tokens: ['picture', 'shows', 'the', 'SIMPLE', 'reference', 'architecture', 'on', 'which', 'it', 'is', 'based', 'Figure', '', 'SIMPLE', 'Reference', 'Architecture', 'for', 'Presence', 'service', 'support', 'based', 'on', 'IETF', '', 'Presence', 'Source', 'The', 'Presence', 'Source', 'is', 'an', 'entity', 'that', 'provides', 'presence', 'information', 'to', 'a', 'Presence', 'Service', '', 'It', 'can', 'be', 'located', 'in', 'the', 'users', 'terminal', 'or', 'within', 'a', 'network', 'entity', 'When', 'the', 'SIPIP', 'Core', 'network', 'corresponds', 'with', 'GPP', 'IMS', 'or', 'GPP', 'MMD', 'networks', 'the', 'Presence', 'Source', 'can', 'be', 'implemented', 'in', 'a', 'UE', 'or', 'an', 'AS', 'as', 'defined', 'in', 'GPP', 'and', 'GPP', 'respectively', 'The', 'following', 'figure', 'shows', 'an', 'architecture', 'for', 'the', 'PNA', 'in', 'the', 'case', 'that', 'no', 'GPPGPP', 'network', 'represents', 'the', 'core', 'Figure', '', 'OMA', 'Reference', 'Architecture', 'for', 'Presence', 'service', 'support', 'in', 'a', 'nonGPPGPP', 'case', '', 'Watcher', 'The', 'watcher', 'is', 'an', 'entity', 'that', 'subscribes', 'to', 'presence', 'information', 'about', 'a', 'presentity', 'or', 'list', 'of', 'presentities', 'ie', 'presence', 'list', '', 'When', 'the', 'SIPIP', 'Core', 'network', 'corresponds', 'with', 'GPP', 'IMS', 'or', 'GPP', 'MMD', 'networks', 'the', 'watcher', 'can', 'be', 'implemented']