Abstract: An apparatus for avoiding head of line blocking in an ATM device includes a scheduler, at least one multicast queue, at least one unicast queue, a multicast session table, a multicast timer, and a problem PHY vector. The methods of the invention include alternate scheduling between multicast queue(s) and unicast queue(s). If a PHY device in a multicast session is inactive, it is skipped and the next PHY in the session is serviced. When the session has serviced all of the active PHYs and there remain only inactive PHYs in the session table, the session is ended. Preferably, a timer is started when only inactive PHYs remain in a session and the session is ended when the timer expires, if not sooner. Preferably, a problem PHY vector is maintained and updated at the end of each multicast session and when PHYs become active. The problem PHY vector includes a list of all of the presently inactive PHYs.

Abstract: An apparatus for avoiding head of line blocking in an ATM device includes a scheduler, at least one multicast queue, at least one unicast queue, a multicast session table, a multicast timer, and a problem PHY vector. The methods of the invention include alternate scheduling between multicast queue(s) and unicast queue(s). If a PHY device in a multicast session is inactive, it is skipped and the next PHY in the session is serviced. When the session has serviced all of the active PHYs and there remain only inactive PHYs in the session table, the session is ended. Preferably, a timer is started when only inactive PHYs remain in a session and the session is ended when the timer expires, if not sooner. Preferably, a problem PHY vector is maintained and updated at the end of each multicast session and when PHYs become active. The problem PHY vector includes a list of all of the presently inactive PHYs.