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Preston University
BS Proram
Distance Learning Assignment
Optical Networks
Q1.
a. Explain the wireless optical system.
This system is multichannel optical wireless system. This system mainly requires a line-of-sight topology. It essentially uses WDM with custom-built telescopes, standard optical transmitters and receivers. This system works as the light signals are sent from a transmitters and receiving telescope and are focused onto the core of an optical fibre using coupling optics within the second telescope. This system is useful as it offers a cost-effective solution to line-of- sight channels in conference and conventional centres as well as it is useful where the deployment of fibre cable is not feasible, for example, across restricted-across terrain, or bodies of water. This system doesn’t require governmental licensing or frequency allocation schemes in most countries.
b. Explain any three key terms for optical cross
connect.
- Optical / Electrical cross-connect (OXC): this means that the optical signals are received, converted to electrical signals and the routing/switching and/or ADM decisions are made. This is noted as O/E/O also and called an opaque operatin.
- Photonic cross-connect (PXC): this means that the functions of the OXC are performed and also performs all operations on optical signals. These operations are called transparent operations and are noted as O/O/O.
- Cross-connect (XC): this term is used when it is not necessary to distinguish between the PXC or the OXC.
Q2.
a. Discuss the layered protocol model in transport
network.
There are different transport models that use the layered protocol system and those are the OSI, 1G,2G,3G transport networks, MPLS and the internet. To understand the layered protocol model we will discuss the OSI model. The first observation is that transport networks operate at the physical layer (layer1) of the model which is defined as one that had restricted, physical functions such as signal generation/reception, clocking and so on. While this statement still holds, the physical layers of the 2G and 3G transport networks are quite powerful and define many other operations such as extensive diagnostics, backup/recovery and bandwidth provisioning. The data link layer (layer2) is the second layer of the OSI model and corresponds with the MPLS layer of the internet. This is followed by the network layer (layer3) usually uses the IP systems for this to work. Then comes the transport layer (layer4) which uses TCP/UDP in order for it to work. Layer5 & layer6 (transport and session) usually do not have corresponding layers on the internet model and re the stairway to the final layer (application) that uses the internet in from of web, Emails etc.
b. Explain the digital signal hierarchies.
The first generation digital transport systems were first developed in Europe, Japan and North America during the past 30years. All of these hierarchies are based on the clocking rate of 125micro-sec, and the basic 64 bit/s signal leading to well basic architectural inter work.
On the DS1 rate of 1.554 Mbit/s are based the multiplexing hierarchies of Japan and North America while Europe uses an E 2.048 Mbit/s multiplexing scheme. The DS1/T1 is basically based on multiplexing 24 users onto one physical TDM circuit. E1 standards were published shortly after the inception of T1.
meanwhile. The amount of energy the electron releases determines the optical wavelength.
- (^) Stimulated emission takes place when the electron enters and stays in high-energy state then changes it spontaneously. During this state it can be stimulated by a photon to emit its energy in the form of other photon The detector:
- (^) When a photon arrives from a fibre, it is absorbed by the material resulting in the creation of an electron/hole pair. Both migrate towards one of the electrical contacts with electron attracted to the positive contact and hole to the negative, thus creating electrical energy.
Q4.
a. Discuss different control planes.
A control plane is a set of software and/or hardware in a node that is used to control several vital operations of the network. An important example of the control plane is the SS7 protocol stack. It’s important to understand the control planes and is also called the signaling plane and its job is to control the data plane of the telephone network. Other examples are the routing protocols (PSPF, IS-IS, BGP) which are used in data networks and they enable the IP to forward traffic correctly. Optical networks perform important tasks that include:
- Exchanging status messages.
- Providing timing messages to keep nodes’ clock in sync with each other.
- Using messages to download information on which wavelengths will be used between two nodes.
- Exchanging hello messages to make certain nodes up and running.
- Setting up and tearing down data plane connections.
- Building forwarding tables to allow the data plane to relay traffic from its input to output link.
Second generation digital transport networks have used network management protocols in data plane. This approach has certain problems including:
- It leads to slow convergence following a failure.
- It complicates the task of inter working equipment.
- It preclude the use of distributed dynamic routing control capabilities.
Third generation digital transport networks can operate in any of the following fashions:
- The control plane message can be exchanged on a separate physical fibre link from those of the user traffic.
- It can also be sent on the same fibre link.
- It can be sent on a separate wavelength on the same fibre.
- It can be sent and received on separate nodes from those that carry the data traffic.
b. Explain the SONET frame structure.
It is The official international standard for second generation digital carrier networks. It’s basic rate is 155.52 Mbit/s. it then uses a N x 155.52 multiplexing scheme. A smaller rate of 51.840 Mbit/s is also available for it. It’s basic transmission unit is the envelope (frame). It is comprised of 8 bit bytes (octets) that are transmitted serially on the optic fibre. For ease of documentation, the payload is depicted as a two-dimensional map which is comprised of n rows and m columns. Each entry in this map represents the individual octets of a synchronous payload (envelope). The octets are transmitted in a sequential order until the last octet is transmitted. The envelopes are sent without interruption and the payload is inserted into the envelope under stringent timing rules. By simply placing the user payload into the payload envelope, the pointer can be set to indicate where the payload is. The payload is then encapsulated into the SONET and sent on its way using the ongoing network clocks. This approach allows the network to operate synchronously while accepting asynchronous traffic.
OUT-OF-BAND signaling is an approach in which the 3G transport networks use a separate channel for for signaling information. It is more efficient and robust and has further two types:
- Physical out-of-band signaling , a separate physical channel is used for signaling.
