Transit is the purchasing of Internet access by a network operator from another network operator (known as their upstream provider).
The two considerations when purchasing transit are:
The vast majority of transit attracts traffic charges. Charges are normally levied on traffic levels (typically measured in US$ per Mbps per month). Charges vary from region to region, and the quantity committed to. For example, committing to 100Mbps will attract a much higher charge per Mbps than committing to 1Gbps would.
Some locations charge based on outright volume (usually of downloads, but some combine both downloads and uploads), which can make Internet access very expensive. This method is reminiscent of legacy telephony plans which permit so many calls per month, or mobile data plans allowing the user so many call minutes and/or Gbytes of download per month.
Choosing a Transit Provider
The detailed process behind choice of a transit provider is beyond the scope of the Peering Toolbox, and often depends on local or regional circumstances too.
However, typical considerations include (in no particular order):
cost of the link (physical as well as traffic)
reliability of the link and of the transit provider
service quality provide by the transit provider
latency to destinations that are important for the end-site operator
bandwidth available through transit provider network
the transit provider's diversity of peering and transit arrangements
All of these considerations are important - it's not just about cost, even though cost minimisation is desirable!
Diagram showing Transit
The diagram below shows the transit link between the upstream provider and their customer network.