1、H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips974. Optical Networks4.1 Optical Network Elements & Topology DesignFunctionally the three main elements of optical networks are:Optical line termination (OLT)o Used at the ends of a point to point link.o Multiplexe
2、s transmitted wavelengths and demultiplexes received wavelengthso The signals it multiplexes, and the network then carries, come from a client network, the signals it demultiplexes are returned to the client networko A transponder performing optical to electronic to optical (OEO) conversion is typic
3、ally needed OEO ITU-T wavelength chanels Optical interface FIGURE: Optical Line Termination (OLT). On the left this will connect with client network, via either OEO or pure optical (managed) interface.The OLT can be thought of as functionally bidirectional. In practice there would be separate wavele
4、ngth mux and demux, with separate connections to receiver and transmitter parts of the OEO (& the pure optical interface needs two fibres/patch cords).H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips98Optical add drop multiplexer (OADM) Allow individual waveleng
5、th channels to be accessed without needing all the others to be detected and retransmitted other channels pass through Can be designed to be reconfigurable use tunable filters, tunable transmitters for add channels and optical switches Can be regarded as back to back demux/mux, and often built that
6、way, but same effect can sometimes be achieved more cost effectively with single channel add & drop filters ITU-T wavelngth chanels OE or pure optical interface to client layer/network FIGURE: Basic OADM. Like the OLT can be considered functionally bidirectional. However, typically, the two differen
7、t directions will be physically separate. A A D D a b a b Through chanels FIGURE: One direction only of a cheaper way of implementing an OADM function when only a few channels are added/dropped. Note the possibility of co-channel crosstalk if some of a or b passes (leaks) through the drop filters.H6
8、4OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips99Optical crossconnect (OXC)o OXC are very large scale switches have optical input and outputo Some ports connected to WDM equipment - OLTs are needed to connect the OXC to fibre and other ports to client network lay
9、erso Can switch input light on different ports between different output portso Enable reconfigurable optical networks - needed for service provisioning and for protection in event of failureo Some OXC definitions include the OLT and it is possible to buy OLT and OXC separately or togethero There are
10、 a variety of different combinations of OLT, OXC core and OLT/OXC interface1.Optical core, pure optical interface (no transponders)2.Optical core, OLTs have OEO (transponders)3.Optical core with OEO, OLTs have OEO4.Electrical core with OE and EO, OLTs have OEOIn all the elements (OLT, OADM, OXC) opt
11、ical amplification can also be deployed. Additionally optical line amplfiers (OLAs) may be deployed separately, but these do not change the basic topology of the network.The different elements can be connected in a variety of ways, leading to different network topologies.H64OCN, H64OCA, H64OCP Optic
12、al Communications (+)Lecture Notes 2010-11 Dr A J Phillips100The primary topologies are:Point to point (linear) mainly uses OLTs (and OLAs) but can have intermediate OADMsRing mainly uses OADMs (and OLAs)Mesh mainly uses OXCs and OLTs (and OLAs). OADMs can be used in the middle of links. There are a
13、 number of different regular mesh topologies but most real networks are not regular.LightpathsThe purpose of the optical network is to provide lightpaths to the client network. The lightpath optically links the source and destination. It is a wavelength on the ITU-T grid, but may be wavelength conve
14、rted in the course of its journey which could be through many intermediate nodes. It is circuit switched (at the optical level) all the information for the connection travels the same path through the network (i.e. the lightpath). This is in contrast to optical packet switching, which is still under
15、 development.o Packet switching can be performed at higher levels e.g. ATM, IPo The rate of change of connectivity is slow (unlike in a conventional circuit switched voice network!) Typically lightpaths are assumed to be bi-directional (as the connection using them typically is)H64OCN, H64OCA, H64OC
16、P Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips101To include wavelength conversion incurs extra costs but typically improves network utilization and blocking probability. They can do this as they get round the wavelength continuity constraint: i.e. the constraint that information s
