Comprehension Issues

Because of the somewhat fuzzy borders between the various layers, it is some- times difficult to apply the OSI Reference Model to an actual problem. That is particularly true for someone who is new to the subject, and misunderstandings frequently occur.

The reason for such problems lies in the theoretical approach of the defi- nition of the model or in the overlapping tasks of the layers. GSM adds even more complexity by switching between layers for the data transfer on the differ- ent interfaces (A, Abis, Air).

For example, when the BTS receives a Layer 2 SABM frame from the MS, it forwards that information as an EST_IND message toward the BSC,

wrapped into an I-frame. The EST_IND message, clearly Layer 3 information, can be regarded as a translation of the SABM frame (Figure 5.3). This “leap” can be explained by the fact that Layers 1 through 3 are valid only on a link- by-link basis.

The remainder of this book frequently refers to the OSI Reference Model. For that reason, it is important to understand the model, its function, and the difference between the layers. The following analogy is presented to try to help give a better understanding of the “theory.”

5.7.1 An Analogy: The Move to Europe

Since we all have different experiences in life and see things from different per- spectives, the relationship to the OSI model is immediately emphasized during the course of this analogy, “The move to Europe.”

5.7.1.1 The Moving Family as the User or Application

A family that has to move wants to have to do as little work as possible, particu- larly tasks like disassembly, packing, unpacking, reassembly, setting up furni- ture, washing dishes, cleaning rooms, and so on.

The moving family is comparable to the user or the application in the OSI model, which is outside the model. The user communicates with the mov- ing company and defines the schedule as to when to make the move, where to move, and when the move should be finished.

5.7.1.2 The Moving Company as OSI Layers 7, 6, and 5

Let us assume, for the purpose of this analogy, that the moving company has local branches all over the world that are governed by the same business rules. The moving company has many employees: some work in the office to

Air-interface Abis-interface FACCH/SABM I/RLM/EST_IND/[ / ]− − BTS TRX BSC

coordinate the whole process, while others work onsite to do all the packing and unpacking.

The moving company has a selection of different packaging materials spe- cifically designed for the purpose of moving household items (seeASN.1in the Glossary). The moving company can be likened to Layers 5, 6, and 7 of the OSI model. The employees in the office, who control the whole process, can be likened to Layer 7. The onsite employees whose function it is to separate the various household items, such as the clothes, dishes, furniture, and so on, and to pack them appropriately can be likened to part of Layer 6.

This task is similar to the processing of parameters and data in the Presen- tation Layer (Layer 6). The onsite employees label the boxes, according to their contents (e.g., books, clothes, dishes), which makes it easier for their counter- parts in Europe to do the opposite task of unpacking. The packing and labeling procedure in Layer 5 ensures that the moving company at the destination side sets up the bookcases before unpacking the books or sets up the bureaus before unpacking the clothes. Otherwise, the books and the dishes would be unpacked before there are places to put them.

The different boxes for the various goods and the labels on the boxes are, technically speaking, peer-to-peer protocols in OSI Layers 5 and 6, which add some overhead to the process of moving household goods.

Neither the employees in the office nor those onsite deal with the actual transportation process. For that, the moving company uses the services of a transportation company.

5.7.1.3 The Transportation Company as OSI Layer 4

The transportation company is responsible for the end-to-end transportation, which is comparable to a Layer 4 task. The people who work for the transporta- tion company count and number the boxes (error detection and segmentation) and write the destination address, based on the information they have received from the moving company (Layer 7) on the boxes. The numbering of the boxes is as requested by the moving company, that is, the employees onsite (Layer 5), who inform the transportation company as to what order (or sequence) the individual boxes have to be shipped to the destination.

Note that the numbering of the boxes creates a new peer-to-peer proto- col. The transportation company does not know what the labels “bookcase,” “dishes,” and so on, mean in particular because they have no knowledge of the specific requirements. It is for that reason that Layer 4 translates the Layer 5 specific information into its own protocol, in this case, the numbering scheme.

When everything is done, the transportation company hands over all the boxes to the selected shipping company (Layer 3), which selects the method of physical transportation according to price and availability.

5.7.1.4 The Shipping Company as OSI Layer 3

The shipping company or its employees are equivalent to Layer 3. They are not concerned about the contents of the shipment, the numbers on the boxes, or the labels that characterize the contents. They take the boxes, process the address (routing information), and arrange for the packaging of the boxes into containers for transportation.

The long distance between the origination in America and the final desti- nation in Europe is taken in a number of smaller steps (truck, railroad, plane, ship) and requires the reloading of the boxes into different containers. That also requires that, for each leg of the journey, addresses for the temporary, interme- diate destinations have to be assigned

5.7.1.5 Truck, Railroad, Boat, and Airplane as OSI Layer 2

The various containers that have to be used, the types of which are determined mainly by the means of transport, correspond to Layer 2.

Starting at the home, a truck is used to transport the boxes to the railway station. A railroad wagon is used to transport them to the airport, harbor, and so on. The larger units correspond to the various Layer 2 protocols that are used over the link between any two nodes of a telecommunications network.

In the telecommunications environment (cargo shipping), it is Layer 2 (the means of transport) that serves the purpose of providing a secure physical transport medium for the actual data (household goods).

The checksum in a telecommunications environment corresponds to the truck driver’s checklist. This illustrates the difference between what Layer 4 does for data security compared with Layer 2. While Layer 4 numbers and accounts for the boxes of one user (moving family), Layer 2 performs that task for one container that, in general, is shared by many users. That is, Layer 2 sees the process from the viewpoint of the shipping company, and Layer 4 sees it from the perspective of the user.

5.7.1.6 The Infrastructure as Layer 1

What remains is indicating the constituent parts of Layer 1. These are the roads, railroad tracks, engines, and people— everything and everyone that con- tributes to the physical transportation process. Just as in a move to Europe, the Physical Layer in telecommunications changes between intermediate nodes.

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