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REVENUE RECOGNITION - Schedule of Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
REVENUE RECOGNITION      
Total revenue $ 844,556 $ 833,881 $ 826,339
United States      
REVENUE RECOGNITION      
Total revenue 402,957 395,984 344,415
Europe, Middle East and Africa      
REVENUE RECOGNITION      
Total revenue 214,151 234,679 261,901
Asia Pacific      
REVENUE RECOGNITION      
Total revenue 180,985 151,570 167,083
Other      
REVENUE RECOGNITION      
Total revenue 46,463 51,648 52,940
Product revenue      
REVENUE RECOGNITION      
Total revenue 434,587 447,229 445,150
Product revenue | Cloud and Edge      
REVENUE RECOGNITION      
Total revenue 198,163 211,001 184,730
Product revenue | IP Optical Networks      
REVENUE RECOGNITION      
Total revenue 236,424 236,228 260,420
Product revenue | Sales to enterprise customers      
REVENUE RECOGNITION      
Total revenue 148,758 176,064 143,853
Product revenue | Sales to service provider customers      
REVENUE RECOGNITION      
Total revenue 285,829 271,165 301,297
Product revenue | Indirect sales through channel partner program      
REVENUE RECOGNITION      
Total revenue 134,154 170,426 157,495
Product revenue | Direct sales      
REVENUE RECOGNITION      
Total revenue 300,433 276,803 287,655
Product revenue | United States      
REVENUE RECOGNITION      
Total revenue 179,660 201,340 161,945
Product revenue | Europe, Middle East and Africa      
REVENUE RECOGNITION      
Total revenue 111,235 129,824 151,938
Product revenue | Asia Pacific      
REVENUE RECOGNITION      
Total revenue 129,298 100,766 115,923
Product revenue | Other      
REVENUE RECOGNITION      
Total revenue 14,394 15,299 15,344
Service      
REVENUE RECOGNITION      
Total revenue 409,969 386,652 381,189
Service revenue (maintenance)      
REVENUE RECOGNITION      
Total revenue 270,551 274,582 279,652
Service revenue (maintenance) | Cloud and Edge      
REVENUE RECOGNITION      
Total revenue 206,604 212,988 219,939
Service revenue (maintenance) | IP Optical Networks      
REVENUE RECOGNITION      
Total revenue 63,947 61,594 59,713
Service revenue (maintenance) | United States      
REVENUE RECOGNITION      
Total revenue 136,452 133,182 133,737
Service revenue (maintenance) | Europe, Middle East and Africa      
REVENUE RECOGNITION      
Total revenue 69,594 71,856 75,478
Service revenue (maintenance) | Asia Pacific      
REVENUE RECOGNITION      
Total revenue 38,474 39,863 39,891
Service revenue (maintenance) | Other      
REVENUE RECOGNITION      
Total revenue 26,031 29,681 30,546
Service revenue (professional services)      
REVENUE RECOGNITION      
Total revenue 139,418 112,070 101,537
Service revenue (professional services) | Cloud and Edge      
REVENUE RECOGNITION      
Total revenue 106,663 81,168 72,979
Service revenue (professional services) | IP Optical Networks      
REVENUE RECOGNITION      
Total revenue 32,755 30,902 28,558
Service revenue (professional services) | United States      
REVENUE RECOGNITION      
Total revenue 86,845 61,462 48,733
Service revenue (professional services) | Europe, Middle East and Africa      
REVENUE RECOGNITION      
Total revenue 33,322 32,999 34,485
Service revenue (professional services) | Asia Pacific      
REVENUE RECOGNITION      
Total revenue 13,213 10,941 11,269
Service revenue (professional services) | Other      
REVENUE RECOGNITION      
Total revenue $ 6,038 $ 6,668 $ 7,050