17、tays on the same wavelength from source to destination instead allowing a connection to be made if a path exists with a free wavelength on every link from source to destination.Regardless of the use of wavelength converters or not one of the major features of an optical network is that of wavelength
18、 reuse. The same wavelength can be assigned to multiple lightpaths as long as those lightpaths do not share a link.Detailed design of topology is mainly necessary for mesh networks.Design Considerations for information onlyConsidering things like: likely immediate and future demands for bandwidth be
19、tween the different nodes capacity of the fibre links (bit rates & no. of wavelengths) full, limited or no wavelength conversion physical constraints of transmission (maximum amount of fibre, amplifiers, nodes in a lightpath) survivability ultimately leads to design the physical interconnections (ph
20、ysical topology) design a routing & wavelength allocation (RWA) algorithmSo have a topology design problem & an RWA problem.These can be optimized in combination (hard) but it is generally better for them to be approached separately. The topology design problem is typically solved using integer line
21、ar programming (ILP) techniques (maths of constraints & objectives where quantities involved (e.g. number of wavelengths, links in a path, etc.) are integers)H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips102Public Telecoms Network Long Haul (inter-exchange net
22、work) Metro/Regional network Aces network FIGURE: The public telecoms network vastly simplified. (Private networks also use a variety of topologies, often making use of parts of the public network)Note: the precise hierarchical nature of the network depends on the country.Long haul networks:Mesh top
23、ologies typically are used for high capacity long haul connections between nodes that are typically very large exchanges or offices (where much of the functionality at the physical layer is now to be provided by the OXC). H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J
24、Phillips103Regional/Metropolitan networks:Ring topologies (of OADMs, connecting up smaller exchanges/offices) are typically used for regional networks and for metropolitan networks, these typically connecting to the nodes of the long haul network. The distinction between regional and metro is blurre
25、d. In some networks metro rings hang off regional rings. In other networks there is no distinction.Access networks:These connect the exchanges (that were interconnected by the metro network) to the ultimate user (home or business) by a variety of techniques. Not all techniques use fibre, and some us
26、e fibre part way but complete the final step (the last mile, or first mile) using either ADSL, VDSL, wireless, coax, etc. When fibre is used the optical network units/terminations (ONUs/ONTs) may be connected point to point or in a tree/branch configuration (often called a passive optical network (P
27、ON) to the OLT in the nearest exchange. Access rings are also sometimes used. PONs see later.Both access networks and metro/regional networks can improve their survivability (ability to function in the event of loss of node or link) by dual homing, which basically means two connections into the larg
28、er network above it in the hierarchy. H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips1044.2 Clients of the optical layerThe optical layer provides the physical interconnections for a variety of higher level networks. As such it is therefore a server layer and t
29、he higher level networks are its clients. This way of looking at the networks is derived from models like the ISO seven layer hierarchy7 Application layer6 Presentation layer5 Session layer4 Transport layer3 Network layer2 Data link layer1 Physical layerFIGURE: The ISO seven layer hierarchyHere we a
30、re mainly concerned with the physical layer and the 3 layers above it. Four higher layer possibilities are of particular interest: internet protocol (IP) Ethernet asynchronous transfer mode (ATM) synchronous digital hierarchy (SDH) very similar to SONET in the USThe different properties of these lay
31、ers, and their ability to pile up (e.g. IP over ATM over SDH over WDM!) means the model is now a bit simplistic ATM, IP, SDH and Carrier Ethernet are very complicated so here give a brief summary only.H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips105Synchronou
32、s digital hierarchy (SDH) SDH is a time division multiplexing protocol for transmitting many bit streams over the same optical channel.The motivation for SDH came from deficiencies of the pleisochronous digital hierarchy (PDH).The PDH was based on multiplexing of streams of data (based on fundamenta
33、l units of 64 kbit/s voice channels).These channels were at very slightly different clock ratesSo some bit stuffing was occasionally necessary when channels multiplexed by interleaving bits.Multiplexed like this (with bit stuffing) it is very difficult to pick out a particular channel without demult
