- a/src/adn_server/application/ports.py
+++ b/src/adn_server/application/ports.py
@@ -27,7 +27,10 @@ from __future__ import annotations
 
 from abc import ABC, abstractmethod
 from collections.abc import Sequence
-from typing import Any, Protocol
+from typing import TYPE_CHECKING, Any, Protocol
+
+if TYPE_CHECKING:
+    from adn_server.domain.subscription import AudioChannel, Subscription, SubscriptionId, SubscriptionPhase, SystemId
 
 
 class ConfigLoader(ABC):
@@ -256,3 +259,47 @@ class TalkerAliasEmblcEncoder(Protocol):
 
     def encode_blocks(self, blocks: dict[int, bytes]) -> tuple[list[dict[int, Any]], int]:
         ...
+
+
+class SubscriptionStore(ABC):
+    """Authoritative in-memory subscription registry (Phase 2; replaces BRIDGES dict over time)."""
+
+    @abstractmethod
+    def get(self, sub_id: "SubscriptionId") -> "Subscription | None":
+        ...
+
+    @abstractmethod
+    def upsert(self, subscription: "Subscription") -> None:
+        ...
+
+    @abstractmethod
+    def remove(self, sub_id: "SubscriptionId") -> bool:
+        ...
+
+    @abstractmethod
+    def clear(self) -> None:
+        ...
+
+    @abstractmethod
+    def replace_all(self, subscriptions: Sequence["Subscription"]) -> None:
+        ...
+
+    @abstractmethod
+    def snapshot(self) -> tuple["Subscription", ...]:
+        ...
+
+    @abstractmethod
+    def list_by_channel(self, channel: "AudioChannel") -> tuple["Subscription", ...]:
+        ...
+
+    @abstractmethod
+    def list_by_system(self, system: "SystemId") -> tuple["Subscription", ...]:
+        ...
+
+    @abstractmethod
+    def list_active(self) -> tuple["Subscription", ...]:
+        ...
+
+    @abstractmethod
+    def list_by_phase(self, phase: "SubscriptionPhase") -> tuple["Subscription", ...]:
+        ...
diff --git a/src/adn_server/domain/__init__.py b/src/adn_server/domain/__init__.py
index 02d873c..5cd07ea 100644
--- a/src/adn_server/domain/__init__.py
+++ b/src/adn_server/domain/__init__.py
@@ -22,6 +22,16 @@
 ###############################################################################
 
 from .entities import BridgeEntry, StreamState, SystemConfig
+from .subscription import (
+    ActivationPolicy,
+    AudioChannel,
+    Subscription,
+    SubscriptionId,
+    SubscriptionPhase,
+    SubscriptionRole,
+    SubscriptionState,
+    SystemId,
+)
 from .errors import ACLError, ConfigError, DomainError
 from .hbp_protocol import (
     HBPF_DATA_SYNC,
@@ -40,6 +50,14 @@ __all__ = [
     "BridgeEntry",
     "StreamState",
     "SystemConfig",
+    "ActivationPolicy",
+    "AudioChannel",
+    "Subscription",
+    "SubscriptionId",
+    "SubscriptionPhase",
+    "SubscriptionRole",
+    "SubscriptionState",
+    "SystemId",
     "DmrId",
     "TgId",
     "Slot",
diff --git a/src/adn_server/domain/subscription.py b/src/adn_server/domain/subscription.py
new file mode 100644
index 0000000..451ffd5
--- /dev/null
+++ b/src/adn_server/domain/subscription.py
@@ -0,0 +1,94 @@
+"""Subscription model: logical TG channels and per-system participation (Phase 2)."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import Enum
+
+from .value_objects import Slot, TgId
+
+
+class SubscriptionRole(str, Enum):
+    """How a system participates in an audio channel."""
+
+    SINK = "sink"
+    SOURCE = "source"
+    PASSIVE_STAT = "passive_stat"
+    ECHO = "echo"
+
+
+class ActivationPolicy(str, Enum):
+    """What may activate this subscription (separate from session state)."""
+
+    USER_ACTIVATED = "user_activated"
+    STATIC = "static"
+    INBAND = "inband"
+    OPENBRIDGE_STAT = "openbridge_stat"
+
+
+class SubscriptionPhase(str, Enum):
+    """Runtime session phase for a subscription leg."""
