EXAMPLE EQUIPMENT AND SERVICE SPECIFICATIONS

The remainder contains examples of specifications that can be used to detail technical requirements for a digital transmission facility for a multiple station group. The examples cover a studio-to-transmitter (STL) network, digital television transmitter, design, erection, and installation services. The material is intended to serve as the technical content of an RFI or RFP and acceptance of deliverables.

STL Network

The STL network will provide and all transmission facilities required to transport content, monitor and control transmitters, and support voice and data communications, site security, and network management functions. The network must be designed to meet very high reliability operations with less than a 1-minute outage per month, cumulative outage of less than 10 minutes per year. Each site must have at least two physical routes between the central master control and each transmitter site. If one path experiences an outage, the network will be required to sense the outage and restore service via the alternate route without human intervention. Alternate route facilities between the central site and each transmitter site will be designated Main and Backup. Both paths will be subject to monitoring and alarm at all times. The normal mode of operation is Main path on air, and Backup path in standby. If the Main path experiences an outage, the network will be required to switch to Standby signal automatically without human intervention. If the Standby path experiences an outage, it will cause an alarm to be initiated and logged. Service restoration will be of the highest priority, and at least equal to any service restoration priority provided to other customers in the class covering public convenience and safety.

Service Provider Technical Qualifications

The service provider shall demonstrate competence through design level expertise with classical and contemporary network technology, systems, facilities, services, and network operations. The successful service provider will own more than 75% of all facilities that the network traverses. For those portions it does not own, it will be required to demonstrate satisfactory contractual business relationships with the owner.

The successful service provider will incorporate network management systems that can be accessed by the buyer for purposes of lodging requests for investigation of alarms and outages. Preference will favor those service providers capable of demonstrating current capabilities to accept requests for investigation of outages or service impairment whereby the user enters information into a terminal or other device with an Internet browser-type interface. It will be an absolute requirement that the successful service provider acknowledge a request within 1 minute of lodging by an automatically generated response, and confirm within certain time frames, depending on priority included in the request.

Responses to this request will be evaluated based on cost and technical trade-offs across performance, robustness, and reliability. Presentation of facts and figures is left to the service provider; however, greater consideration and eventual award will be given to the proposal with meaningful, detailed design characteristics that are related directly to service cost. Highly summarized, the most credible proposal containing best performance, robustness, and reliability constrained by most favorable start-up and long-term cost is most likely to be accepted. The buyer reserves full and final judgment on all matters of the selection and award process and will not under any circumstances pay for cost estimates, proposal preparation, or a similar effort, material, or services except by formal agreement to do so in advance of a formal purchase order.

Due diligence will follow initial selection. Due diligence includes physical examination of a cross-section or example of network elements making up the service provider’s core network. Terminals and central offices serving all sites will be examined closely and must include face-to-face meetings with technicians, supervisors, and managers who will be supporting the network. The successful supplier will exhibit confidence in its ability to isolate and resolve issues and concerns of all levels of severity in a competent, logical, and straightforward manner. An examination of power systems, heating, ventilation, air handling, and safety systems and procedures will be conducted.

Network Topology

Network topology will cover sites 1 through N. Service providers initial responses should be based on eight sites as depicted in Figure 1. After examination of the initial proposals and selection of two or three finalists, additional sites may be added for revised technical proposals and pricing.

 

Figure 1: STL Network Topology

Linking in the final network design will be a mix of round robin and point-to-point links between sites. Fully meshed network design is not technically prohibited; however, cost of such networks is likely not attractive. Furthermore, network management and utilization is unattractive because of operational complexity.

Service providers are requested to study the topology and capacity requirements of the STL network and propose at least two approaches. Each approach should be presented with a spreadsheet containing link numbers, site designation capacity, and pricing for each link. Specific routing details for each link are required to show active and passive network elements on each alternate route between sites. An analysis showing minimum and maximum time to repair a single failure of each active and passive element in each path or route will be included on a separate spreadsheet in the same file as linking and pricing.

Network Interface and Capacity

For the initial 3-year period, core network interface and capacity needs include diverse STS1 interface (51.8 Mbs) at each transmitter site and diverse OC12/STM4 (622 Mbs) interface at the central site. The service provider will include an estimate of the time, cost, and required notice for making additional capacity available in diverse STS1 increments.

Service interface will be DVB-ASI across a BNC connector on either side of a bulkhead or patch panel of the service provider’s choosing. In parallel with each service interface, the service provider will include an Ethernet interface to the network management system to facilitate voice and data communications required to support network management. The service provider may provide access to the public network at their option. If such is included, the service provider must demonstrate adequate and satisfactory firewall capabilities between the network interface point and the outside world.