34、iplexing all the channels expensive when only one channel needed.This leads to multiplexer mountains.SDH improved on this by:o All clocks are perfectly synchronized to a master clock so no bit stuffing needed.o As no bit stuffing simply need to identify the relevant bits in the data stream (by point
35、ers) and can extract/insert a particular channel without disturbing others. This is done using an SDH add drop multiplexer (ADM).o SDH also provided improvements in interoperability, management and protection switching.H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phi
36、llips106Many SDH network structures are similar to analogous optical layer structures (e.g. add drop rings) Some models regard SDH as on layer 2 (data link layer). However SDH is also often treated as the physical layer by higher order layers like ATM, though in fact the optical layer can be a serve
37、r layer for SDH. From this perspective we can also view the physical layer as itself comprised of several layers!SDH is basically the same as SONET (in the US) apart from data rate designators and that the basic rate of SONET is one third of the basic rate of SDHo STM-1 in SDH is 155.52 Mbit/s, equi
38、valent to OC-3 in SONET. STM-4 (OC-12) is 622.08 Mbit/s, STM-16 is approx. 2.5 Gbit/s, STM-64 approx. 10 Gbit/s, STM-256 approx. 40 Gbit/so Hence the choice of bit rates in many optical communication systems (though sometimes slightly higher rates may be used to allow for forward error correction (F
39、EC) overhead)o STM is synchronous transfer moduleH64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips107Asynchronous transfer mode (ATM)ATM is a Layer 2 (data link layer) protocol originally planned as a way of integrating separate voice and data networks. It can al
40、so be viewed as a network layer (layer 3) when SDH below it is regarded as a data link layerATM was at one point seen as likely to be the dominant layer 2/3 solution particularly given the care taken in its design. However the already present IP network has won out mainly because it seems to have be
41、en easier to adapt what is already present than to completely change it!ATM uses packets or cells of length 53 bytes (5 bytes header, 48 bytes payload) the length is a compromise between the desire to minimize overhead in data networks (which would prefer longer packets i.e. same header length for m
42、ore payload) and the desire to minimize delay and delay variation in networks providing constant bit rate services like voice header contains overhead like virtual path and channel information, error check fields, priority bits, etc.H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010
43、-11 Dr A J Phillips108ATM sets up a “virtual circuit” between the two endpoints before the data exchange commences. Despite being packet based ATM does establish connections (unlike the connectionless IP)ATM has a number of adaptation layers which detail how to map user data at the source into cells
44、, that is segmented, and how it is reassembled at the destination.ATM can provide quality of service (QoS) guarantees (e.g. on cell loss, delay and delay variation).For information only: It does this by traffic shaping and by admission control. Traffic shaping ensures that users of an ATM network st
45、ick to a contract (detailing things like peak cell rate, average cell rate, length of burst) Admission control uses the information of traffic shaping to ensure that new connections will only be allowed if they do not lead to the possibility that existing connections will be adversely affected Queui
46、ng policies are also used to guarantee QoSATM has various service classes based on the QoS and traffic parametersH64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2010-11 Dr A J Phillips109Internet Protocol (IP)o IP is a network layer (layer 3) which can use a variety of different data
47、link layers (which perform the framing & multiplexing of data)o IP is a packet based network and is truly connectionless packets being routed via routing tables held at IP routers, which say what the next hop is for a particular ultimate destination (determined from packet header). o Up to date rout
48、ing tables are key, provided by using a distributed routing protocol which makes use of the knowledge each router has of its own immediate links to neighbouring routers. Change in link status is communicated by flooding.o IP does not really offer QoS (unlike ATM) so voice (now being realized by VoIP) was some time coming.o Instead it offers a “best effort” service. Packets go different routes and sometimes arrive out of sequence. Some packets just get lost.o The development of multiprotocol label switching (MPLS) has brought some possibility of QoS guarantees to IP.o Label
