T1/E1 in the Gigabit Ethernet MAN

Definitions vary, but the Metropolitan Area Network, or MAN, is a term used to describe data networks that are wider than LANs but more local than WANs. MANs usually cover an entire metropolitan area, such as a large city and its suburbs.

Until the 1990s, control of the Local Loop by monopolistic PTTs and the high cost of implementation throttled extensive development of MANs. MAN technology was typically based on FDDI, and data rates ran from T3 speeds (45 Mbps) up to 200 Mbps. With the onset of deregulation and liberalization, alternative carriers began to compete with the incumbent operators by offering specialized data services based on SDH/SONET backbones and an ATM switch layer. Initially, the driving force of these new MANs was the need to support LAN-to-LAN connectivity. With the proliferation of the Internet/intranet, however, the model is becoming more IP-centric, including VoIP.

The concept of city-based MANs that support both voice and Internet/IP service applications for the enterprise and residence is gaining popularity. Both in the public and private sectors, there is considerable interest in the construction of MANs to achieve high speed connectivity and to save costs in a way that LAN/WAN combinations cannot.

Another part of the service market that has also been dramatically affected by the trend of liberalization is the utility sector. Power, water and gas companies are looking for ways to augment their slimmer profit margins in the new era of fierce competition. In the U.S., and to a lesser extent in Europe, utility companies have been busily laying fiber along regional water mains maintenance systems or wrapping it around high voltage power cables, with the knowledge that fiber means bandwidth that can be turned into revenue.

In terms of physical connectivity in the MAN, technology options include: fiber optic cabling; cable TV and associated modems; microwave and Wireless Local Loop; xDSL; cellular telephony; and even large airships located 20 kilometers in the atmosphere above cities. In terms of networking architectures and mechanisms, costs have been dropping for all products associated with IP, ATM, SONET/SDH, and now, Gigabit Ethernet.

MANs based on fiber optic cabling and Gigabit Ethernet/IP hardware switches are beginning to appear out of this complex soup of technologies, business models and market needs. They are being created by newly deregulated carriers and utility companies, eager to supply voice/data services to businesses and residences, as well as reduce their transmission costs and convert their backbones to converged IP-oriented networks. Still unknown is the potential size of this revitalized MAN market, and what proportion of MANs being implemented are using Gigabit Ethernet instead of ATM and SONET/SDH. What is clear is that it is now feasible to provide services with sufficient Quality of Service support (using 802.3 p/q) over dark fiber, in networks of at least 5 kilometers in circumference (and often far more) using Gigabit Ethernet switches. In fact, such MANs have already been established in the U.S., Scandinavia, UK, China and other locations around the world.

If we assume that Gigabit Ethernet MANs will grow in popularity as dark fiber becomes more readily available and Gigabit Ethernet product prices plunge (as have those of Ethernet and Fast Ethernet products), then the next question is: How will voice services be provided over such networks? The immediate answer is the Voice over IP (VoIP) gateway. In the last two years, vendors and voice carriers alike have been investing heavily in developing, testing and implementing VoIP technology, which is starting to replace TDM-based international and enterprise voice trunks.

The suitability of VoIP gateways in this environment, however, is not altogether certain. The two primary categories of users of Gigabit Ethernet MAN voice services today are enterprises using the Gigabit Ethernet MAN data services, and voice carriers planning to implement voice capacity growth using packet-based technology instead of legacy TDM solutions. Both types of customers demand reliable and high quality voice services. They also require basic functionality (such as call back and caller identification), with cost an important factor, both in terms of purchase price and maintenance overhead. It is important to understand that VoIP solutions include the switching functionality required for voice services. This means that, in many ways, the VoIP gateway and gatekeeper are a case of reinventing the telephony switch and the PBX. Although this is the natural consequence of converging voice and data services over IP networks, it is not a trivial or smooth process, as Voice over Frame Relay vendors (and their customers) have learned. Most VoIP products are a long way from supporting the many hundreds, sometimes thousands, of features supported in SS7 signaling environments. H.323, the standard for VoIP gateways, is now facing competition from SGCP/MGCP, which claims to support SS7 environments more efficiently. Customers will think carefully before swapping out working, reliable, high quality, fully featured legacy voice equipment with VoIP products.

One way to overcome these issues when connecting voice devices to packet-based networks such as Gigabit Ethernet MANs is not to reinvent the telephony switch and the PBX at Layer 4 of the OSI model, but to convert the Layer 2 and 3 TDM output to packet streams.

RAD Data Communications has developed the IPmux-4T1/E1, designed to convert up to four T1/E1 circuits into IP packets, which are then forwarded via a 100BaseT link to the Ethernet/IP network. At the destination, a second IPmux converts the IP packets back into TDM and reconstitutes the T1/E1 circuits. Aimed at point-to-point applications, it leaves the voice switching issues to the telephony switches and PBXs. It is therefore transparent to SS7 and all its features, which are transported in the T1/E1 circuits. Based on hardware, the IPmux provides very high performance, a low cost per T1/E1 port and low delay for processing time-sensitive voice traffic. Because of its inherent transparency to the T1/E1 passing through it, the IPmux is simple, and therefore inexpensive, to configure and maintain.

The predictions of TDM's demise have proven premature, as evidenced by the growing number of worldwide installations of T1/E1 ports. TDM is a proven technology with a large knowledge base and steadily dropping prices. It will therefore take some time before telephony switches, PBXs and telephones are replaced by their IP counterparts. However, Ethernet and IP backbones and services in "greenfield" environments will be implemented in increasing numbers.

The IPmux is a high performance, low cost and simple device for transparently transporting T1/E1 circuits over Ethernet/IP packet networks. It will play an important role in Gigabit Ethernet MANs, both at customer premises receiving Ethernet/IP services, and in the carrier network, where T1/E1 ports are being added to existing telephony switches in a packet-based backbone environment.