Corporate LANs employ Cat 5 or Cat 6 twisted copper, Ethernet switches and wiring closets with battery backup and air conditioning. These networks are in place, they're not broke, so nobody is going to rip them out and replace copper with fiber.
OK. We've got that one out of the way.
Fiber will be deployed and is being deployed in greenfield sites. The business case is primarily based on OPEX and space savings; having lots of bandwidth is a bonus. Savings come from the fact that fiber infrastructures are simple and they’re passive. With minimal electronics, there is less power and air conditioning and the use of passive splitters means fewer failures.
Alcatel-Lucent has indicated that most of the interest they've seen comes from customers who are putting up new buildings. The ability to showcase green credentials is seen as a bonus. The other main area of interest has been US Government authorities. In this case the security of fiber networks is an important additional feature.
It's hard to get case references, particularly those from military authorities, but the US Department of Defense Commissary Agency (DECA) has gone public about a fiber LAN. DECA's operations used to spread around the Pentagon and various regional offices, but when they were consolidated in a new building, fiber-optic cabling was deployed. The solution serves 278 employees on four floors.
The Fiber-Copper Combination
The fiber infrastructure terminates (no pun intended) in an ONT (Optical Network Terminal). This is a small box, shown below, which sits near or under the desk. It has four Gigabit Ethernet ports. Regular Cat 5 or Cat 6 cables are used to connect to four PCs or two PCs and two VoIP phones. Note: the Cat 5/6 cable can support data rates up to 1 Gbps.
Figure 1 The ONTs (Optical Network Terminals) are devices that convert fiber-optic/light signals to copper/electric signals. (Photo source Alcatel-Lucent).
Thus, it's fiber to the desk and a short copper connection to the desktop.
The Architecture
GPON (Gigabit Passive Optical Networking) has emerged as the preferred delivery mechanism for IPTV/Triple Play services to consumers and is equally applicable in a business environment, i.e. as a replacement for traditional copper based Ethernet LANs.
Starting from the user's desktop (see figure below), the ONTs convert electrical traffic from the PCs and phones into optical traffic. This optical signal is sent to an Optical Line Terminal (OLT), which aggregates traffic from multiple ONTs and sends it up higher into the network. In between the ONT and OLT there are a series of optical splitters.
Figure 2 This is a much simpler architecture than that of a regular LAN. Splitters are small and passive so they can be deployed in convenient places on individual floors. Ethernet switches are relatively large and are stacked in wiring closets in a daisy-chain configuration. This can result in an inequitable sharing of bandwidth. (Source Alcatel-Lucent).
Optical splitters are essentially glass prisms that divide (split) the incoming signal into a maximum of 64 output connections. However, instead of running 64 long lines out to the ONTs, a typical configuration would cascade the splitters, e.g. use a 1:4 splitter with 1:6 splitters, thereby creating a 32-way split closer to the offices. Thus, a single strand can connect up to 128 devices (32 ONTs x 4 ports on each ONT).
Today's glass fiber is robust and it can be bent. New advances allow it to be wrapped around a pole or stapled like a copper wire. Therefore it’s easy to install in offices, where sharp turns are the norm.
That used to be an issue; so too did the tricky business of splicing and terminating fiber cables. Splitters are now pre-cabled and come in configurations that go from 1:4 to 1:64. In addition, pre-loaded patch panels facilitate installation.
Desktop Benefits
Lots of bandwidth is an obvious benefit, but that statement has to be qualified. An enterprise typically has a 20 to 50 Mbps pipe (uplink) back to the Internet and this is a shared resource, so this uplink can become a bottleneck. Therefore a large enterprise or organization would typically deploy fiber between buildings in a campus in order to enable the internal exchange of very large files.
Although fiber has essentially unlimited bandwidth, over short distances copper can handle data rates up to 1 Gbps, which is more than enough for most enterprises. The important differences that fiber introduces are (a) the ability of GPON technology to allocate bandwidth dynamically and (b) to deliver a high Quality of Service.
Dynamic Bandwidth Allocation
A GPON bandwidth allocation algorithm allows the OLT to control upstream bandwidth allocation to the ONTs. It can statistically allocate the bandwidth for services that need constant upstream bandwidth, e.g. VoIP or native TDM.
Services such as Internet browsing, streaming video, file sharing and file download, are bursty by nature and in this case the OLT allocates the upstream bandwidth dynamically. The algorithm determines the length and location of each ONT upstream burst using "transmission containers" that package the traffic.
Determining the requisite upstream bandwidth takes place at the container level. The dynamic bandwidth allocation engine assigns a time slot to a specific container--not an ONT or individual ports. This allows a subscriber to burst up to the line rate of the Ethernet port, i.e. get up to 1 Gbps on demand. On regular Ethernet LANs, subscribers only get a best effort service.
Low Latency
GPON provides fine granular control of bandwidth and QoS. The latter allows GPON to prioritize different service types and ensure low latency for voice and video traffic. This is done through the use of "P-bits" that are encoded into the packets' frames. Each Ethernet port on the ONT is provided with eight queues or services. A high P-bit ensures that packets encoded with that value are given the highest priority at the corresponding ONT or OLT.
The same feature ensures that voice traffic transits between ONT and OLT without any delays even if the same users are sending large files simultaneously.
Conclusions
GPON is a robust technology that is working well in the consumer sector. For example, in the States, Verizon's FiOS service reaches around 3 million households. The only issue for deployment in enterprises is the fact that regular LANs are also working well.
Enterprises are not going to rip and replace copper infrastructures; deployment will start in greenfield sites. In this case, the benefits are significant: easy installation; lower OPEX; dynamic allocation of bandwidth and prioritized traffic types.
Bob Emmerson is a freelance writer who lives in The Netherlands. Email: [email protected]. Web: www.electric-words.org
