Regarding the physical layer, fibre optics dominates the core and metro networks. 99% of core networks are already optical. The remaining 1% is satellite and point-to-point microwave used in well defined specific situations, usually in geographically remote areas, which are sparsely populated and have very rough terrain [6.1].
In the next 15 years the number of optical channels is expected to increase from the presently common 40-80 channels to 200 channels and the bitrate per optical channels is expected to increase from the presently common 2.5-10 Gbits/s to 40-160 Gbit/s.
In parallel with the above outlined development of pure "volume" increase, the optical layer will become smarter, and the functionality implemented in the optical layer will also increase.
For example, in many instances protection is already realised in the optical layer.
The protocol stack will continue to converge (eg. from IP-over-ATM-over-SDH-over-WDM to IP-over-WDM). This will bring increased efficiency through reduced functionality
duplication/redundancy.
Optical Transport Networking (OTN) represents a natural next step in the evolution of transport networking. For evolutionary reasons, OTNs will follow many of the same high-level architectures as followed by SONET/SDH, ie. optical networks will remain connection-oriented, multiplexed networks. The major differences will derive from the form of
multiplexing technology used: TDM for SONET/SDH vs. wavelength division for OTN. To satisfy the short-term need for capacity gain, the large-scale deployment of WDM point-to-point line systems will continue. As the number of wavelengths grows, and as the distance between terminals grows, there will be an increasing need to add or drop wavelengths at intermediate sites. Hence, flexible, reconfigurable Optical Add-Drop Multiplexers (OADMs), will become an integral part of WDM networks. As more wavelengths become deployed in carrier networks, there will be an increasing demand to manage capacity. In much the same way that digital cross-connects emerged to manage capacity into the electrical layer, Optical cross-connects (OXCs) will emerge to manage capacity at the optical layer.
Figure 6.1.1 depicts an OTN architecture covering the core, metro, and high-capacity access domains. Initially the need for optical-layer bandwidth management was most acute in the core environment, but increasingly the access network at the client or server is becoming the bottleneck for data transfer. The logical mesh-based connectivity found in the core will be supported by way of physical topologies, including OADM-based shared protection-rings, and OXC-based mesh restoration architectures. As bandwidth requirements grow for the metro and access environments, OADMs will be used there too.
It is expected that the core and metro network will evolve to consist only of IP- and WDM- technologies. The architecture of the next generation network will take advantage of the provision of an integrated IP network layer directly on top of a WDM transport layer. The encapsulation of IP over WDM can be accomplished in different ways with simplified network stacks deploying protocols such as Packet over SONET/SDH, Gigabit Ethernet or Simple Data Link.
The basic guideline for the integrated IP/WDM architecture is that WDM is considered as a backbone technology and IP is interconnected to the WDM equipment at the edges of the Core network. Such a network is mainly considered by ISPs and in particular, Competitive Operators, deploying optical infrastructure, leased or owned, willing to provide IP services on top of it using IP Points of Presence (PoPs).
The optical infrastructure will gradually evolve from ATM/SDH. Different topologies of WDM equipment may be deployed in the metropolitan and backbone areas. Incumbent operators could also deploy such a network, where in that case they integrate their existing ATM and SDH infrastructure with the DWDM equipment by using the WDM backbone or core to carry the ATM and SDH traffic.
Figure 6.1.1 Optical Transport Network Architecture
Three main areas are considered in this integrated IP over WDM network architecture:
- Backbone area, consisting of core level IP PoPs, which are interconnected via the WDM backbone network. WDM backbone network topologies heavily depend on the distances of the IP PoPs. For long distances with significant power losses (partial) mesh networks or concatenated rings of point-to-point WDM systems are most common, while for smaller distances similar topologies to the Metro area (eg. rings) are applicable.
- Metro area, consisting of an optical WDM metro core with ring topologies dominating, and metro access area, where the IP PoPs are located. IP PoPs can be of 2 categories:
o edge level ones are the gateways to the Customer Premises IP equipment o core or transit ones are used to groom traffic and forward it to the IP backbone - Access area, where main Business/Enterprise customers or smaller Residential/Small
Office/ Home Office IP customers are interconnected to the ISP acquiring Internet access.
Figure 6.1.2 depicts a future ISP’s metropolitan network consisting of a WDM optical Metro core and IP Metro access. The IP section is composed of a number of IP PoPs, where
customers can access the IP network services and traffic is groomed and forwarded to other PoPs or networks through the backbone. Access is facilitated to customers through the interconnection of the ISP’s Provider Edge (PE) IP routers with the Customer Edge (CE) IP routers. Existing ATM and SDH equipment is shown for completeness. Provider equipment can be collocated or not with the customer equipment, depending upon the distance between customer and provider premises and on the amount of traffic generated by the customer, and the tele-housing policies.
Figure 6.1.2. Metropolitan Area IP over WDM Example
The optical WDM metro core is usually composed of a ring of re-configurable OADMs, while additional point-to-point WDM links with Terminal Multiplexers can be considered for large customers. OADMs offer management interfaces so that they can be remotely re-configured to add and drop wavelengths (optical channels) to the ring through the tributary cards and multiplex them in the form of optical line signal in the corresponding line cards of the ring in each direction.
In the case where there are two WDM metro core rings, then an optical cross-connect is needed, to route wavelengths from one ring to the other supporting all-optical networking.
Such cross-connects are the most expensive pieces of optical networking equipment, capable of performing additional tasks, such as wavelength switching and conversion for hundreds of ports in an all-optical form without O-E conversion.
The metropolitan network should extend the transparency and the scalability of the LAN through to the optical core network. The IP Metro access is composed of a set of PE routers interconnected via optical interfaces with OADMs. At the access side of the metropolitan network, Fast Ethernet is becoming commonplace.
However, a more-compatible methodology would be the use of optical Ethernet (40-Gigabit speeds (SONET OC–768) have already been demonstrated). Network operators may limit their customers to a few Mbit/s, but the links are gigabit-capable; and someday the fees for gigabit-scale Ethernet services will be affordable. In the meantime, the protocols and
techniques for bandwidth segregation over shared links exist, work well, and are used in thousands of sites. It is a simple step to run parallel optical Ethernet trunks, each on a separate wavelength, all multiplexed over a single fibre pair using DWDM technology. In this way, a point-to-point Ethernet link could have scores of 10 Gbit/s channels, with an aggregate Ethernet bandwidth of perhaps 400 Gbit/s. Of course, this kind of network requires very large Ethernet switches at the ends of the fibres.
The limits on optical Ethernet bandwidth may be only the limit of fibre optic bandwidth (perhaps 25 Tbit/s per second for the available spectrum on today’s fibre) which is still well beyond the capabilities of today’s lasers and electronics. However, extrapolating from recent trends brings us to that level in only 5 or 10 years..
In the case that the router provides interfaces working in 15xx nm for transmission and reception, there is no need for a transponder in the OADM. The usual case, however, is that the routers’ optical interfaces work in 1310 nm and there is a need to adapt this wavelength to the 15xx, which is done by the corresponding two-way transponder. The transponder converts the optical signal of 1310 nm to electrical and back to optical.
The Wide Area Network is usually composed of a partial mesh-type optical WDM network.
Transmission rates of more than 10 Gbit/s per wavelength are providing access to terabits of bandwidth between metropolitan areas. The power budget is generally sufficient for distances up to 1000km without regeneration, reshaping and retiming. Optical Amplification is deployed either to boost the aggregate multiplexed optical line signal (eg. with an Erbium Doped Fibre Amplifier) or to separately regenerate each optical channel at the corresponding tributary.
6.1.2 Access Network Technology