The service between the central master control and each transmitter site will consist of a minimum of four 20 Mbs channels, net of any overhead, but not including any forward error correction. Forward error correction will be discussed in detail during proposal reviews and a determination made as to its necessity and value.

Premises Architecture and Network Interface

Architecture of the premises equipment includes capabilities to support program content transport, voice, data communications, Internet access, and similar applications such as video conferencing and visual site monitoring. Interface on the network side at SONET/SDH level STS1 or STS4 is preferred. All plesiochronous (PDH) service interfaces will be on the service side of the network interface equipment.

Service Interfaces include SMPTE 259, SMPTE 292, SMPTE 305, DVBASI, T1, DS3, and IEEE 10/100/1000BT in accordance with applicable standards. Suppliers are requested to provide a detailed list of applicable standards of official standards bodies such as SMPTE, IETF, ANSI, ITU, or other and relevance and applicability of each reference within the context of each standard’s use in their products and services. Figure 2 shows a functional block diagram of the general requirements. Potential suppliers are requested to include one or more block diagrams showing the next level of detail along with a matching spreadsheet for each item of equipment proposed.

 

Figure 2: Premises Equipment Architecture and Network Interface

The spreadsheet must include quantity, description unit price, and total price and discount applicable to each item based on total package price. Separate sheets in the file covering equipment, installation, and maintenance cost during the first and second through the fifth year of operational life are requested.

The service provider may offer network interface device equipment; however, it may not be part of an overall service contract, depending on initial and ongoing maintenance cost. Potential equipment suppliers are encouraged to quote stand-alone equipment with pricing separate from maintenance cost. Preference will be given to potential suppliers who can demonstrate successful, satisfactory business relationships with similar class customers.

Design and Installation Services

Services and material of the type required and described in this document are complex and risky. This SOW covers lifting, assembly, and installation of a passive transmission system on and at or near the top of tall tower structures. The major components of the system include a gas stop, transmission line components, hangers, an elbow complex, and an antenna. Successful completion of the work requires specialized technical knowledge and heavy lifting equipment. Preference will be given to service providers deemed in possession of these attributes:

• An established business of providing services such as contemplated
• A strong, accident-free safety record
• The continuous employment of key management and site crew personnel
• A stable financial condition with insurance adequate to cover all risks at each site
• Good relationships with tower designers and manufacturers capable of providing material and design knowledge sufficient to support modifications to existing towers, foundations, and guy cable
• An established relationship with third-party structural engineering firms, or regular employment of professional engineers licensed to do business and practice their profession in the state or states where the work will be performed

TECHNICAL REQUIREMENTS | Defining Network Applications

Depending on organization and project size, design resources, budgets, and facilities management practices, natural divisions of responsibility may occur between facilities management, communications, and computer work groups and require close collaboration and cooperation. Defining communications applications is a matter of considering the technical aspects and characteristics of equipment, facilities, and services needed to satisfy the business requirements.

Like the layering models, it’s useful to picture communications network applications at the lower levels, and business or organizational functions at the higher levels. Each location where communications network facilities and services will be used will require physical layer network access and transport. Physical layer access may only appear in one single room in the establishment, but higher layer functions are likely to be spread around the entire building or campus with appearances in each office, cube, conference room, operations center, equipment room, or even outside in parking lots or production areas.

Once an understanding of a business need is clear and mapped to equipment, facilities, and services, it’s helpful and enlightening to analyze the traffic by breaking it out by type across organizational and/or operational function. Voice, data, and Internet access are likely to be at all workstations and many items of shared equipment; however, it’s unlikely that anyone except key individuals in payroll, accounting, and human resources would have access to payroll information. Core content transport may be limited to certain operational functions and even compartmentalized to limit individual access in certain cases. It is also helpful to characterize each application with a priority. For example, all the traffic from all the users necessary to get a commercial on the air would logically take priority over downloading a new commercial from an ad agency website.

The selection and invoking of standards helps to define applications and facilitates acceptance. For example, once a supplier has been qualified, inspection and verification of compliance can be conducted on a periodic sample basis instead of dealing with each and every telephone instrument, network interface card, or other high quantity items. Circuit and facility acceptance test and verification can be reduced from 72 to 48 or 36 hours. Simply stating that compliance with a particular standard, engineering guideline, or recommended practice can serve as a shortcut or alternative to multiple pages of detailed specifications.

