LINKING IN COMMUNICATIONS NETWORKS

Linking involves two types of paths through the network: physical and virtual. Paths are created through the network when physical links are connected in series between two terminal sites. A receive path output from equipment in a terminal is connected to transmit path input on another facility. For a full-duplex or two-way path, the receive path from the opposite direction must be connected to the transmit path in the opposite direction. This type of linking process in digital networks can be extended many times without undue service impairment, except for the accumulation of transmission delay and errors. The latter can be mitigated by good link engineering practice common in radio and optical link budgeting.

When any network is made up of three or more sites, another term comes in to play called meshing. Networks are either fully meshed or partially meshed. The four-site network depicted in Figure 1 is classified as partially meshed because there is no direct path or link between sites 1 and 3 or between sites 2 and 4. Figure 2 shows a fully meshed four-site network.

Figure 1: Four-Site Fully Meshed Network

The difference in meshing has implications in terms of economics, reliability, and robustness. Economically, the partially meshed network connected in a round-robin fashion requires four links. A fully meshed network would require six links. The two additional links might increase the monthly charges for leased private line facilities by 50% when comparing a fully meshed, four-site network to a partially meshed network. Such a choice or decision is the eternal dilemma of network architects and designers. The way out of the woods requires economic analysis and judgment to resolve. For now, simply note that two-site networks need not be concerned about meshing issues. Three-site networks require four links to be fully meshed, four-site networks require six links to fully mesh, and a five-site network requires 10 links. Figure 2 shows a five-site fully meshed network.

Figure 2: Five-Site Fully Meshed Network

As the number of sites in a network increase, the number of links soars, dramatically increasing complexity. The more complex network architecture becomes, the greater the need for more detailed documentation required to manage cost, reliability, robustness, and network performance. Another factor mitigating the need for fully meshed networks is use. For example, consider that when one site connects to another, it is for the purpose of passing traffic. That traffic may be flow only one way or both ways, and if the equipment or people at the site are busy or engaged with one site, it may not be possible to connect to a third site simultaneously. From a slightly different perspective, if two sites are busy, then the likelihood of either of them becoming engaged with a third site is limited. Therefore, use of the other possible links is significantly less likely, so why spend money on all that capacity. However, without a fully meshed network, ways to communicate between all the sites must be established, and the answer is switching. The carrier might provide the switching function, or it can be done with premises equipment.

One last point is that meshing might occur at any one of the communications layers, all the way from the physical layer through and including layer 4 of the OSI stack.

Virtual paths at layer 1 are created in the electrical domain, across disparate physical facilities. Linking of virtual paths involves a combination of several factors such as physical aspects of the connector, a match between transmit and receive pins or pairs, signal polarity, and clocking. For example, digital cross connect switching systems are used to create a virtual path through the transmission network when provisioning private line services. POTS/ISDN switches and their signaling systems create virtual paths through the same transmission network to support voice grade connections enabling telephone calls, facsimile, and modem transmission. The mechanics of the connection involve connecting a transmit/receive pair on one side of the switching system to a receive/transmit pair on another facility.

ATM network architecture includes a virtual path layer, inside which virtual channels or circuits are created and placed. Routing and switching are performed according to information in the virtual path identifier and virtual circuit identifier sections of the ATM cell header.

The linking term is also applied to a process or protocol to create a path for data or information between disparate media. A link created with IEEE 802.2 logical link control (LLC) at layer 2 is at the highest layer of LAN architecture. It defines a set of protocols that support services between the media access layer and the transport layer. LLC is functionally equivalent to the telephone hook-switch and DTMF keypad used to control setup and teardown of a voice connection or link. Most LAN cards, and many other devices supporting LAN connections, have a green indicator light. If the light is illuminated, it indicates physical connectivity between two devices. Many telephones, especially those with two or more lines, have an indicator to show off-hook, active line, or in use. Most equipment with a wide area physical connection is equipped with some kind of indicator as well as alarm to indicate status of the link.

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.

Network Architecture, Facilities, and Services

If network architecture can be thought of as analogous to a framework, then facilities and services can be thought of as the bits and pieces that give the architecture detailed substance and a useful purpose. In communications networks, as in broadcast networks, there is an input and output—the basic function. Both must be managed and their assets and cost of operation must be accounted for. Both are enabled with interfaces and protocols. Both tend to evolve and adapt to regulatory and technology forces. More importantly, those that respond favorably to market demand, survive, grow, and prosper.

This chapter is not about how to design and build a telecom network. The intent is to provide information about the nature and characteristics of networks sufficient to communicate desires and requirements to potential equipment and service providers using Internet and Telecom terms, standards, and symbols. The overall goal is to help the reader understand communications network architecture, facilities, and services. The secondary objective is to provide a framework with which to evaluate potential service provider’s ability to deliver facilities and services capable of meeting their requirements, and ultimately assessing and measuring performance against the terms in a contract.

