High speed switching of communications links without interrupting ATM cell traffic

A handoff method for a mobile ATM communications network, wherein upstream ATM cells and downstream ATM cells are transmitted between a mobile site and a mobile switching center over a first communication link and a handoff request message is sent from the mobile site to the mobile switching center when the first communication link is likely to become unavailable, the method comprising the steps of:

• a) holding the upstream ATM cells in a first buffer located in said mobile site immediately following the transmission of said handoff request message and determining the location of one of the upstream cells within said first buffer which is to be transmitted first when transmission of upstream cells is resumed and storing an address pointer indicating said location into a first address memory;
• b) transmitting an end-of-stream OAM (operations, administration and maintenance) cell from the mobile site to the mobile switching center over the first communication link;
• c) holding the downstream ATM cells in a second buffer located in said mobile switching center in response to the end-of-stream OAM cell and determining the location of one of the downstream cells within said second buffer which is to be transmitted first when transmission of downstream cells is resumed and storing an address pointer indicating said location into a second address memory;
• d) establishing a second communication link between said mobile site and said mobile switching center;
• and
• e) resuming transmission of upstream ATM cells from said mobile site, starting with a location of said first buffer specified by the address pointer in said first address memory and resuming transmission of downstream ATM cells from said mobile switching center, starting with a location of said second buffer specified by the address pointer in said second address memory.

An end user system for an ATM communications network, comprising:

• an interface for establishing a wireless link to the network;
• a transmit buffer for transmitting upstream ATM cells to the network via the interface;
• a receive buffer for receiving downstream ATM cells from the network via the interface;
• monitor circuitry for monitoring said wireless link and transmitting a request message to said network when the link is likely to become unavailable for informing that the end user system is establishing a new wireless link;
• an address memory;
• and
• a controller for (a) holding the upstream ATM cells in said transmit buffer immediately following the transmission of said request message, (b) determining the location of one of the upstream cells within said transmit buffer which is to be transmitted first when transmission of upstream cells is resumed, (c) storing an address pointer indicating said location in said address memory, (d) transmitting an end-of-stream OAM (operations, administration and maintenance) cell to the network before said new link is established, and (e) resuming transmission of upstream ATM cells when said new wireless link is established, starting with a location of said transmit buffer specified by the address pointer in said address memory.

A communications network comprising:

• an end user system;
• first and second base stations for establishing a wireless link to said end user system;
• an ATM switching system connected to said first and second base stations for establishing a communication link to said end user system via said first base station;
• and
• a mobile switching center connected to said ATM switching system, comprising:
• a transmit buffer for transmitting downstream ATM cells to the end user system via said ATM switching system;
• a receive buffer for receiving upstream ATM cells from the end user system via said ATM switching system;
• a processor for receiving a request message from said end user system when the wireless link is likely to become unavailable and determining a second communication link via said second base station to said end user system;
• an address memory;
• and
• a controller for (a) holding the downstream ATM cells in the transmit buffer in response to an end-of-stream OAM (operations, administration and maintenance) cell from the end user system, (b) determining the location of one of the downstream cells within the transmit buffer which is to be transmitted first when transmission of downstream cells is resumed, (c) storing an address pointer indicating the location of said one downstream cell into the address memory, (d) controlling the ATM switching system to establish said second communication link, and (e) resuming transmission of downstream ATM cells via the second communication link, starting with a location of the transmit buffer specified by the address pointer in the address memory.