If the requirement is for additional facilities at an existing service location, circuit and facility inventory records should be studied for potential use of spare or unused capacity. New circuits and facilities should be planned with the idea that it may be possible to migrate and consolidate all traffic on new facilities, allowing decommission of part or all of older facilities, and reducing operating expense.

NETWORK PERFORMANCE

Network performance metrics include availability, reliability, robustness, and bit error rate. Obviously, these terms are subjective as single words. Including them in a contract for service is wise. Defining them in words that can be used to set performance expectations where money is to be paid in exchange is even wiser. Drawing a diagram and making a list of locations and facility or circuit identification numbers that the definitions cover is absolutely necessary to ensure services received meet expectations; otherwise, money may be paid for inferior or unsatisfactory services.

Here are some generalized statements in layman language to serve as a starting point for definitions in a service contract.

Availability is a measure of the time network resources being paid for that are actually available for use. The time period may be as specified as 24 hours a day, 7 days a week, 365 days a year.

Reliability is a measure of the time the network is performing in accordance with specifications when it is available.

Robustness is a measure of the network’s ability to recover from loss of service caused by a malfunction of one or more elements making up the network.

Bit error rate, also referred to as bit error ratio, is a measure of bit errors received, compared to the total number of bits received over a given time. Packet networks measure lost packets instead of measuring bit errors. The packet either makes it through the network, or it doesn’t. Obviously, if the network doesn’t successfully carry a packet, there will be errors in a file.

Transmission facilities in communications networks are built on terrestrial and satellite radio wave (wireless), wire, and light wave media.

In general, wire has significant distance limitations depending on the electrical bandwidth and level of signal it’s expected to carry. Radio waves are better than wire in both respects, but not nearly as capable as light waves. Light waves have many times the bandwidth capacity as radio waves.

For example, broadcast satellite service (BSS) facilities are designed around an architecture having 32 transponder channels, each with 32MHz occupied bandwidth. Each channel carries a single quadrature phase shift-keyed bitstream capable of about 30 Mbs payload. Direct satellite service (DSS) facilities occupying a single slot are typically configured with 36-, 54-, or 72-MHz transponders and are typically loaded with subcarriers. Each subcarrier is separated from its neighbors, depending on the amount of bandwidth in the payload it’s required to carry. 

Generally, a 36MHz transponder can accommodate a DS-3 (44.736 Mbs), depending on uplink antenna size, transmit power and receive antenna size. These variables all accumulate in a calculation involving reliability, robustness and bit error rate (BER). The lower the error requirement, the more expensive the facility becomes. Terrestrial radio operating in the same band, for example, 4 to 6 GHz or 11 to 12 GHz exhibit similar payload capacity and BER characteristics. Radio channel baseband BER ranges from 10 7 to 10 9.

Wireless LANs are proliferating as well, using the 1Ghz spectrum. Bandwidth ranges from 10 to approximately 50 Mbs, depending on equipment and physical free space path distance.

Single mode fibers running SONET/SDH optical carrier–based protocol has many times the payload capacity and significantly better BER. An OC-48/STM-16 runs at 2.5Gbs and has a capacity of 48 STS-1 (51.84 Mbs). Each STS-1 can carry 1 DS-3 and several more T1s. Baseband link performance is in the range of 10 9 to 10 11 BER.

When differentiating between satellite and terrestrial transmission, it’s important to consider several factors. These include the nature or topology of the network, reliability, robustness, and error rate performance. All things being equal, which of course they aren’t in every detail, but at a high level, satellite networks tend to be more cost-effective and reliable in point-to-multiple-point topology than terrestrial networks. This is not a fiber vs. radio technology consideration; it’s a physical consideration. Fundamentally, the satellite transponder is the common source for multiple receivers of the same information. The physical aspect is one transmitter (albeit an expensive one) to many inexpensive receivers. If the satellite could radiate light waves instead of, or in addition to, radio waves, the consideration wouldn’t change. What would change is the payload capacity for a given radiated source. It’s well known that light waves are capable of carrying far more information than radio waves. The difference is primarily the fact that free space propagation or attenuation is drastically different than physical conductor propagation or attenuation.

Network architecture is best described with a topology map or block diagram. Depending on the business requirements and nature of the traffic, the topology map and block diagram can be combined in a single figure and be used as a conceptual tool or have greater detail added and be used as a design detail document in test and acceptance work, network performance evaluation, or fault diagnosis. The topology map or block diagram is used to capture and convey locations, number of sites, type of facilities, number and type of connections, service demarcation points, bandwidth, phone numbers, network domains, IP address, service provider boundaries and interface points, type and class of service, etc.

