Architecture Comparison: Present Mode of MAN Operations

To provide enterprises with broadband services, Telecom carriers have deployed SONET/SDH rings in the MANs. With the promise of ATM (Asynchronous Transfer Mode) providing necessary QoS (Quality of Service) for multi-media services, ATM and SONET/SDH rings have become the choice of network architecture for most MAN carriers (www.extremenetworks.com).

In order to accommodate the burst nature of data traffic, customers are asking for features such as flexible bandwidth allocation and short service provisioning time. However, due to the TDM nature of the SONET/SDH network architecture, telecom carriers can offer only leased lines in fixed increments of DS0s (64kb/s), e.g. DS1 (24 DS0s) and DS3 (28 DS1s) or fixed bandwidths in optical carriers (Finneran, 2001). Variable bandwidth leased line service is not possible under the SONET/SDH architecture. There exists the throughput mismatch issue of fitting Ethernet service rates into SONET/SDH transmission rates. If a client wants a 100Mb/s Ethernet service, a carrier must provision a 155 Mb/s SONET channel to carry it, often throwing away the other 55 Mb/s. This inefficiency doesn't exist with Ethernet as the transmission layer. Furthermore, to provide an end-to-end SONET/SDH service requires a long lead-time for the coordination of equipment and facilities before service can be activated.

At times, the provisioning time for a SONET/SDH service can range from weeks to months.

This long service provisioning time has become detrimental in the competitive business environment (Information Technology, 2000).

Because of the lack of bandwidth flexibility and the long service provisioning time of SONET/SDH equipment, service carriers face the following business risks:

1.Revenue Loss: Since SONET/SDH can not offer variable bandwidth service, customers who need such flexibility, for example 10 Mb/s bandwidth, turn to other service providers who can meet this demand. This means a revenue loss for the SONET/SDH carriers.

2.High Operating Cost: The complexity of the ATM and SONET/SDH equipment provisioning not only requires a skilled workforce to do planning, engineering, installation and maintenance, but also needs comprehensive backend OSSs (Operations Support Systems) support in order to offer ATM over SONET/SDH services. Maintaining a well-trained workforce and sophisticated backend OSSs translates into high operating costs. In addition, the relatively high cost of ATM equipment further erodes a carrier’s profitability.

To address the above business challenges, a solution with flexible bandwidth allocation, that is data/IP friendly and quick provisioning in the MAN is essential. Several competing technologies potentially address the above objectives. For example, ATM VP Ring, SONET/SDH MSPP, RPR (Resilient Protection Ring), and Gigabit Ethernet over WDM optical network are all emerging solutions for the MAN network. This section focuses on exploring the feasibility of adopting the Gigabit Ethernet technology in the MAN environment. Challenges as well as pros and cons of deploying Gigabit Ethernet technology in the MAN network will also be discussed (Clavenna, et al., 2001).

3.2 Evolution of Ethernet Technology: from LAN to MAN and WAN

Ethernet is the most widely used local area network (LAN) technology for data communications applications. The most popular version of Ethernet supports a data transmission rate of 10 Mb/s, while newer versions of Ethernet called “Fast Ethernet” and

“Gigabit Ethernet” support data rates up to 100 Mb/s and 1Gb/s (1000 Mb/s). Ten gigabit Ethernet has been proposed and is under development. In addition to the traditional physical twisted pair copper cable, Gigabit Ethernet can also use fiber optical cable as the transmission

media to take advantage of its large bandwidth capability. Today, Ethernet-based LANs already own 95% of the corporate desktops market share (Clavenna, et al., 2001). The share is steadily increasing as Ethernet continues to enhance its features and transmission rates.

Ethernet success in the LAN environment can be attributed to its continued technology enhancements to meet users’ needs.

1. Initially, Ethernet employed basic technology and simple protocols to provide fundamental LAN functionalities. This simplicity translated into high reliability and low maintenance cost for Ethernet users. As a result, Ethernet became widely accepted in the LAN market.

2. Because each Ethernet node must first “sense” the medium before starting to transmit data, this requires the transmission delay to be small enough so that the node at the other end of the cable knows when to transmit in order to avoid a collision. This effectively limits the cable length of Ethernet LANs. As a result, Ethernet “islands” proliferated in the business environment. As more Ethernet islands were deployed, connecting Ethernet islands together for inter-communications became a challenge.

