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Transmission System Identifiers and Logical Address Mapping: A View from the Bottom


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Transmission System Identifiers and Logical Address Mapping: A View from the Bottom



Standards Architectures and Protocol Development



The Influence of WAN History on Modern Networks



Connectionless Services



The Art of Services Formerly Known as WAN



Addressing Checklist for Dedicated Lines



Circuit-Switched Transmission Systems



Addressing Checklist for POTS Dial Access



ISDN Addressing Checklist



Packet, Frame, and Cell Switched Transmission Systems



Addressing Checklist for X.25 Service



Addressing Checklist for Frame Relay



Addressing Checklist for ATM



Emulated and Virtual LANs



Looking Ahead

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Designing Addressing Architectures for Routing and Switching

From: Designing Addressing Architectures for Routing and Switching
Author: Howard Berkowitz
Publisher: MTP
More Information

4. Transmission System Identifiers and Logical Address Mapping: A View from the Bottom

Avian carriers can provide high delay, low throughput, and low altitude service. The connection topology is limited to a single point-to-point path for each carrier, used with standard carriers, but many carriers can be used without significant interference with each other, outside of early spring.

—D. Weitzmann

One policy, one system, and universal service.

—Theodore Vail

Well if I called the wrong number, why did you answer the phone?

—James Thurber

In designing internetworks, the usual emphasis is on logical addressing. But logical addresses are abstractions that travel over lower-level transmission systems, and these transmission systems often have their own systems of addressing. This chapter deals with those lower-level addresses and their relationships to logical addresses.

The bottom is always a fine place to start. Underlying this chapter is the little-known but useful architectural extension of the OSI Reference Model called the Internal Organization of the Network Layer, which provides a basis for systematic mapping between logical and transmission system addresses.

Standards Architectures and Protocol Development

Regardless of what one thinks of OSI as a solution, it does contain useful architectural methods. These methods did not freeze with the original OSI Reference Model.

Remember that OSI evolved in formal international standards bodies, whose roots were in telecommunications. These roots were largely in governmental national monopolies. Telecommunications providers classically interface to subscribers at the edge of the provider's “cloud,” the details of which are hidden from the subscriber. Subscribers use signaling protocols to set up and receive connections from the cloud. Subscriber endpoints are identified by telephone numbers.

Do not become overly concerned because this information is hidden. Simply assume, for the initial analysis, that the provider network is magical and provides appropriate connectivity. The endpoint addresses in the wide area network (WAN) are the conventions by which user networks access it.

Also understand the underlying protocol model. In a local area network (LAN), the lower-layer protocols often are end-to-end, or at least map to real media. In a WAN, the lower-layer protocols are consciously not end-to-end, but intended to manage access between a provider and customer.

Taking a slightly tongue-in-cheek view of OSI development in the late 1970s, these organizations viewed all communications media as belonging to them, and they would tell subscribers how to connect to them. LAN development in the Institute for Electrical and Electronic Engineers (IEEE) was separate and largely did not affect the original OSI Reference Model published in 1984. Serious coordination between the two cultures was more of a mid-1980s matter. IEEE LAN standards work is IEEE Project 802.

A result of that coordination was the Internal Organization of the Network Layer, a document that relaxed the rigid boundaries between Layer 2 and Layer 3 of the OSI Reference Model and came up with a much more useful picture.

Ignoring the rather obscure OSI terminology for these new layers, the model, shown in Figure 4.1, is split into three parts:

  • Logical addressing

  • Medium or transmission system addressing

  • Mapping between logical and transmission system addresses

Figure 4.1. Revised OSI model reflecting Internal Organization of the Network Layer extensions. The mapping function translates between medium-independent logical addresses and medium-dependent transmission system addresses.

Logical addressing is what we usually think of as Layer 3: largely transmission system independent such as IP. The transmission medium address is the next level down.

As shown in Figure 4.2, IEEE further subdivided the transmission system for the LAN environment into Media Access Control (MAC) and Logical Link Control (LLC) at the data link layer. IEEE also divided the physical layer into media-independent (MDI) and physical-media dependent (PMD) sublayers. General physical layer and PMD functions tend to be implemented in different hardware chip sets.

WAN services, on the other hand, were usefully split into persistent and transient levels. Think of a five-button telephone to which is assigned a five-line hunt group, a group of five telephone lines with a single telephone number. The telephone number is the persistent identifier while the button that lights for a specific call is the transient identifier.

The distinction between LLC and MAC has allowed LLC to be a general buffer management mechanism that hides the timing-critical aspects of the MAC sublayer. LLC also provided a place for more general mechanisms such as upper layer protocol identification and optional retransmission.

Figure 4.2. WAN and LAN sublayering of basic OSI layers consistent with the Internal Organization and IEEE extensions to the OSI model.

Transmission systems have their own addressing, distinct from the network layer. Their addressing models tend to fall into one of two categories, LAN or WAN. This categorization is not really based on the geographic scope of the transmission system, but on the underlying administrative model.

Figure 4.3 illustrates a typical WAN model, as might be seen with Frame Relay, X.25, or ATM. User organizations use access mechanisms to interact with a service provider cloud, the details of which are usually not visible to the calling or called user. Users have virtual circuits across the cloud.

Figure 4.3. Points of reference in WAN models. The DCE is the boundary between provider and customer responsibility.

Even if the WAN service is of physical rather than virtual circuits, there still is a clear demarcation point between carrier/provider and user responsibility.

This demarcation point is logically inside the data circuit terminating equipment (DCE). DCE is often assumed to stand for data communications equipment, but this is the formal standard meaning of the acronym. Special contractual arrangements are needed, in many cases, for the user organization to know the path taken through the provider network.

If, for reliability reasons, the user wants to have multiple circuits with no common point of failure, this takes specific engineering from the carrier, called facility diversity. This can be quite expensive. Some users have made the extremely dangerous assumption that leasing circuits from multiple providers inherently gives diversity. In practice, providers lease transmission facilities from each other and from “wholesale” providers. Diversity can be engineered when a specific organization is responsible for assuring it, but if the different carriers are not aware of each other, they may wind up leasing a common facility from a third party—a facility that can be affected by a single failure.

In contrast, all components of a LAN are operated by the same organization. Internetworking has its roots in the WAN culture, and it is worth reviewing the WAN concepts that shape communications.


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