Layer 7: Application- serves as the window for users and application processes to access the network services

Layer 6: Presentation- Formats the data to be presented to the application layer. It can be viewed as the "translator" for the network.

Layer 5: Session- Allows session establishment between processes running on different stations.

Layer 4: Transport- Ensure that messages are delivered error free in sequence and with no losses or duplications

Layer 3: Network- Controls the operations of the subnet, deciding when physical path the data takes

Layer 2: Data link- Provides error-free transfer of data frames from one node to another over physical layer

Layer 1: Physical- concerned with the transmission & reception of the unstructured raw bit stream over physical medium

1. IWB- International Wholesale Bandwidth
2. IPLC- International Private Lease Circuit
3. IEPL- International Ethernet Private Lease
4. International Transit
5. IRU- Indefeasible Right of Use
6. Global Interconnect
7. Backhaul

1. Global VSAT (Very Small Aperture Terminal)
2. Dark Fibre

Examples
Layer 1 Physical- Bandwidth (IPLC, IEPL, Backhaul, Interconnect)

Definition:
OSI layers is a conceptual framework for how applications communicate over a network.

The purpose of the OSI reference model/conceptual framework is to guide vendors and developers so the digital communication products and software programs they create can interoperate, and to facilitate a clear framework that describes the function of a networking or telecommunication system.

Most vendors involve in telecommunications make an attempt to describe their products and services in relation to the OSI model.

And although it is useful for guiding discussion and evaluation, OSI is rarely actuallly implemented as is. That is because few network products or standard tools keep related functions together in well-defined layers, as is the case in the OSI model. The TCP/IP protocol suite, which defined the internet, does not map cleanly to the OSI model.

An ISP is how you access the Internet, whether you are a business or a residential customer. Phone companies, cable companies, and satellite companies all work to service given areas for Internet access.

Tier 1 Eg Hibernia Networks, Cogent Communications, Level 3 Communications
A tier 1 ISP is an Internet provider who exchanges Internet traffic with other tier 1 providers. These ISPs exchange traffic strictly through peering agreements.

Tier 1 Internet providers are the networks that provide the backbone of the Internet. We call them backbone Internet providers. These providers build infrastructure such as the Atlantic Internet sea cables. They provide traffic to all other Internet providers, not end users.

Without tier 1 Internet providers, Internet traffic could not be exchanged between continents and countries.

Tier 2 Eg British Telecom, Vodafone, Easynet
A tier 2 ISP is a service provider who connects between tier 1 and tier 3 Internet service providers. Tier 2 providers will exchange Internet traffic through peering agreements, as well as purchase Internet transit.

Tier 2 and tier 3 internet providers are sometimes used interchangeably. A tier 2 supplier tends to find it easier to purchase transit than to work out a peering agreement with a tier 1 provider. The reason for this is the level of transit. The tier 2 provider may not have enough transit, or capabilities, for it to make sense to peer with the tier 1 provider.

Tier 3 Eg Comcast, Deutsche Telekom, Verizon Communications
A tier 3 ISP is a provider who strictly purchases Internet transit. A tier 3 provider is the last mile provider who delivers Internet access to residential homes and businesses.

Although the tier 3 ISP is becoming a blurred definition, it is safe to say they operate strictly on a purchasing model.

Internet service providers are how you gain access to the Internet. Downloading documents from a server in California to a user in Germany means all three levels of ISPs have to work together. This is done through a combination of peering and paid Internet transit agreements.