COMMUNICATIONS NETWORK SWITCHING AND TRANSMISSION ARCHITECTURE

Telecom network architecture in 2004 is very much a child of many years of growth and technical evolution driven by telephone calls. Telephone calls require 64 Kbs and are point-to-point connections used for short periods. Telephone service economics where the longer the distance between two points of a call costs more money influenced most of the traffic into local and regional calling patterns. These regional calling patterns led to hierarchical network architecture with high capacity switch sites in fewer numbers piecing together circuits to enable regional and national telephone calls.

Figure 1 is an illustration of the hierarchical nature of the telephone network.

 

Figure 1: Classical Telecom Hierarchical Architecture

The access tandems and certain designated transmission facilities are generally referred to as the access network. These switches formed the technical and economic basis for a philosophy the FCC dubbed equal access. Next time you pay a telephone bill, look at the term FCC access charge. Nope, the money doesn’t go to the FCC. It goes into the owner of the access facilities. And according to FCC regulations, the owner of the access facilities in each LATA must afford equal access to any inter-exchange carrier (IXC) willing to arrange for a connection between their facilities and the local exchange carrier’s (LEC) facilities.

On the other side of that equation, equal access means the LEC must offer their subscribers the ability to select their long distance, or IXC.

The higher levels in the hierarchy vary, depending on the size and extent of the individual long distance carriers. AT&T operates much as it did before divestiture. Others, such as MCI and Sprint, have flatter or fewer levels in their switching hierarchy. All the long distance carriers have gateways to international networks and their own shares in international transmission facilities. Piecing together two or more SONET links end to end creates an extension of the physical boundaries of the network with the addition of more links (Figure 2).

 

Figure 2: SONET Transmission Segment

All the layers are extended across the interface. Each layer becomes an extension of itself each time a SONET transmission facility is connected to another. Linking two or more in series along a path or route, extends the boundary or domain as defined in the facility specification. Figure 2 shows a block diagram of SONET transmission infrastructure linked to provide continuous bandwidth that could be used to provision an E-1/T-1/J1 or E3/DS3/J3 private line. Such a facility could be used as a link in a data communications network or to connect two routers in an inter-network or intra-network link.

Another method of linking facilities is the mid-span meet, commonly used by carriers to link or interface between networks. This is depicted in Figure 3 and shows links between LECs and IXCs.

 

Figure 3: Linked SONET Transmission Facility

A variation on the light wave terminal equipment (LTE) is the add-drop multiplexer (ADM). SONET transmission capacity is built around the 51 Mbs STS. An LTE breaks out all the STS in the optical carrier (OC) and makes them available. If the LTE is used at a location where all STS are not terminated in local routing/switching equipment, then any transit streams must be connected directly to other LTE facing other routes. The ADM breaks out and terminates a limited quantity of STS from the optical carrier. For example, the office in which the terminal is located might only require 1 or 2 STS from an OC-3, OC-12, or higher rate transmission facility. Using an ADM would be less costly in terms of capital investment and operations expense.

CLOCKING CONSIDERATIONS

Clocking and timing in the public network is based on four levels of accuracy, traceable to worldwide time standards. These highly accurate clocking sources are generically referred to as stratum 1 through stratum 4, where stratum 1 is the most accurate. Before 1984, the US portion of the network was primarily the responsibility of AT&T. Network clocking information was distributed throughout the network infrastructure whereby a single clock signal source was sent through the network hierarchy. The most accurate clock signal was distributed to a set of switches that passed it on to lower levels in the hierarchy with the lowest and least accurate clock signal reaching end office switches serving subscribers.

This all changed in 1984 when the network was broken up into the seven regions and equal access to the long distance network was declared. The seven regions were divided into 200 plus local access and transport areas (LATAs) where traffic originating and terminating in different LATAs had to be transported by the long distance carriers. By the time of the breakup, the other long distance carriers already had networks running from independent clocks. With AT&T’s network running on its own clock, this left each of the seven local exchange carriers no choice except to design and build their own clock systems into each LATA’s network.

Original T-carrier or T-1 clocking was a simple matter of two channel banks inter-operating with each other over a four-wire connection. As long as clock stability and recovery were within range, there was no problem. When the clock signal ranged out of bounds of clock recovery, something called a slip, meaning an out-of-lock condition occurred, followed by re-synchronization of the receiving clock. From a practical perspective, this causes a barely audible click in speech. The impact is hardly noticeable in telephone conversation and will likely cause a re-send in data applications, but it can be a killer in content transport applications, especially where the application is something like an MPEG transport stream.

