Which layer of the OSI model is concerned with routers and routing tables?

Layer 1 – Physical

The physical layer is layer 1 of the OSI model. Layer 1 describes units of data such as bits represented by energy (such as light, electricity, or radio waves) and the medium used to carry them (such as copper or fiber optic cables). WLANs have a physical layer, even though we cannot physically touch it.

Cabling standards such as Thinnet, Thicknet, and Unshielded Twisted Pair (UTP) exist at layer 1, among many others. Layer 1 devices include hubs and repeaters.

Layer 2 – Data Link

The Data Link Layer handles access to the physical layer as well as local area network communication. An Ethernet card and its MAC (Media Access Control) address are at Layer 2, as are switches and bridges.

Layer 2 is divided into two sub-layers: Media Access Control (MAC) and Logical Link Control (LLC). The MAC layer transfers data to and from the physical layer. LLC handles LAN communications. MAC touches Layer 1, and LLC touches Layer 3.

Layer 3 – Network

The Network layer describes routing: moving data from a system on one LAN to a system on another. IP addresses and routers exist at Layer 3. Layer 3 protocols include IPv4 and IPv6, among others.

Layer 4 – Transport

The Transport layer handles packet sequencing, flow control, and error detection. TCP and UDP are Layer 4 protocols.

Layer 4 makes a number of features available, such as resending or re-sequencing packets. Taking advantage of these features is a protocol implementation decision. As we will see later, TCP takes advantage of these features, at the expense of speed. Many of these features are not implemented in UDP, which chooses speed over reliability.

Layer 5 – Session

The Session Layer manages sessions, which provide maintenance on connections. Mounting a file share via a network requires a number of maintenance sessions, such as Remote Procedure Calls (RPCs); these exist at the session layer. A good way to remember the session layer’s function is “connections between applications.” The Session Layer uses simplex, half-duplex, and full-duplex communication.

Note

The transport and session layers are often confused. For example, is “maintenance of connections” a transport layer or session layer issue? Packets are sequenced at the transport layer, and network file shares can be remounted at the session layer: you may consider either to be maintenance. Words like “maintenance” imply more work than packet sequencing or retransmission: it requires “heavier lifting,” like remounting a network share that has been un-mounted, so session layer is the best answer.

Layer 6 – Presentation

The Presentation Layer presents data to the application (and user) in a comprehensible way. Presentation Layer concepts include data conversion, character sets such as ASCII, and image formats such as GIF (Graphics Interchange Format), JPEG (Joint Photographic Experts Group), and TIFF (Tagged Image File Format).

Layer 7 – Application

The Application Layer is where you interface with your computer application. Your Web browser, word processor, and instant messaging client exist at Layer 7. The protocols Telnet and FTP are Application Layer protocols.

Note

Many mnemonics exist to help remember the OSI model. From bottom to top, “Please Do Not Throw Sausage Pizza Away” (Physical Data-Link Network Transport Session Presentation Application) is a bit silly, but that makes it more memorable. Also silly: “Please Do Not Tell Sales People Anything.” From top to bottom, “All People Seem To Need Data Processing” is also popular.

Understanding Networking

The OSI model is used as a reference for all networking protocols.

TCP/IP is used as the core networking protocol today on both the Internet and in the enterprise.

The TCP protocol has clearly defined points where a session begins and ends.

Understanding Security Basics

Security is a process that is never finished; security needs are constantly changing as well as the needs for new technologies.

There is no single product or solution that can be used to ensure your network's security.

Each organization has its own specific needs to best help it minimize security risks.

Understanding Firewall Basics

Juniper NetScreen firewalls use stateful inspection to ensure the security of connections passing though the firewall.

Firewall technology is constantly changing to meet the security needs of today's organizations.

DMZ design depends on the designer's ability to accurately assess the actual risks in order to design an adequate structure.

50,000 Foot View of the OSI Model

The OSI model is not a protocol. It is only a reference model.

The higher you go in the stack, the closer you are to the application. Conversely, as you travel down the stack, you're getting closer to the layers that deal with specific network functionality.

The application layer is handed the data with instructions of what to do with it.

The presentation layer converts the format and syntax of the data. It also encrypts and compresses it.

The session layer manages conversations between applications.

The transport layer builds session between end devices and ensures that data is received.

The network layer provides a path between end devices.

The data link layer converts packets from the network layer into frames.

The physical layer converts bits to voltage, frequency, or light before sending these bits across the media.

How to Use the OSI Model in Troubleshooting

The OSI model fits into an overall troubleshooting strategy. Depending on the layer that you begin your troubleshooting, different techniques may be required to determine the root cause of the issue. The following are some of the purposes behind having a networking model:

It provides a standard to utilize and reference, thereby helping to isolate network issues to specific layers of a protocol stack.

It allows to identify the types of components that may be involved in an issue, thus helping in troubleshooting the problem.

Open standards enable the consumer to patronize multiple vendors.

Vendors produce products that can be used in various networks, including those that started out using a different vendor's products.

The layered approach provides a logical division of responsibility, where each layer handles only the functions that are specific to that layer.

Since the networking protocols are typically divided into layers, trouble-shooting is easier because you are better able to narrow down the source of the problem to a specific layer.

