The Data Link layer sits between the Physical layer (raw bit transmission) and the Network layer, and offers the following core services to the layer above it:

1. Framing
Organizes the raw bitstream received from the Physical layer into discrete, delimited frames, each carrying its own header (addressing, control info) and trailer (error-check bits), giving the Network layer a well-defined unit of data to work with rather than an undifferentiated bit stream.

2. Physical (MAC) Addressing
Attaches source and destination hardware addresses to each frame, allowing correct delivery within a local network segment — distinct from the logical (IP) addressing handled at the Network layer.

3. Flow Control
Regulates the rate at which frames are sent so a fast sender doesn't overwhelm a slower receiver's buffering capacity, typically using sliding-window mechanisms.

4. Error Control
Detects (and sometimes corrects) bit errors introduced during transmission, using techniques like parity, checksums, or CRC, combined with ACK/retransmission (ARQ) schemes to recover from lost or corrupted frames.

5. Access Control (Media Access Control)
On a shared medium (e.g. a LAN), determines which device gets to use the channel at any given time, avoiding or resolving collisions — handled by protocols like CSMA/CD or CSMA/CA within the MAC sublayer.

Two broad service types offered upward to the Network layer:

• Connectionless service — each frame is sent independently, with no prior handshake, and delivery is not guaranteed (e.g. Ethernet's basic frame delivery).
• Connection-oriented service — involves an establishment phase, in-order delivery, and reliable acknowledgement of every frame before the connection is later terminated (e.g. some WAN link-layer protocols like older HDLC-based links).