- Physical in-band/logical out-of-band signaling, signaling and user traffic use the same physical channel, but part of the channel capacity is reserved only for signaling traffic and the remainder is used for user traffic such as IP payloads.
Q6. Discuss the followings
- Erbium doped fiber: It is a type of fibre that has amplifiers. The EDFA are transparent to a date rate. They provide high gain and experience low noise. The major attraction is that all the optical signal channels can be amplified simultaneously. EDFA are found in amplifiers, optical cross-connects, wavelength add-drop multiplexers and
broadcast networks.
- Add and drop multiplexer: These are devices that are considered the key tools to support the management of the fiber capacity by the selective adding, inserting and removal of the WDM channels at intermediate points in the network. The requirements range from rearrangeable add-drop of 1- channels in a small business to 40 channels in an inter office ring. Each WADM channel should be capable of carrying a different data rate and channel mix.
- Higher dispersion: When a phenomenon of light spreading out within the fiber occurs, this is called ‘modal dispersion’. In single mode fibre, modal dispersion is eliminated but different frequency wavelengths travel at different velocities giving rise to a (chromatic dispersion). A mew type of fibers is called Advanced NZDF , has been developed and it exhibits a maximum dispersion of 10 spec/ nm-km at the far end of the C band. It tends to suppress the non-linearities that are introduced by the tighter channel
spacing. It will allow the dispersion to be high enough to minimize non-linearities and low enough to minimize the need for dispersion compensation.
- Optical channel concatenation: Optical channel concatenation is actually the protection channel sharing topology used for managing parallel links. There is a term used (superchannel) to a concatenation of multiple links and/or wavelengths within the links. It can appear as one interface on a router. The superchannel has multiple subchannels within it and are connecting two nodes at either ends. The job of these two nodes is to manage the individual subchannels. Messages are sent between these two nodes if all subchannels are operating without any problems. Each message is a bit map that reflects the state of each port whereas the bit map can reflect the state of each wavelength that is being received as well as the port interfaces. The four 1s in the message signify that all four subchannels ore operating satisfactorily. When there is a problem, the affected node uses the subchannels that are still operating to report on the problem. The receiving node then alters the bit map in the message to identify the specific problem.
- Meshed topologies: It is a local network topology in which the infrastructure nodes connect directly, dynamically and non-hierarchically to as many other nodes as possible and cooperate with one another to efficiently route data from/to clients. The network nodes are called signaling transfer points (STPs) which are large switches that are responsible principally for keeping the backbone network operational. Each node has a direct optical link with all other nodes. An example of this is the SS7 which also uses shorthand notations to describe the location of the link sets. During normal operations, routing traffic goes through the most direct route and if a failure occurs between certain nodal pair, no routing change occurs. It is possible for this topology to break down to such an extent that traffic is not routable for a particular STP and it provides 100%
There are three types of MPLS nodes:
- Ingress LSR: Receives native-mode user traffic and classifies it into an FEC. It then generates an MPLS header and assigns it an initial label. The IP data gram is encapsulated into the MPLS packet, with the MPLS header attached to the data gram. if it is integrated with a QOS operation, the ingress LSR will condition the traffic in accordance with the DiffServ rules.
- (^) Transit, interior or core LSR: It receives the packet and uses the MPLS header to make forwarding decisions. It wil also perform label swapping. It is not concerned with processing the IP header, only the label header
- Egress LSR: It performs the decapsulation operations.
Q8. Explain the complete framework for IP over
optical networks.
The authors in the industry state that the optical network control plane should stiles IP-based protocols for dynamic provisioning and restoration of light paths within and across optical sub-networks. Two major issues are discussed Regarding the use of revisions to exiting protocols; the first is the adaptation and reuse of IP control plane protocols within the optical network control plane. The second is ransport of IP traffic through an optical network together with the control and coordination issues that arise therein.
There are 2 general models. Both are following:
A. Domin Service Model: The optical network primarily offers high bandwidth connectivity in the form of lightaths. The following 4 services are invoked by standardized signaling across the UNI. I. Lightpath creation: This allows a lightpath with specified attributes to be created between a pair of termination points in the optical network. Such as security. II. Lightpath deletion: This service allows an existing lightpath to be deleted. III. Lightpath modification: This allows certain parameters of the lightpath to be modified.
IV. Lightpath status enquiry: This allows the status of certain parameters of the lightpath to be queried by the router that created the lightpath. Service discovery allows a client to determine the static parameters of the interconnection with the optical network. The signaling protocols requirements are minimal and is required to convey a few messages with certain attributes in a point-to-point manner between the router and the optical network. Theses services do not deal with the type and nature of routing protocols within and across the optical network. The ODS model results in the establishment of a lightpath topology between routers at the edge of optical networks.
B. Unified Service Model: The IP and optical networks are treated asa single integrated network from a control plane view. There is no distinction between the UNI, NNIs and any other router-to-router interface from a routing and signaling point of view. An edge router can creat a lightpath with specified attributes or delete and modify lightpaths as it creates MPLS LSPs. The services obtained maybe invoked in a more seamless manner as compared to the domain services model. It can then establish an LSP across the optical internetworks. ‘Forwarding adjacency’ can be used to specify virtual links across optical inter networks in routing protocols such as OSPF.
Interconnections for IP over Optical Given that IP over optical, the transport of the IP datagrams over an optical network can occur through three kinds of interconnections: A. Peer: The IP/MPLS layers act as a peer of optical transport networks. When there is a single optical network involved with appropriate extensions can be used to distribute topology information over the integrated IP-optical network. B. (^) Overlay: The IP/MPLS routing, topology distribution and signaling protocols are independent of the routing topology distribution and signaling protocols at the optical layer. They are defined for the optical domain. Everything is accomplished through UNI-defined procedures.
