LAYERING AS USED IN COMPUTERS AND COMMUNICATIONS NETWORKS

The basic idea behind layering is that computer equipment and system functions are bound by, or within, layers. Each layer is bound to, or interacts with, its neighbor immediately above and below. If each layer up and down the stack interacts, or interoperates, with its neighbor successfully, then the system or process making up the overall system or network is likely to succeed in performing all the functions it was designed to accomplish.

The layering concept was created by the International Standards Organization to serve as a standard definition of computer industry structure dealing with communications issues in computer environments. The standard is named the open systems interconnect (OSI) model and typically referred to as the OSI stack. The communications industry adopted the technique for use in standards and design documents. Two examples of communications stacks are shown in Figure 1 along with the OSI stack. The OSI stack has seven layers; the synchronous optical network/synchronous digital hierarchy (SONET/SDH) and Internet stacks are both four layers.

Figure 1: Layering Models Used in Computing and Communications

Taken in isolated display, there doesn’t appear to be much of a match or direct relationship between the three. However, we know that SONET/SDH is an example of the physical layer of the OSI stack, and only the physical layer.

The four-layer Internet stack used by many rests on an IP layer, which is a peer to the network layer in the OSI stack.

Each of the layers in the OSI stack is unique in function and behavior, and has a specific relationship with its neighbors above and below. The OSI model is subdivided with layer 1 and 2 classified as being hardware-oriented, whereas the upper layers are said to be software-oriented. As an entity, the lower layers are communications-oriented; the upper layers are user- or application-oriented.

Another commonly used way of illustrating the layering concept and relating it directly to linking is to draw an end-to-end service model with multiple layers showing multiple virtual channels on top of a physical channel. Figure 2 is another example of the use of layering models to show relationships between the layers in communications networks and computers.

Figure 2: Example of Three Layering Models

Figure 2 has been redrawn from the three separate models shown in Figure 1. It shows these three models and how they relate to each other.

The OSI model has been separated between layer 2 and layer 3. This is the division between hardware on the lower level and software on the higher level. The Internet model is left intact and placed above the SONET model. The VT layer has been added to the SONET model.

Layers 2, 3, and 4 have been shaded with four different levels or shades of gray, reflecting the dividing layer between hardware and software in the OSI model. There is a similar division between the IP, and lowest layer in the Internet model and the VT/STS, or highest layer, in the SONET/SDH model. The data link layer is a point of commonality across all three. That is, voice and data networks use exactly the same physical layer network interfaces and protocols as the Internet does. For example, an unchannelized E1/T1, E3/DS3, or OC3/STM1 facility can be used to support all types of traffic defined in the site location architecture.

Most depictions of SONET/SDH layering don’t include the virtual tributary (VT) layer. If this layer is included on top of the SONET/SDH model, it becomes a direct fit, or interface to the Internet model for plesiochronous (PDH) point-to-point links or leased/private line facilities. Another point worth mentioning is the fact that many purveyors of network equipment and services are offering direct interface between IP and SONET/SDH synchronous transport streams. In and of itself, it’s an incremental step. However, if this physical layer capability is combined with differentiated services in the network, traffic aggregation, and type of service (TOS) capabilities in new and emerging network equipment, the result is a potentially dramatic and profound impact on ISDN/PSTN and plesiochronous (PDH) network facilities and services.

Consolidating mixed or disparate traffic requires knowledge of all three layers and a detailed understanding of how they relate to each other when integrating equipment and software. More importantly, understanding how the traffic payloads are organized and structured is key to successfully mapping the traffic to the network to get maximum use of the network. Much has been said and written about convergence or converged networks. Occasionally, the term multi-service network is used. Layer 2 and 3 is the place in the layering models where multiple, disparate traffic types are converged and mapped to a common access and/or transport facility. Physical placement of routers and switching equipment and its configuration determines physically where, and in what sequence, the so-called convergence (of disparate traffic) takes place.

Applying the layering concept to communications networks seems quite natural and logical. Two types of layering are commonly used to depict communications networks. Classical telephone networks are structured around a multiplexing, switching, and transport hierarchy, while computer networking and the Internet are structured around protocols and interfaces.

Another term that creeps into the lingo from time to time is overlay network. For example, the larger multi-service carrier networks share transmission facilities between voice grade services that require 64 Kbs transmission links with ATM switches and IP routers that use raw transmission bandwidth in varying amounts. Since the ATM and IP networks came into existence after voice grade services, they were built and are said to overlay voice grade services. Occasionally this term is also understood to mean the ATM and IP networks are above voice grade services in the OSI stack, where voice grade services are seen at layer 2, ATM at layer 2/3, and IP at layer 3/4.

Continuing the evolution, the Internet community has taken these two separate models and created a separate but related structure dubbed Inter-networks. The Internet is a completely different structure with its own unique behavior in terms of how it moves a payload, otherwise called content or information. The concept is built around an idea that combines payload information with address information, hands it to the network, and the network not only carries the information, but when it gets to its destination, the network communicates that fact back to the sender. Overall result: A third network is built using the same kinds of standard layer 1 and layer 2 facilities as voice and data networks.

Continuing to add communications facilities for separate network applications has led many to question and wonder if there may not be a better way to organize their traffic to get better use of all the facilities being paid for. Attempts to address these issues have led to solutions called converged or multi-service networks. A more appropriate label might start with the traffic whereby disparate traffic types are converged into a common access facility or across a common transport facility. Viewing the three layered models in a single context can be a constructive and instructive step to defining requirements for a network capable of carrying disparate traffic.