+
+    IDLE = "idle"
+    ACTIVE = "active"
+    HANGTIME = "hangtime"
+
+
+@dataclass(frozen=True, slots=True)
+class SystemId:
+    """Enabled system name (legacy BRIDGES row ``SYSTEM``)."""
+
+    value: str
+
+    def __str__(self) -> str:
+        return self.value
+
+
+@dataclass(frozen=True, slots=True)
+class AudioChannel:
+    """Logical talkgroup channel: ``(tgid, slot)``."""
+
+    tgid: TgId
+    slot: Slot
+
+
+@dataclass(frozen=True, slots=True)
+class SubscriptionId:
+    """Stable key for one system leg on a channel."""
+
+    channel: AudioChannel
+    system: SystemId
+
+
+@dataclass(slots=True)
+class SubscriptionState:
+    """Mutable session state (legacy ACTIVE / TIMER / hangtime)."""
+
+    phase: SubscriptionPhase = SubscriptionPhase.IDLE
+    timer_expires_at: float | None = None
+
+
+@dataclass(slots=True)
+class Subscription:
+    """One system participating in a channel with LC rewrite and activation rules."""
+
+    channel: AudioChannel
+    system: SystemId
+    target_tgid: TgId
+    role: SubscriptionRole
+    policy: ActivationPolicy
+    state: SubscriptionState
+    bridge_key: str | None = None
+    timeout_seconds: float | None = None
+
+    @property
+    def subscription_id(self) -> SubscriptionId:
+        return SubscriptionId(channel=self.channel, system=self.system)
+
+    def is_active(self) -> bool:
+        return self.state.phase == SubscriptionPhase.ACTIVE
+
+    def table_key(self) -> str:
+        if self.bridge_key is not None:
+            return self.bridge_key
+        return str(self.channel.tgid.value)
diff --git a/src/adn_server/infrastructure/subscription_store.py b/src/adn_server/infrastructure/subscription_store.py
new file mode 100644
index 0000000..44bfb4b
--- /dev/null
+++ b/src/adn_server/infrastructure/subscription_store.py
@@ -0,0 +1,51 @@
+"""In-memory subscription store (Phase 2 authority; not wired to BRIDGES yet)."""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+from adn_server.application.ports import SubscriptionStore
+from adn_server.domain.subscription import (
+    AudioChannel,
+    Subscription,
+    SubscriptionId,
+    SubscriptionPhase,
+    SystemId,
+)
+
+
+class InMemorySubscriptionStore(SubscriptionStore):
+    """Hold subscriptions keyed by ``SubscriptionId``; no Twisted or YAML."""
+
+    def __init__(self) -> None:
+        self._items: dict[SubscriptionId, Subscription] = {}
+
+    def get(self, sub_id: SubscriptionId) -> Subscription | None:
+        return self._items.get(sub_id)
+
+    def upsert(self, subscription: Subscription) -> None:
+        self._items[subscription.subscription_id] = subscription
+
+    def remove(self, sub_id: SubscriptionId) -> bool:
+        return self._items.pop(sub_id, None) is not None
+
+    def clear(self) -> None:
+        self._items.clear()
+
+    def replace_all(self, subscriptions: Sequence[Subscription]) -> None:
+        self._items = {sub.subscription_id: sub for sub in subscriptions}
+
+    def snapshot(self) -> tuple[Subscription, ...]:
+        return tuple(self._items.values())
+
+    def list_by_channel(self, channel: AudioChannel) -> tuple[Subscription, ...]:
+        return tuple(sub for sub in self._items.values() if sub.channel == channel)
+
+    def list_by_system(self, system: SystemId) -> tuple[Subscription, ...]:
+        return tuple(sub for sub in self._items.values() if sub.system == system)
+
+    def list_active(self) -> tuple[Subscription, ...]:
+        return tuple(sub for sub in self._items.values() if sub.is_active())
+
+    def list_by_phase(self, phase: SubscriptionPhase) -> tuple[Subscription, ...]:
+        return tuple(sub for sub in self._items.values() if sub.state.phase == phase)
diff --git a/tests/domain/test_subscription.py b/tests/domain/test_subscription.py
new file mode 100644
index 0000000..80c7734
--- /dev/null
+++ b/tests/domain/test_subscription.py
@@ -0,0 +1,40 @@
+"""Domain subscription entities."""