Over time as digital components made their way into switching systems and higher-order multiplex equipment, clocking signals were distributed through the network itself. When that happened, all of a sudden, network clocking was dependent on the network. If the network failed, then clocking failed. To solve this issue, the concept of clock hold-over was created. Hold-over simply set some design and operational rules in place that said, ‘‘When clocking signal is lost, switch to a local source and hold the stability within specified range until the master clock signal is restored, then switch back to the master clock signal.’’

Most pieces of equipment in use have at least one clock. Typically, the design includes external reference capability and configuration parameters that allow the user to control the operation so that it either stays on internal reference, or alternatively locks to an external source. If it is programmed to lock to an external source and it fails, the equipment automatically reverts to the internal clock source.

Communications network clocking architecture is multi-level or strata. There are four levels: stratum 1, 2, 3, or 4. Stratum 1 is the most accurate and stable. Stratum 1 clocks, by definition, do not rely on and may not have or need external references but simply an ability to be calibrated on some periodic basis—say on 1-year intervals. Stratum 2, 3, and 4 are less accurate and stable than stratum 1, and they typically have provisions for external reference input.

From a strict timing perspective, stratum 1 clocks are at the core of the network, while stratum 4 clocks are at the edge of the network such as in a PBX, channel bank, data multiplex equipment or router. Going back through the chain, stratum 4 clocks depend on a stratum 3 clock, which depends on a stratum 2 clock, which depends on a stratum 1 clock, the master clock for the network.

It’s a given that long-term, undisturbed connections through a network depend on continuing, long-term, undisturbed common clock signals and their distribution (or linking to every single network element), which carry a digitized payload. Numerous strategies are used or chosen from to formulate clocking in a network. Regardless, the network operates synchronous or non-synchronous. T-carrier networks are said to run plesiochronous. Taken literally, and in subjective practical terms, it means nearly synchronous. Therefore, it is not synchronous, as understood by an experienced radio or television engineer.

Such operation is characterized by the fact that it experiences clock slip behavior sooner or later. Clock slips mean that the clock and everything that uses it vary from its reference by an amount sufficient to cause the payload to be out of coincidence with its original clock by one clock cycle. It’s not a matter of whether or not clock slips occur, but when. Even when all the network elements run with external reference to a common, higher order accuracy and stability clock, slips can happen from various effects such as temperature changes and atmospheric variations in the case of wireless media. Truly synchronous transmission media became a reality with SONET/SDH network transmission standards and along with that came a significant improvement in network stability.

How to deal with clock distribution and stability is an overall, end-toend network issue. Dealing with it from a design level perspective requires obtaining a set of facts about the capabilities and limitations of the common network equipment and the network service provider’s clocking and distribution infrastructure, and the specific part of that infrastructure providing use of facilities and services to all the sites included in the overall network. More specifically, decisions about network interface equipment should not be made without a clear definition and understanding of the performance characteristics of the equipment with and without external clock reference. The second most important piece or set of information is a definition and understanding of the communications network-timing source and its relationship to the network channel interface at each site.

Once a picture as outlined above has been obtained, it can be assessed in light of the requirements for content transport. The broad perspective includes a situation on the one hand where it may be acceptable to rely totally on service provider network timing sources as the master timing and clocking reference, while the opposite extreme is to equip each site with a set of stratum 1 clock references. Obviously, somewhere in between is likely the practical approach for the enterprise. Don’t forget that network clocking and synchronization is an entirely different matter than station clocking and synchronization. As long as the communications network runs on an accurate enough clock and assuming it is configured to carry live or real-time program content, the content will be clocked from the originating site, with its embedded clock intact, into the communications network, and then moving it from there to one or more sites, extracting it from the telecom clock time base and transferring it to the time base of the receiving site.

One last tip is the fact that a communications network referenced to a global positioning source (GPS) or other source of universal time becomes attractive if the content being transported is also GPS referenced. A good middle of the road, practical design trade off is to reference the MPEG codec-to-station sync and reference the network interface equipment to the service provider facility. If the network bit-error rate is of sufficient level, then the network will be stable and always timed to the same source as the station is. If clock reference is lost, then the network becomes subject to the internal references of the interface equipment, and clock slips become more of a problem because they cause disturbances in picture and sound quality and may cause loss or corruption of closed caption or other similar content.

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.