Reviewing the OSI Model

The OSI model consists of seven layers, and when one computer communicates with another one, data at the sending computer is passed from one layer to the next until the final layer puts it out onto the network cable. At the receiving end, it travels back up in reverse order. Although the data travels down the layers on one side and up the layers on the other, the logical communication link is between each layer and its matching counterpart, as shown in Figure 10.1.

FIGURE 10.1. Each layer of the OSI model communicates with the corresponding layer

As the data goes down through the layers, it is encapsulated or enclosed within a larger unit as each layer adds its own header information. When it reaches the receiving computer, the process occurs in reverse; the information is passed upward through each layer, and as it does so, the encapsulation information is evaluated and then stripped off one layer at a time.

8.4.1 The OSI model

Networks are complex systems. Ideally, they provide high-level services while hiding many of the details of data transmission from the other components in the system. To help understand (and design) networks, the International Standards Organization (ISO) has developed a seven-layer model for networks known as Open System Interconnection (OSI) models [Sta97A]. Understanding the OSI layers will help us to understand the details of real networks.

The seven layers of the OSI model, shown in Fig. 8.4, are intended to cover a broad spectrum of networks and their uses. Some networks may not need the services of one or more layers because the higher layers may be totally missing or an intermediate layer may not be necessary. However, any data network should fit into the OSI model.

OSI layers

The OSI model includes seven levels of abstraction known as layers:

Physical: The physical layer defines the basic properties of the interface between systems, including the physical connections (plugs and wires), electrical properties, basic functions of the electrical and physical components, and the basic procedures for exchanging bits.

Data link: The primary purpose of this layer is error detection and control across a single link. However, if the network requires multiple hops over several data links, the data link layer does not define the mechanism for data integrity between hops, but only within a single hop.

Network: This layer defines the basic end-to-end data transmission service. The network layer is particularly important in multihop networks.

Transport: The transport layer defines connection-oriented services that ensure that data are delivered in the proper order and without errors across multiple links. This layer may also try to optimize network resource utilization.

Session: A session provides mechanisms for controlling the interaction of end-user services across a network, such as data grouping and checkpointing.

Presentation: This layer defines data exchange formats and provides transformation utilities to application programs.

Application: The application layer provides the application interface between the network and end-user programs.

Although it may seem that embedded systems would be too simple to require use of the OSI model, the model is in fact quite useful. Even relatively simple embedded networks provide physical, data link, and network services. An increasing number of embedded systems provide Internet service that requires implementing the full range of functions in the OSI model.

The OSI Model

The Open System Interconnection (OSI) Reference Model is a layered network model that was developed by the International Organization for Standardization. It is abstract—that is, we do not run it directly in our systems (most now use the TCP/IP model); rather, we use it as a reference point, so “Layer 1” (physical) is universally understood, whether, for example, we are running Ethernet or Asynchronous Transfer Mode (ATM).

The OSI model has seven layers, as shown in Table 7.1. The layers may be listed in top-to-bottom or bottom-to-top order.

Table 7.1. The OSI Model

The OSI Model

The OSI (open system interconnection) reference model is a layered network model. The model is abstract; we do not directly run the OSI model in our systems (most now use the TCP/IP model). Rather, it is used as a reference point, so “Layer 1” (physical) is universally understood, whether you are running Ethernet or ATM, for example. “Layer X” in this book refers to the OSI model.

The OSI model has seven layers, as shown in Table 4.1. The layers may be listed in a top-to-bottom or bottom to top order. Using the latter, they are Physical, Data Link, Network, Transport, Session, Presentation, and Application.

Table 4.1. The OSI Model

7Application6Presentation5Session4Transport3Network2Data link1Physical

Wireless Network Architecture

Wireless networks operate at the physical and data link layers of the open system interconnection (OSI) model. Both FHSS and DSSS are implemented at the physical layer of the OSI model. The data link layer is divided into two sublayers: the media access control (MAC) and logical link control (LLC) layers.

The MAC layer is responsible for such things as framing data, error control, synchronization, and collision detection and avoidance. Wireless networks utilize carrier sense multiple access with collision avoidance (CSMA/CA) method for protecting against data loss. Using CSMA/CA, a wireless workstation first tries to detect if any other device is communicating on the network. If it senses it is clear to send, it initiates communication.

Router

A router is a hardware routing device. A router works at layer 3 of the OSI model—the Network layer—and so can communicate between various networks. Layer 3 is where the IP protocol examines the source and destination of packets between IP addresses. In the event that the destination is not listed, the router will either send the packet to a default router or drop the packets as lost. Routers are usually used to connect a LAN to a wide area network (WAN) because routers prevent broadcast traffic from bleeding between networks by isolating their own broadcast domain. If the network is being bombarded by IP broadcasts, the router allows you to segregate traffic using subnetting. Only the router allows broadcast traffic to flow between subnets. Because routers prohibit broadcast traffic and give you control over the TCP and UDP ports, they make exceptional firewalls.

What OSI layer is routing table?

Which layer of the OSI model is responsible for addressing and routing?

What is Layer 3 in the OSI model responsible for?

What is Layer 4 in the OSI model?