Editing Internet service provider (section)

===Transit ISPs===
[[File:Internet Connectivity Distribution & Core.svg|thumb|upright=2.0|Tiers 1 and 2 ISP interconnections]]
Just as their customers pay them for Internet access, ISPs themselves pay upstream ISPs for Internet access. An upstream ISP such as a tier 2 or tier 1 ISP usually has a larger network than the contracting ISP or is able to provide the contracting ISP with access to parts of the Internet the contracting ISP by itself has no access to.<ref name="ssrn">[http://ssrn.com/abstract=2128103 Gerson & Ryan A Primer on Internet Exchange Points for Policymakers and Non-Engineers] {{Webarchive|url=https://web.archive.org/web/20200405083326/https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2128103 |date=2020-04-05 }} Working Paper, August 11, 2012</ref>

In the simplest case, a single connection is established to an upstream ISP and is used to transmit data to or from areas of the Internet beyond the home network; this mode of interconnection is often cascaded multiple times until reaching a [[tier 1 carrier]]. In reality, the situation is often more complex. ISPs with more than one [[point of presence]] (PoP)<ref>{{cite book | url=https://books.google.com/books?id=N5DJJXoLPDQC&dq=tier+1+mesh&pg=PA283 | title=Networked Life: 20 Questions and Answers | isbn=978-1-107-02494-6 | last1=Chiang | first1=Mung | date=10 September 2012 | publisher=Cambridge University Press }}</ref><ref>{{cite book | url=https://books.google.com/books?id=5OCcBAAAQBAJ&dq=tier+1+point+of+presence&pg=PA360 | title=Network Routing: Algorithms, Protocols, and Architectures | isbn=978-0-12-800829-4 | last1=Medhi | first1=Deep | last2=Ramasamy | first2=Karthik | date=6 September 2017 | publisher=Morgan Kaufmann }}</ref> may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and may have connections to each one of them at one or more point of presence.<ref name="ssrn" /> Transit ISPs provide large amounts of [[Bandwidth (computing)|bandwidth]] for connecting hosting ISPs and access ISPs.<ref>[http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml#intro cisco.com Sample Configuration for BGP with Two Different Service Providers (Multihoming)] {{Webarchive|url=https://web.archive.org/web/20140119045246/http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a008009456d.shtml#intro |date=2014-01-19 }} BGP article</ref>