+
+from __future__ import annotations
+
+from adn_server.domain.subscription import (
+    ActivationPolicy,
+    AudioChannel,
+    Subscription,
+    SubscriptionPhase,
+    SubscriptionRole,
+    SubscriptionState,
+    SystemId,
+    TgId,
+)
+
+
+def _sub(*, phase: SubscriptionPhase = SubscriptionPhase.IDLE, active: bool = False) -> Subscription:
+    state = SubscriptionState(
+        phase=SubscriptionPhase.ACTIVE if active else phase,
+        timer_expires_at=100.0 if active else None,
+    )
+    return Subscription(
+        channel=AudioChannel(tgid=TgId(730444), slot=2),
+        system=SystemId("OBP-CL"),
+        target_tgid=TgId(730444),
+        role=SubscriptionRole.SINK,
+        policy=ActivationPolicy.INBAND,
+        state=state,
+    )
+
+
+def test_subscription_id_stable():
+    sub = _sub()
+    assert sub.subscription_id.channel == sub.channel
+    assert sub.subscription_id.system == sub.system
+
+
+def test_is_active():
+    assert not _sub().is_active()
+    assert _sub(active=True).is_active()
diff --git a/tests/infrastructure/test_subscription_store.py b/tests/infrastructure/test_subscription_store.py
new file mode 100644
index 0000000..99dba82
--- /dev/null
+++ b/tests/infrastructure/test_subscription_store.py
@@ -0,0 +1,84 @@
+"""In-memory subscription store."""
+
+from __future__ import annotations
+
+from adn_server.domain.subscription import (
+    ActivationPolicy,
+    AudioChannel,
+    Subscription,
+    SubscriptionId,
+    SubscriptionPhase,
+    SubscriptionRole,
+    SubscriptionState,
+    SystemId,
+    TgId,
+)
+from adn_server.infrastructure.subscription_store import InMemorySubscriptionStore
+
+
+def _leg(
+    *,
+    tgid: int,
+    slot: int,
+    system: str,
+    target: int | None = None,
+    phase: SubscriptionPhase = SubscriptionPhase.IDLE,
+) -> Subscription:
+    return Subscription(
+        channel=AudioChannel(tgid=TgId(tgid), slot=slot),  # type: ignore[arg-type]
+        system=SystemId(system),
+        target_tgid=TgId(target if target is not None else tgid),
+        role=SubscriptionRole.SINK,
+        policy=ActivationPolicy.STATIC,
+        state=SubscriptionState(phase=phase),
+    )
+
+
+def test_upsert_get_remove():
+    store = InMemorySubscriptionStore()
+    sub = _leg(tgid=730444, slot=2, system="MASTER-A")
+    store.upsert(sub)
+    sub_id = SubscriptionId(channel=sub.channel, system=sub.system)
+    assert store.get(sub_id) is sub
+    assert store.remove(sub_id) is True
+    assert store.get(sub_id) is None
+    assert store.remove(sub_id) is False
+
+
+def test_replace_all_and_snapshot():
+    store = InMemorySubscriptionStore()
+    a = _leg(tgid=1, slot=1, system="A")
+    b = _leg(tgid=2, slot=2, system="B")
+    store.replace_all([a, b])
+    snap = store.snapshot()
+    assert len(snap) == 2
+    assert a in snap and b in snap
+    store.replace_all([a])
+    assert store.snapshot() == (a,)
+
+
+def test_list_by_channel_system_and_active():
+    store = InMemorySubscriptionStore()
+    channel = AudioChannel(tgid=TgId(730444), slot=2)
+    idle = _leg(tgid=730444, slot=2, system="OBP-CL", phase=SubscriptionPhase.IDLE)
+    active = _leg(tgid=730444, slot=2, system="ECHO", phase=SubscriptionPhase.ACTIVE)
+    other = _leg(tgid=9990, slot=1, system="ECHO")
+    store.replace_all([idle, active, other])
+
+    by_channel = store.list_by_channel(channel)
+    assert len(by_channel) == 2
+    assert idle in by_channel and active in by_channel
+
+    by_system = store.list_by_system(SystemId("ECHO"))
+    assert len(by_system) == 2
+    assert active in by_system and other in by_system
+
+    assert store.list_active() == (active,)
+    assert store.list_by_phase(SubscriptionPhase.IDLE) == (idle, other)
+
+
+def test_clear():
+    store = InMemorySubscriptionStore()
+    store.upsert(_leg(tgid=1, slot=1, system="X"))
+    store.clear()
+    assert store.snapshot() == ()