3. In 1984, the Ethernet switch (also known as Bridge) was introduced. It provided a simple, fast, self-learning algorithm, enabling multiple Ethernet networks to be transparently interconnected. This allows the expansion of the Ethernet LAN coverage in the enterprise environment. Furthermore, an Ethernet switch can increase the bandwidth efficiency of individual LANs by assigning users that don’t communicate with each other very often to separate LANs, and users that do communicate with each other frequently to the same LAN.

By doing so, the total effective LAN bandwidth can be increased. However, a basic Ethernet switch is limited to a single spanning tree environment and requires that no loops exist in the LAN topology.

4. In 1985, VLAN (Virtual LAN), also known as the multi-tree Bridge, was invented. VLAN

removes the single spanning tree limitation and enables arbitrary LAN topology.

Furthermore, VLAN offers unlimited capacity and redundancy, traffic priority, and easy reconfiguration advantages for Ethernet users. With the introduction of the VLAN, Ethernet switches began to dominate routers in the LAN environment (Seifert).

Due to the characteristics of CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol, Ethernet LAN transmission distance has to be proportionally reduced as the transmission rate increases. For example, for a minimum packet size of 64 bytes, a 10 Mb/s Ethernet can reach a maximum length of 2500 meters. Under the same packet size, a 100 Mb/s Ethernet LAN can only extend 250 meters. This distance limitation is of a particular concern for the introduction of Gigabit Ethernet. With the high data rate of a Gigabit Ethernet LAN, the applicable coverage is too small relative to the physical environment of an enterprise. Any resolution to this distance shrinking issue has to keep the integrity of the Ethernet frame structure in order to maintain backward compatibility with the large number of existing Ethernet equipment (Information Technology, 2000).

To solve this problem, while keeping the same Ethernet frame format, the concept of full-duplex Ethernet was developed. In a full-full-duplex Ethernet, the CSMA/CD protocol is replaced by the function of a switch. By removing the CSMA/CD protocol constraints and replacing it with a switch, Ethernet transmission distance is no longer limited. For example, using fiber optic transmission media, an Ethernet LAN can cover up to 80 kilometers. With this enhancement, Gigabit Ethernet can be extended to cover a metropolitan area. As a result, Ethernet LAN can expand into an Ethernet MAN (Metropolitan Area Network) (Information Technology, 2000). The evolution of Ethernet technologies is summarized in Table 8.

Table 8. Ethernet Technology Evolution

Ethernet Provide LAN functions

Ethernet switch (bridge)

Connecting and providing switching functions for Ethernet LANs, and increasing Ethernet LAN bandwidth efficiency

VLAN (multi-tree bridge) Eliminating LAN loop-free topology limitation, Enabling arbitrary LAN topology and capacity.

Gigabit Ethernet without CSMA/CD

Ethernet transmission distance is only limited by the transmission characteristics of the physical media.

The advantages of Ethernet are summarized below:

1. Plug and play fast provisioning time

2. Unlimited transmission distance (the removal of CSMA/CD) 3. No network topology limitation (the introduction of VLAN)

4. Multiple kinds of transmission media (fiber optical or coaxial twisted cable) 5. High transmission rate (10 Mb/s, 100 Mb/s, 1 Gb/s, 10 Gb/s and higher rates) 6. Good price/performance (line speed switching)

7. Enormous installed base worldwide 8. Internet IP friendly

9. Scaleable bandwidth

10. Applicable for LAN, MAN and WAN applications

In light of the above attributes, Gigabit Ethernet that uses fiber optical cable as the transmission medium can be used for MAN or even WAN applications. Because Ethernet has the attributes of easy provisioning and bandwidth flexibility, it appears that Gigabit Ethernet

could be a candidate for MAN access to alleviate the bandwidth bottleneck in broadband access.

3.3 Benefits of Using Gigabit Ethernet Technology for MAN Access

Since Gigabit Ethernet is ideal for extension to the MAN environment, telecom carriers can integrate it into a broadband solution that offers bandwidth scalability and easy deployment.

In addition to fast provisioning and bandwidth scalability, additional benefits of simplified network architecture and reduced cost can also be realized (Clavenna, et al., 2001).

3.3.1 Simplified network architecture

If carriers offer conventional DS3 (45 Mb/s) broadband service, they will have to provision DCS (Digital Cross-Connect System) and SONET/SDH ADM (Add-Drop Multiplex) ring in their existing network. With Gigabit Ethernet technology, a telecom carrier could provide broadband connection to users by deploying a series of Ethernet switches linked with leased fiber cables. The elimination of DCS and SONET/SDH ADM equipment in the network architecture not only translates into equipment cost savings, but also simplifies network operations. It is because service provisioning and activation process now require fewer operations tasks with Gigabit Ethernet architecture. This, in turn, could result in substantial operations savings for the telecom carriers.

