The local distribution network is typically high-quality routed network with very tightly controlled latency, jitter, and packet loss. The local distribution network is typically comprised of a metropolitan core tier and a consumer distribution tier (Fig. 5.7).
In the metropolitan core tier, IPTV is generally transmitted using the telco’s private “carrier-grade” IP network. The network engine can be pure IP-based, MPLS-based (layer “2.5”), metro Ethernet-based (layer 2), or optical SONET/OTN based (layer 1), or a combination thereof. A (private) wireless network such as WiMAx can also be used. The backbone network supports IP Multicast, very typically PIM-DM or PIM Sparse–Dense.
It is important to keep the telco-level IP network (the metropolitan core tier) streamlined with as few routed hops as possible, and with plenty of bandwidth between links and with high-power nodal routers in order to meet the QoS
requirements of IPTV. Otherwise pixilation, tiling, waterfall effects, and even blank screens will be an issue. It is important to properly size all Layer 2 and Layer 3 devices in the network. It is also important to keep multicast traffic from “splashing back” and flooding unrelated ports. IGMP snooping and other techniques may be appropriate.
The consumer distribution tier, the final leg, is generally (but not always) DSL-based at this time (e.g., VDSL or ADSL2+); other technologies such as PON (Passive Optical Network) may also be used (Table 5.2). A bandwidth in the 20–50 Mbps is generally desirable for delivery of IPTV services. For
example, the simultaneous viewing of an HD channel along with two SD channels would require about 17 Mbps; Internet access would require additional bandwidth. Therefore, the 20 Mbps is seen as a lower bound on the bandwidth. In the United States Verizon is implementing Fiber-to-the-Premises (FTTP) technologies, delivering fiber to the subscriber’s domicile; this supports high bandwidth
but it requires significant investments. AT&T is implementing Fiber-to-the-Curb (FTTC) in some markets, using existing copper for only the last 1/10th of a mile, and Fiber-to-the-Node (FTTN) in other markets terminating the fiber run within a few thousand feet of the subscriber. These approaches lower the up-front cost but limit the total bandwidth.
As noted, IPTV as delivered by the telephone carriers may use PON technology, as an FTTH implementation technology, or perhaps VDSL2. However, if loop investments are made by these carriers it is likely that it will be in favor of FTTH. VDSL2 may find a use in Multidwelling Unit (MDUs), as we note below.
The VDSL2 standard ITU G.993.2 is an enhancement to G.993.1 (VDSL). It uses about 30 MHz of spectrum (versus 12 MHz in VDSL) and thus allows more data to be sent at higher speeds and over longer distances. VDSL2 utilizes up to 30 MHz of bandwidth to provide speeds of 100 Mbps both downstream and upstream within 1000 ft. Data rates in excess of 25 Mbps are available for distances up to 4000 ft (Fig. 5.8); Fig. 5.9, depicts, for illustrative purposes, test results for Zhone’s VDSL2 products [2] VDSL2 technology can handle, say, three simultaneous HDTV streams (for example, according to the firm GigaOm Research, the average US home has 3.1 televisions). Of course, there is the issue
that many homes in the United States are too far from the Central Office. The VDSL2 standard defines a set of profiles (Table 5.3) that can be used in different VDSL deployment architectures; ITU G.992.3 extends the North American frequency range from 12 to 30 MHz.
For example, carriers such as Verizon Communications may use VDSL2 for risers in MDUs to bring FTTH service in these buildings. The carrier has been using relatively inexpensive Optical Network Terminal or ONTs (also called Optical Network Units (ONUs)2) for Single Family Unit (SFUs) (using Broadband Passive Optical Network or BPON) initially, but now also seeing Gigabit Passive Optical Network or GPON deployment). Using this arrangement, it is not excessively expensive to bring the fiber service to the living unit of an SFU.
