HS-DSCH ( High-Speed Downlink Shared Channel )

High-Speed Downlink Shared Channel (HS-DSCH) is used in HSDPA (UMTS) to send packets on the downlink to the UEs. It is called High-Speed to distinguish it from the more general definition of the downlink shared channel (DSCH) in UMTS. (The latter channel is sometimes referred to as Release'99 DSCH.)

HDLC Transparency

The flag sequence must never occur within the content of a frame otherwise it could be confused with an intentionally sent flag. A technique known as 0-bit insertion is used to prevent random data synthesising a flag. The technique is said to make HDLC transparent, since any stream of bits may be present between the open and closing flag of a frame. The transparency is achieved by encoding the data by inserting a 0-bit after any sequence of 5 consecutive 1's within the payload, as shown:

Insertion of a "zero-bit" into the content of a frame to ensure transparency

HDLC Flags

HDLC is a data link protocol which uses a unique bit sequence to delimit the start and end of each PDU transported by the data link layer service. In HDLC, frames are delimited by a sequence of bits known as a "flag". The flag sequence is a unique 8-bit sequence of the form 0111 1110. The way in which this is performed is described in the diagram

(fig)The flags before and after an HDLC frame indicate the start and end of the frame

HDLC Hunt Mode

Normally a HDLC receiver starts in the idle state, waiting for the start of a frame. This is called "Hunt" mode, since the receiver is said to be hunting for a non-flag sequence. This may be achieved through a shift register and combinational logic as shown (a Finite State Machine (FSM) is usually used to implement this):









(fig) Detection of a start of frame