Network Layer & Routing

Note by Liam-Beckwith, updated more than 1 year ago
Created by Liam-Beckwith about 5 years ago


This set of notes will briefly look at Layer 2 of the OSI Model, the Data Link Layer, as well as Layer 3, the Network Layer. We will focus on Routing and IP Addressing as well a variety of protocols and concepts related to these layers.

Resource summary

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Data Link LayerThe Data Link Layer takes packets from the Network Layer above, it then creates Frames before sending them to the Physical Layer below them.It is also responsible for detecting and correcting errors in receiving data.Multiple access protocols reside within the Data Link Layer. Firstly, there is Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).TDMA is a technology for delivering digital wireless service using Time-Division Multiplexing (TDM). TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels.Other protocols which reside in the Data Link Layer are Address Resolution Protocol (ARP). This converts an IP address into a physical address, called a DLC address, such as an Ethernet Address.Token Ring operates on the Data Link Layer as the layer assigns the permission token which travels around the network. Point-to-Point Protocol (PPP) is also used between two nodes.

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Network LayerLayer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, inter-networking, error handing, congestion control and packet sequencing.The Network Layer has an end to end delivery of data between two computers. This is not 'devices' but an actual entire route. This layer is also interested in Addressing and Routing, something we'll touch on later.The Network Layer is responsible for packet forwarding including routing through intermediate routers. Whereas, the Data Link Layer is responsible for media access control, flow control and error checking.Network examples include AppleTalk, DDP, IP and IPX.There are three main features in this layer; Routing, Logical Addressing, Path Determination. Routing defines how devices (typically routers) forward packets to their final destination. Logical Addressing defines how each device can have an address that can be used by the routing process. Path Determination refers to the work done by the routing protocol by which all possible routers are learnt, but the best route is chosen for use.The Routing Protocol aids routers by dynamically learning about the groups of addresses in the network which allows for forwarding and routing of packets. Other utilities the Network Layer relies on is the Domain Name System (DNS), Dynamic Host Configuration Protocol (DHCP) and the Address Resolution Protocol (ARP).

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Transmission Control Protocol/Internet Protocol (TCP/IP)The Internet Protocol (IP) has the purpose to deliver packets with as little per-packet effort as possible. This means there is no error checking involved. The current version of IP is IPv4. A new version, called IPv6 or IPng, is under development.The Transmission Control Protocol (TCP) performs the useful networking functions such as error recovery and resending lost packets. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent.

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RoutingIn a network, the transmitting computer doesn't know much about the network, only very local information such as the connected Router is known.So it must be known how to get a packet from one computer to another. This is managed through the Router's local knowledge and routing tables which built automatically.The concept of Hop-by-Hop lists the next device in the path to the destination. This is a fundamental concept in the IP/Network Layer.

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RoutersA Router acts a default gateway that moves data between networks and restricts network broadcasts. A Switch, on the other hand, creates a network whilst a Router links computers to the Internet, so users can share the connection. It also acts as a dispatcher, choosing the best path for information to travel so it's received.When a computer wants to talk to a computer on another network, it does so by sending data to the default gateway or the local router. Data is then passed between routers until the data reaches the destination network.

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