GPON's ability to double both the capacity and split ratio make it a logical choice for today's aggressive deployments of FTTP.
By SAYEED RASHID
Many fiber-to-the-premise (FTTP) deployments today use either Broadband passive optical network (BPON) or Ethernet PON (EPON) technology to deliver between 600 Mbits/sec and 1.2 Gbits/sec of capacity with up to 1:32 split ratios in the distribution network. But now there's a new kid on the block that offers network architects significant cost savings while doubling deliverable capacity and enabling higher split ratios.
Using recent innovations in optical transceiver products, Gigabit PON (GPON) delivers twice the bandwidth of EPON at its full speed of 2.5 Gbits/sec. At the same time, GPON-capable transceivers provide an adequate loss budget to enable higher split ratios--up to 1:64 splits--and the ability to achieve the necessary optical loop length distances. Thus, the attributes of GPON make it a logical choice for all FTTP deployments. Its bandwidth and split ratios are particularly suited for multi-dwelling or multi-tenant building applications.
How did we get here?
The Full Service Access Network (FSAN), an industry organization made up of network operators and suppliers, has been the primary force behind the development of a common set of standards for PON since the mid 1990s. FSAN specifications are developed to promote wide interoperability and mass deployment of triple-play services over fiber, and they are adopted by the ITU as international standards. FSAN's work has led to the successful mass deployment of BPON, and that work now continues with GPON.
As BPON gained momentum, the IEEE saw an opportunity to complement BPON with an Ethernet-based PON technology as part of the Ethernet in the First Mile (EFM) initiative. The IEEE developed EPON to deliver higher speed Internet access services; its line rate, at 1.2 Gbits/sec, is twice as fast as BPON.
BPON deployments continue today for most triple-play service rollouts, particularly in North America. EPON, meanwhile, has gained traction in markets like Japan, where the focus has been on delivering a bigger pipe for highly competitive Internet access service.
The FSAN's overall efforts have been focused on building integrated access networks--not just to deliver more bandwidth for high-speed Internet access service, but also to include a complete solution set for all voice, data, and video services. While BPON offers this complete solution, it is limited in two important aspects. First, it provides only 600 Mbits/sec of bandwidth. Second, it uses ATM as the Layer 2 technology, which requires unnecessary overhead in an increasingly Ethernet-centric world.
EPON has several key limitations as well. For instance, it uses native Ethernet at Layer 2, but adds a highly inefficient PON layer that reduces usable bandwidth. Additionally, EPON has a limited scope in terms of its specified capabilities, including quality of service (QoS) and management.
GPON is designed to address the limitations of EPON and BPON. The ITU ratified the initial set of GPON specifications in 2003, and most of the FSAN work was complete by March 2005. Since both BPON and EPON pre-date GPON, their specifications were limited by the state of technology at the time.
The GPON advantage
At full speed, GPON offers twice the bandwidth and split ratio compared to other PONs. The higher speed and split ratio translates into cost advantages for network operators, who can service twice the number of subscribers using the same number of central office (CO) electronics and feeder fiber. In essence, the CO and feeder costs are divided by a factor of two and reduce the overall per-subscriber cost.
GPON's higher bandwidth and split ratios are only achievable using "smarter" transceivers. Innovations in higher-speed optical transceiver technology enable the bandwidth and split advantages of GPON without compromising the optical loop length, or the distance from CO to subscriber. Typically, there is a trade-off between longer distance and number of splits; the more splits, the more power loss and less achievable distance.
GPON-capable optical transceivers feature a 28-dB optical loss budget to enable 1:64 split ratios and provide adequate optical loop lengths for most subscribers. Most EPON and some BPON systems deployed today use 25-dB optics, which limit the split ratio and loop length. Since the new transceivers can deliver 2.5 Gbits/sec of bandwidth, a 64-way split still provides plenty of bandwidth per subscriber, at about 35-Mbits/sec sustained.
GPON also addresses one of the key limitations of BPON: Lack of native Ethernet transport. Like EPON, GPON uses Ethernet as the Layer 2 technology, but it goes much further. The PON protocol it uses adds very little overhead at the PON layer compared to EPON. Moreover, it specifies QoS and management protocols required for triple-play services over an Ethernet-based PON network.
Bandwidth efficiency depends on how much overhead (non-user bits) is needed to carry the revenue-generating user bits. Greater bandwidth efficiency combined with a higher line rate makes GPON the ideal choice for service providers looking to deliver more and more bandwidth.
Bandwidth sweet spot
The demand for bandwidth continues to grow. What seems to be plenty of bandwidth today may not be enough tomorrow. But the search for that bandwidth sweet spot where network costs and revenue are optimized is really about building flexibility in the network architecture.
GPON offers maximum flexibility because its 2.5-Gbits/sec bandwidth can be split up to 64 times to keep costs down. If needed, GPON also can use lower split ratios (i.e. 32, 16) to deliver more bandwidth. For example, GPON allows 2,300 Mbits/sec (i.e. 2.5-Gbits/sec line rate x efficiency) of bandwidth to be shared by 64 customers, or about 35 Mbits/sec per customer. EPON, by contrast, enables a maximum 900 Mbits/sec (i.e. 1.2-Gbits/sec line rate x efficiency) of bandwidth split between 32 customers, or about 28 Mbits/sec per customer.
The ability to double the split ratio, or serve twice the customers, while delivering equal or more bandwidth reduces per-subscriber costs for GPON. Significant capital expenditure (capex) savings are realized at the optical line terminal (OLT) end of the network while costs at the optical network terminal (ONT) end remain the same.
The higher bandwidth and split ratio makes GPON technology particularly suitable for multi-tenant or multi-dwelling buildings. These buildings can be accessed by a single fiber that delivers high-bandwidth services for multiple customers over existing in-building copper using technologies such as VDSL2 and Ethernet.
By all indications, the costs of ONTs and OLT ports will be about the same for the different types of PONs. Even if there is a price premium for GPON ports, GPON can offer capex savings over EPON thanks to its higher bandwidth and split ratios. Of course, capex is only part of any PON business case; increased revenue from services is the key element.
While EPON networks are typically designed for high-speed Internet access service only, GPON deployments are designed for triple-play services. High-speed Internet revenue can be as much as $50 per month, but when voice and video are bundled with it, the total can be as high as $190 per month. In addition, bundling means higher take rates for all services and lower customer churn--which adds up to a compelling business case for GPON technology as network providers look to transform their networks.
Sayeed Rashid is senior marketing manager in Alcatel's Access Networks Division (Raleigh, NC). He may be reached via the company's Web site at www.alcatel.com.