[[Border Gateway Protocol]] is used by routers to connect to other networks, which are identified by their [[Autonomous system (Internet)|autonomous system]] number.<ref>{{cite book | url=https://books.google.com/books?id=5OCcBAAAQBAJ&dq=internet+exchange+point+tier+1&pg=PA351 | title=Network Routing: Algorithms, Protocols, and Architectures | isbn=978-0-12-800829-4 | last1=Medhi | first1=Deep | last2=Ramasamy | first2=Karthik | date=6 September 2017 | publisher=Morgan Kaufmann }}</ref> [[Tier 2 network|Tier 2 ISPs]] depend on Tier 1 ISPs and often have their own networks, but must pay for transit or internet access to Tier 1 ISPs, but may peer or send transit without paying, to other Tier 2 and/or some Tier 1 ISPs. Tier 3 ISPs do not engage in peering and only purchase transit from Tier 2 and Tier 1 ISPs, and often specialize in offering internet service to end customers such as businesses and individuals. Some organizations act as their own ISPs and purchase transit directly from a Tier 1 ISP.<ref name="Springer">{{cite book | url=https://books.google.com/books?id=MgDQDwAAQBAJ&dq=tier+1+peering&pg=PA88 | title=Internet Computing: Principles of Distributed Systems and Emerging Internet-Based Technologies | isbn=978-3-030-34957-8 | last1=Sunyaev | first1=Ali | date=12 February 2020 | publisher=Springer }}</ref> Transit ISPs may use OTN ([[Optical transport network]]) or SDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical Networking)<ref name="ciscopress.com"/> with a DWDM ([[Dense wavelength-division multiplexing]]) system<ref>https://www.cisco.com/c/dam/global/de_at/assets/docs/dwdm.pdf {{Bare URL PDF|date=May 2025}}</ref> for transmitting data through optical fiber over long distances such as across a city or between cities.<ref>{{cite book | url=https://books.google.com/books?id=U1M_46OltXAC&dq=dwdm+otn&pg=PA49 | isbn=978-0-387-24063-3 | title=Optical Networking Standards: A Comprehensive Guide for Professionals | date=13 April 2007 | publisher=Springer }}</ref><ref>{{cite book | url=https://books.google.com/books?id=ZO9RWurSS0IC&dq=dwdm+otn&pg=PA93 | isbn=978-1-58053-708-7 | title=Telecommunications Technology Handbook | date=2003 | publisher=Artech House }}</ref><ref>{{cite book | url=https://books.google.com/books?id=EisDEAAAQBAJ&dq=dwdm+otn&pg=PA409 | isbn=978-3-030-16250-4 | title=Springer Handbook of Optical Networks | date=15 October 2020 | publisher=Springer }}</ref> For transmissions in a metro area such as a city<ref>{{cite web|url=https://peering-forum.eu/2022/wp-content/uploads/sites/5/2020/05/P4-400G-ZR...-Florian-Hibler.pdf |date=2022 |publisher=Arista |via=Peering Forum |title=400G-ZR(+) Real World Examples|author=Florian Hibler}}</ref> and for large customers such as data centers,<ref>{{cite web|url=https://pc.nanog.org/static/published/meetings/NANOG75/1954/20190220_Martin_Building_The_400G_v1.pdf |publisher=Arista  |via=NANOG |title=400G and 800G Ethernet and Optics|author=Andreas Bechtolsheim|author-link=Andreas Bechtolsheim}}</ref> special pluggable modules in routers, conforming to standards such as [[C form-factor pluggable|CFP]],<ref>{{cite book | url=https://books.google.com/books?id=dmrOBQAAQBAJ&dq=cfp2+module&pg=PA147 | title=High-Speed Devices and Circuits with THZ Applications | isbn=978-1-4665-9012-0 | last1=Choi | first1=Jung Han | date=19 September 2017 | publisher=CRC Press }}</ref><ref>{{cite book | url=https://books.google.com/books?id=QHa7DwAAQBAJ&dq=cfp2+module&pg=PA42 | title=Coherent Optics for Access Networks | isbn=978-1-000-73650-2 | last1=Jia | first1=Zhensheng | last2=Campos | first2=Luis Alberto | date=28 October 2019 | publisher=CRC Press }}</ref> QSFP-DD, OSFP,<ref>{{cite web |url=https://www.lacnic.net/innovaportal/file/3635/1/christian-urricariet-y-andre-guimaraes.pdf |title=Trends in 400G Optics for the Data Center |author1=Christian Urricariet |author2=André Guimarães |date=May 2019}}</ref> 400ZR or OpenZR+ may be used alongside DWDM<ref>{{cite web|url=https://www.oiforum.com/wp-content/uploads/OIF-400ZR-01.0_reduced2.pdf|title=Implementation Agreement 400ZR|id=OIF-400ZR-01.0|publisher=Optical Internetworking Forum|date=March 10, 2020}}</ref> and many vendors have proprietary offerings.<ref>{{cite web | url=https://www.lightwaveonline.com/optical-tech/transmission/article/14184990/openzr-msa-group-releases-first-specification | title=OpenZR+ MSA Group releases first specification | date=8 October 2020 }}</ref><ref>{{cite web | url=https://www.lightwaveonline.com/optical-tech/transmission/article/14188934/understanding-400zr-openzr-400zr-optics | title=Understanding 400ZR/OpenZR+/400ZR+ Optics | date=11 December 2020 }}</ref><ref>{{cite book | url=https://books.google.com/books?id=QYEqEAAAQBAJ&dq=400zr+dwdm&pg=PA164 | isbn=978-0-12-823134-0 | title=Optical Communications in the 5G Era | date=23 October 2021 | publisher=Academic Press }}</ref> Long-haul networks transport data across longer distances than metro networks, such as through [[Submarine communications cable|submarine cable]]s,<ref name="ciscopress.com"/><ref>{{cite book | url=https://books.google.com/books?id=7PJbEAAAQBAJ&dq=long+haul+network&pg=PA371 | isbn=978-0-19-883422-9 | title=Optical Networks | date=2021 | publisher=Oxford University Press }}</ref> or connecting several metropolitan networks.<ref>{{cite web|url=https://www.cisco.com/c/dam/global/de_at/assets/docs/dwdm.pdf|title=Introduction to DWDM Technology|publisher=Cisco}}</ref> Optical line systems and packet optical transport systems<ref>{{cite web | url=https://www.lightwaveonline.com/network-design/packet-transport/article/16665596/packet-optical-transport-systems-platforms-for-metro-transformation | title=Packet-optical transport systems: Platforms for metro transformation | date=May 2012 }}</ref> can also be used for data transmission in metro areas, long haul connections and data center interconnect.<ref>{{cite book | url=https://books.google.com/books?id=M6vRQRKrMsAC&dq=optical+line+system&pg=PA335 | isbn=978-0-08-051318-8 | title=Optical Fiber Telecommunications IV-A: Components | date=22 May 2002 | publisher=Elsevier }}</ref><ref>{{cite book | url=https://books.google.com/books?id=FZaUPj3yYr4C&dq=optical+line+system+transmission&pg=PA296 | isbn=978-0-12-395172-4 | title=Optical Fiber Telecommunications IV | date=2002 | publisher=Academic Press }}</ref> Ultra long haul transmission transports data over distances of over 1500 kilometers.<ref>{{cite web | url=https://www.lightwaveonline.com/optical-tech/transport/article/16647297/the-search-for-ultra-long-haul-transmission | title=The search for ultra-long-haul transmission | date=May 2001 }}</ref> ISPs connect to each other and to customers via [[data center]]s hosting meet-me rooms.<ref name="auto"/>