OSI Model

OSI Model 

The Open Systems Interconnection (OSI) model is a conceptual model that describes the universal standard of communication functions of a telecommunication system or computing system, without any regard to the system's underlying internal technology and specific protocol suites. The OSI model has seven layers described below, and helps: 

• Determine the required hardware and software to build their network. 

• Understand and communicate the process followed by components communicating across a network. 

• Perform troubleshooting, by identifying which network layer is causing an issue and focusing efforts on that layer.

Fiber Optic

Transmits data via light waves. Optical cable currently has the highest capacity and distance of any transmission medium.

Coaxial Cable

Coaxial Cable Transmits data over longer distances than other copper cables. Coax is most commonly used in cable TV networks.

Twisted Pair Cable

Commonly used for telephone and Ethernet. Usually seen inside houses 

Satellite 

Transmits data via radio waves through an orbiting satellite. Traditionally used for remote locations.  

Microwave 

Transmits data via radio waves over a clear line of sight

OSI Model – Data Link Layer L2 

The data link layer establishes and terminates a connection between two physically-connected nodes on a network. It breaks up packets into frames and sends them from source to destination. This layer is composed of two parts—Logical Link Control (LLC), which identifies network protocols, performs error checking and synchronizes frames, and Media Access Control (MAC) which uses MAC addresses to connect devices and define permissions to transmit and receive data.

Network Switch is the device associated to Data link Layer .

A network switch connects devices within a network and forwards data frames to and from those devices. Unlike a router, Layer 2 switch only understands communication with devices directly connected to it; Layer 2 has no concept of sources or destinations "farther away" on the network. Network switches can operate at either OSI layer 2 (the data link layer) or layer 3 (the network layer). Layer 2 switches forward data based on the destination MAC address. Layer 3 switches forward data based on the destination IP address, effectively making them routers. Some switches can do both, operating at both OSI layers 2 and 3.

OSI Model – Network Layer L3 

The network layer is responsible for the end-to-end delivery of data between the originating device and the final destination. Routers forward packets by discovering the best path across a physical networks. The network layer uses network addresses (typically Internet Protocol addresses) to route packets 

Network Router is the device associated to Network Layer 

A router is a device that connects two or more packet-switched networks or subnetworks. Routers manage traffic between these networks by forwarding data packets towards their intended destination IP addresses. In order to direct packets effectively, a router uses an internal routing table — a list of paths to various network destinations. The router reads a packet's header to determine where it is going, then consults the routing table to figure out the most efficient path to that destination. It then forwards the packet to the next network in the path.

Network and Connectivity

Network Topologies

The term network topology describes how devices are connected to each other within the network. It describes how many
connections each device has, in what order, and what sort of hierarchy. Typical network configurations include mesh
topology, ring topology, star topology, and tree topology. Each topology has a different level of fault tolerance. 

The topology in each node is directly connected to some or all the other nodes present in the

network. This redundancy makes the network highly faulttolerant, but the escalated costs

may limit this topology to highly critical networks.High Redundancy

All network nodes are connected sequentially to a backbone, except that the backbone ends at the starting node, forming a ring. The failure of one node will result in changing the direction of the data traffic since the topology is Bi-directional . High Redundancy. 

A root node is connected to two or more sub-level nodes, which themselves are connected hierarchically to sub-level nodes. Physically.Medium Redundancy.

Also known as hub and spoke. All the nodes in the network are connected to a central device like a hub or switch via cables. Failure of individual nodes or cables does not necessarily create downtime in the network, but the failure of a central device can. Low Redundancy.

source : https://broadbandusa.ntia.doc.gov/sites/default/files/2022-12/Introduction_to_Broadband_and_High_Speed_Internet_FINAL_0.pdf

What is Dark Fiber ?

Dark fiber refers to optical fiber infrastructure that has not yet been lit (*lit fiber), meaning it is installed but is not yet being used (no data transmission). 

Dark fiber is rented by broadband providers who need to control their own network, so instead of leasing a service, they lease infrastructure that allows them to build their own network with their own equipment. 

Dark fiber gives clients the capacity to increase bandwidth as needed without paying any additional monthly costs and decreases dependence on carrier response times during events and for upgrades. 

 Dark fiber is usually priced per strand per mile for a set period. The dark fiber providers often offer their client the option of an indefeasible right of use (IRU)** which can range between 20 to 30 years, paid upfront along with annual payments for maintenance.

*Lit Fiber: This term is the opposite of dark fiber and refers to a fiber optic cable that is active and able to transmit data. ** Indefeasible Right of Use (IRU) : A contractual agreement (temporary ownership) of a portion of the capacity of a fiber optic cable.  

Symmetric vs Asymmetric for Upload and Download Data

Symmetrical Communication 

Symmetrical communication links are just as they sound. They have the same download and upload rates. For example, a 500/500 Mbps fiber Internet link offers download and upload speeds of 500 Mbps.

Asymmetrical Communication 

Asymmetrical communication links, on the other hand, do not have the same download/upload rates. For example, 25/3 denotes a download speed of 25 Mbps and an upload speed of 3 Mbps. Asymmetrical data communication can make more efficient use of bandwidth than symmetrical data flows, since generally, most users will download more data than they upload

Middle Mile & Last Mile Networks

Interconnected networks that transmit data between and across countries and continents.

Middle Mile The electronics and circuitry that carry a signal from interconnection points and aggregation facilities to a Central Office or Point of Presence. 

Last Mile The electronics and circuitry that deliver the Internet service to a household.

*Point of Presence(POP): Demarcation point, access point, or physical location at which two or more networks or communication devices share a connection. 

**Example of Middle Mile Aggregation Facility is An Internet Exchange Point (IXP), which is a physical location through which Internet infrastructure companies such as ISPs connect with each other.

 ***Example of Middle Mile Interconnect Facility is a Data Center, which is a facility responsible for driving Internet content delivery and managing network resources.

source : https://broadbandusa.ntia.doc.gov/sites/default/files/2022-12/Introduction_to_Broadband_and_High_Speed_Internet_FINAL_0.pdf