LANGUAGE AND TERMINOLOGY OF COMMUNICATIONS NETWORKS

Communications network architecture is most often taught and thought of as being layered and linked. Layering breaks the network into a set of logically related components. Linking connects the components and makes up an end-to-end facility or service. The network becomes the link between all the equipment at all the locations making up the network. Successful use of network facilities and services always includes a set of common equipment at each location where facilities are terminated, enabling service delivery. Still, equipment, facilities, and services alone are insufficient to ensure successful business use. Man cannot live by bread alone; he must have peanut butter. The key ingredient required for success is a walking, breathing human being, qualified to be responsible and held accountable for making sure all the equipment, facilities, and services are configured, maintained, and changed as day-to-day demands of the enterprise change.

If you are a designer, manager, or senior staff responsible and/or accountable for making or approving decisions related to design, planning, and management of assets and operating expenses related to communications network asset and expense management, the material in this section is one of the more important, if not the most important, part in the book. It can be a technical foundation for a process whereby you and your organization can take firm control of communications equipment and service vendors by first defining your needs in their terms and lingo so you can establish a competitive procurement environment as the first step in building a long-term mutually beneficial relationship between your organization and the vendors. You can benefit greatly if you take the time to study and understand the technical and economic aspects of how to order and piece together end-to-end facilities and services to build the network best suited to your business.

The level of detail is structured around the common equipment at each location and its interface to network facilities and services. One side of the equipment is connected to the network facility that provides access to network services. The other side of the equipment interfaces to local area network (LAN), private branch exchange (PBX), and Moving Picture Experts Group (MPEG) compression and decompression equipment.

Figure 1 shows an example of the common equipment found at a typical operations site engaged in production, post-production, and on-air transmission operations.

Figure 1: Typical Operations Site Common Network Equipment

Figure 1: Typical Operations Site Common Network Equipment

Basic service requirements include telephone or dial-up service, data transmission, Internet access, and content transport within and outside each site. Group ownership operations may range from half a dozen operational sites to 50 or more. The geographic scope might range from purely local stations to statewide or regional or national in size.

This level of detail may seem insufficient when considered from a broadcast operations point of view; however, it is quite satisfactory for network planning and design purposes where the objective is to inform a potential equipment and/or service provider. It can be used to delineate equipment from services, and it can also be used to inform potential consultants or network designers about their responsibilities in terms of a design, project, or program management.

On the communications network side, the level of detail goes substantially deeper in the form of definition, description and use of various network elements, and several alternatives for linking them together. Figure 2 is a simple sketch showing four sites with links between each.

Figure 2: A Four-Site Network Topology Diagram

Figure 2: A Four-Site Network Topology Diagram

The link arrangement depicted in Figure 3 is only one of several possibilities. Obviously if there were only two sites, there would only be one link between the two. Traffic requirements would drive the selection, but it’s quite likely the links would be bidirectional, and of equal capacity, also called full-duplex, asymmetrical bandwidth.

The arrangement shown is called a ‘‘round-robin’’ because it connects all the sites in a series arrangement, with only one link between any two. This arrangement has operational advantages in terms of reliability and robustness; however, it may have disadvantages because the traffic may be more than it can handle in some places and more than needed in others. For now, keep in mind that there are three types of traffic: voice, data, and program content.

While not explicitly mentioned, the Internet is becoming much more important in many ways; however, it will be included as a potential resource when the traffic is segmented into content creation, distribution, and delivery. Also, don’t forget that the Internet is nothing more than just another resource built on a set of technology. The technology is the family of Internet protocols (IP) that can be used separate and apart from the public Internet to build a private network based on IP standards and techniques.

Regardless of the basic technologies and all the architectural considerations, it is the end-to-end service between any two sites that we seek. Figure 3 is simply a reminder of our reference model.

Figure 3: End-to-End Service Reference Model

Figure 3: End-to-End Service Reference Model

Any seemingly complex, multisite network can be decomposed or decoupled into single, defined point-to-point paths, which can be observed, monitored, and measured in many ways. Once end-to-end measurements are made and a profile of its characteristics recorded, it becomes a benchmark for future operations.

One last point is that the single path can be broken apart and individual component performance measured and characterized. One simple obvious characterization is connecting two sets of equipment with wire or fiber, sometimes referred to as the ‘‘perfect network,’’ and measure performance. And of course there are ways to measure and characterize the performance of the facilities and service making up the link between the sites.

Now we go to details of network architecture, facilities, and services.