3.3.2 Eliminate protocol conversion

Compared to ATM-based xDSL service, IP-based xDSL service can simplify the network protocol conversion process by using Gigabit Ethernet as the transport vehicle. For ATM-based xDSL service, an ATM PVC circuit has to be set up between the xDSL remote terminal and the ATM network. This requires the layer 2 ATM and layer 3 AAL5 protocol stacks to be established before an IP layer protocol can be transmitted. However, if the xDSL service is offered via the IP-based equipment such as Gigabit Ethernet, an ATM PVC connection is not needed. Therefore, there is no need to perform ATM and AAL5 protocol conversions.

3.3.3 Asynchronous network vs. Synchronous network

Because the SONET/SDH network operates in synchronous mode, it requires highly accurate network timing. Complex network synchronization mechanisms based on stratum 1 to stratum 4 timing clock is needed to keep SONET/SDH equipment inter-operating synchronously. Without strict synchronization in timing, transmission impairments such as jitter and slip occur. Therefore, it is critical to have a comprehensive network synchronization plan, in terms of timing signal redundancy and protection, to ensure the network integrity.

The cost of engineering and maintaining such a network synchronization mechanism, particularly for the accuracy and stability of stratum 1 clock, is prohibitively high. In addition, at each equipment line card a PLL (phase locked loop) is needed to extract the timing signal from the core network. The cost of PLLs, which constitutes a significant portion of the line card cost, adds to the total capital investment of a synchronous network (GR-253, 2000).

Since Gigabit Ethernet operates in asynchronous mode, it does not require network synchronization nor sophisticated PLL in each line card. This can reduce overall network equipment costs as well as operating expenses.

3.3.4 Ethernet equipment cost advantage

A study conducted by Dell’Oro Group (Clavenna, et al., 2001) reported that Ethernet bandwidth is approximately 85 percent cheaper than SONET bandwidth. Telecom carriers need to spend only $150,000 on Ethernet equipment to get bandwidth equivalent to a $1 million SONET network.

The Dell’Oro study compared the average selling price of Fast Ethernet (100 Mb/s), Gigabit Ethernet and 10-Gigabit Ethernet switches with that of the OC-3 (155 Mb/s), OC-12 (622 Mb/s), OC-48 (2.5 Gb/s) and OC-192 (10 Gb/s) SONET equipment. A common price/performance metric can be used; these prices are converted into dollars per Gigabit of

bandwidth. As shown in Figure 2, by 2004, the cost difference in deploying one Gigabit of bandwidth using 10 Gb/s Ethernet versus SONET OC-192 was several thousand dollars.

With over 100 million installed Ethernet LANs in the world, there are a large group of users and skillful technicians who are familiar with Ethernet technology. The convenience of the “plug and play” feature and the backward compatibility to lower speed Ethernet LANs suggest a smooth migration path for enterprise IT applications which might push the demand for Gigabit Ethernet high and eventually drive its cost down. In addition to applications such as fast Internet access and low cost transport of IP data among multiple sites within a metro area, other advanced applications of MANs will be possible through leveraging Gigabit Ethernet capabilities.

3.3.5 Global end-to-end LAN connection

Because Gigabit Ethernet maintains the Ethernet frame format throughout the core network, an end-to-end Ethernet LAN connection becomes possible for enterprise customers. The advantages include no necessity for protocol conversion at the network edge, improved network performance, and efficiency in bandwidth utilization. The transparent LAN-to-LAN connection will allow enterprise LANs to exchange data and files without the worry of losing data integrity. Implied in this is the large number of the installed enterprise Ethernet LANs will not become obsolete with the emergence of Gigabit Ethernet technology (www.appliancom.com).

Figure 2. Dollar Per Gigabit of Bandwidth in 2004, by Dell’Oro Group

3.3.6 High speed ASP and SSP access

Most ASPs (Application Service providers) and SSPs (Storage Service Providers) experience slow transmission speed and throughput bottlenecks in the access network. To prevent service quality from degrading, ASPs and SSPs have to put a limit on the number of simultaneous users they can serve. Since Gigabit Ethernet offers gigabit per second access bandwidth, the network access bottleneck is alleviated and the ASP and SSP should have no problem servicing all of their customers. The bandwidth flexibility of Gigabit Ethernet offers much better broadband access than hard-wired private lines. It should be noted that for SSPs the storage devices could either be distributed over locations connected via Gigabit Ethernet WAN or MAN or attached to a Gigabit Ethernet LAN within a single premise (www.appliancom.com).