However, tenants of MDUs are more expensive to service because of the cost in pulling the fiber up the risers. Here is where DSL technologies still have some play: on the link between the basement and the apartment unit. For BPON, it is all VDSL1; for GPON, it is all VDSL2. The carrier will set the locations of the MDUs so that the furthest tenant is around 500 ft.; this achieves speeds of around 35 Mbps downstream, 10 Mbps upstream on VDSL1 and BPON. On GPON/VDSL2 the carrier expects to achieve 75 Mbps downstream (Fig. 5.10).
PON is the leading FTTH technology3 (Fig. 5.11). This approach differs from most of the telecommunications networks in place today by featuring “passive” operation. Active networks such as DSL, VDSL, and cable have active components in the network backbone equipment, in the central office, neighborhood network infrastructure, and customer premises equipment. PONs employ only passive light transmission components in the neighborhood infrastructure; active components are located only in the central office and the customer premises equipment. The elimination of active components means that the access network consists of one bidirectional light source and a number of passive splitters that divide the data stream into the individual links to each customer. At the central office, the termination point is in the PON’s Optical Line Terminal (OLT) equipment. Between the OLT and the customer’s ONT/ONUs, one finds the PON; the PON is comprised of fiber links and passive splitters and couplers.
APON, BPON, GPON, EPON, and GE-PON. These represent various flavors of PON technology. Asynchronous Transfer Mode Passive Optical Network (APON) and BPON are the same specification, which is commonly referred to as BPON. BPON is the oldest PON standard, defined in the mid- 1990s and while there is an installed base of BPON, most of the new market deployment focus is now on Ethernet Passive Optical Network (EPON)/Gigabit Ethernet Passive Optical Network (GE-PON). GE-PON and EPON are different names for the same specification, which is defined by the IEEE 802.3 ah Ethernet in the First Mile (EFM) standard ratified in 2004. This is the current standardized high-volume solution for GPON technologies. GPON was being standardized as the ITU-T G.984 recommendation and is attracting interest in North America and elsewhere, but with no final standard. GPON devices have just been announced, and there is no volume deployment as yet.
Differences between BPON, GPON and GE-PON. One important distinction between the standards is operational speed. BPON is relatively low speed with a 155 Mbps upstream/622 Mbps downstream operation. GEPON/EPON supports 1.0 Gbps symmetrical operation. GPON supports 2.5/1.25 Gbps asymmetrical operation. Another key distinction is the protocol support for transport of data packets between access network equipment. BPON is based on ATM, GE-PON uses native Ethernet and GPON supports ATM, Ethernet, and Wavelength Division Multiplexing (WDM) using a superset multiprotocol layer. BPON suffers from the very aggressive optical timing of ATM and the high complexity of the ATM transport layer. ATM-based FTTH solutions face a number of problems posed by (i) the provisioning process (which requires ATM-based central office equipment); (ii) by the complexity (in timing requirements and protocol
complexity); and (iii) by the cost of components. This cost is exacerbated by the relatively small market for traditional ATM equipment used in the backbone telecommunications network. GPON is still evolving; the final specification of GPON is still being discussed by the ITU-T and Full Service Access Network
(FSAN) bodies. By definition, it requires the complexity of supporting multiple protocols through translation to the native Generic Encapsulation Method (GEM) transport layer that through emulation, provides support for ATM, Ethernet, and WDM protocols. This added complexity and lack of standard low-cost 2.5/1.25 Gbps optical components has delayed industry development of low-cost, highvolume
GPON devices. GE-PON or EFM has been ratified as the IEEE 802.3 ah EFM standard and is already widely deployed in Asia. It uses Ethernet as its native protocol and simplifies timing and lowers costs by using symmetrical 1 Gbps data streams using standard 1 Gbps Ethernet optical components. Similar to other Ethernet equipment found in the extended network, Ethernet-based FTTH equipment is much lower in cost relative to ATM-based equipment, and the streamlined protocol support for an extended Ethernet protocol simplifies development. Table 5.4 compares the technologies.