Which type of service provider is responsible for offering backbone connectivity services to other service providers?

via wikipedia.org

Communications service provider (CSP) – deliver IP VPN and Internet access services

Telecommunications service provider (TSP) – provide access to telephone and related communications services; generally excludes Internet service providers (ISPs), cable companies, satellite TV, and managed service providers

Network service provider (NSP) – a business or organization that sells bandwidth or network access by providing direct Internet backbone access to the Internet and usually access to its network access points (NAPs); network service providers are sometimes referred to as backbone providers or Internet providers

Internet service provider (ISP) – provides services for accessing, using, or participating in the Internet; Internet services typically provided by ISPs include Internet access, Internet transit, domain name registration, web hosting, Usenet service, colocation

Application service provider (ASP) – computer-based services provided to customers over a network; such as access to a particular software application