Signaling Messages THE JUMPSTART PROJECT
258 OPTICAL BURST SWITCHING
paths between JITPAC controllers to enable the efficient exchange of control messages. To that effect a link-state protocol such as OSPF or IS-IS can be used to establish paths
for control messages. Each JITPAC broadcasts link-state advertisements LSAs to all other JITPACs in its
domain. An LSA is a message that contains information about the status and attributes of the interfaces of a JITPAC. LSAs are flooded reliably to all of the JITPACs within the
domain. As a result, each JITPAC obtains complete information regarding the network topology of the JITPACs. Each JITPAC stores this information in its own control routing
database, and runs a routing algorithm to determine how to reach other nodes in its domain. The routing algorithm is typically a variant of Dijkstra’s shortest path algorithm, and the
link costs are based on an administratively selected metric. The results of the routing algorithm are used by a JITPAC controller to populate its control forwarding table which
contains the output interface of the JITPAC for each JITPAC destination address.
Following the initial flooding of LSAs, each JITPAC broadcasts an LSA whenever there is a change in the status of its control interfaces. The link-state protocol can also
dictate a periodic LSA broadcast, even in the absence of changes. This process ensures that each JITPAC not only establishes shortest paths to all other JITPACs over the control
network, but also that these paths are periodically updated to reflect the current state of the network.
Routing architecture for data bursts In Jumpstart, it was assumed that transmission quality for the optical fiber links in an OBS
network is not the same for all fibers. That is, different fibers might have different linear and non-linear impairments see Section 8.2.2. To improve the quality of the transmitted
signals, service circuits can be strategically located at some of the OBS nodes. These service circuits can provide gain compensation, chromatic compensation, and polarization
mode dispersion. The required QoS for the transmitted signal, referred to as the optical QoS OQoS
, is indicated in the SETUP message. Determining a path involves both the selection of a set of physical links between the ingress and egress OBS nodes which
provide the requested OQoS, and the assignment of a wavelength, or a set of wavelengths if we assume that OBS nodes are capable of wavelength conversion.
Jumpstart employs a centralized architecture for computing paths for data bursts within a network domain. The forwarding component of the architecture uses a burst forwarding
table stored locally at each OBS node. The burst forwarding table contains information not only about the output interface for a burst, but possibly also about the output wavelength,
the OQoS degradation expected on this output interface, the offset for the burst needed at the ingress node, as well as other information necessary to forward a burst. The
availability of this information ensures that each JITPAC controller can make a forwarding decision locally using the information provided in the SETUP message.
The path computation is the responsibility of the routing data node RDN, a server attached to one of the OBS nodes. It is responsible for collecting routing information
regarding the data plane, computing the burst forwarding tables for each JITPAC con- troller, and downloading the tables to the JITPAC controllers.
Each JITPAC controller monitors the status of its outgoing optical data interfaces, and summarizes this information into an optical LSA OLSA. The link information in an
OLSA consists of all link attributes that are necessary for computing paths which provide
PROBLEMS 259
OQoS guarantees for the optical signal carrying the data bursts, including the status of the interface, the availability of optical resources such as wavelengths and converters, and
optical layer impairments that are relevant to routing. Each JITPAC controller transmits its OLSA to the RDN via a reliable connection over the control plane.
Once the RDN has collected the OLSA from each JITPAC controller in the domain, it uses an algorithm to compute data paths between pairs of OXCs in its domain. Sub-
sequently, the burst forwarding table for each JITPAC controller is constructed and downloaded to the controller via a reliable connection over the control plane. Along
with a route and a wavelength, the RDN also provides an estimate of the offset to be used for a burst transmission. An ingress OBS node returns the offset value stored in its local
forwarding table in the SETUP ACK to an end device in response to a SETUP message.
In both the control and data plane routing architectures, the forwarding of a burst or a control message can be done by using either the next hop information as in the IP
network or the labels as in the MPLS scheme.
PROBLEMS
1. Note that optical packet switching and OBS are conceptually similar. Their difference lies in the implementation Identify which conceptual features are common to these two schemes. Then
describe their main implementation differences. 2. An OBS end device can have a single transmitter to transmit bursts to its ingress OBS node.
However, it should be equipped with W receivers in order to be able to receive W bursts simultaneously. Assume that each fiber can carry W wavelengths. Give an intuitive argument
to justify this. 3. Consider the diagram given in Figure 10.6. Assume that t
proc
= 1 msec, t
conf
= 1 msec, and a
1-hop propagation delay of 5 msec. The control packet is sent by A at time 0. Calculate the offset and the time at which the control packet will arrive at each OXC, the time at which it
will depart from each OXC, and the time at which the burst will arrive at each OXC. 4. Explain why the timed setup with timed release is the most efficient scheme for the reservation
and release of resources in an OXC. 5. Show through a numerical example that the offset gets smaller as the control packet moves away
from the transmitting end device. 6. A number of OBS end devices are connected to a single OBS node in a star configuration. That
is, there is a fiber from each end device to the OBS node, and from the OBS node there is a fiber to each end device. Assume that each fiber carries a single wavelength and that the OBS node
has full conversion. Describe a simple scheduling algorithm that guarantees zero burst loss. 7. In Jumpstart, why are there two forwarding tables in a JITPAC?
8. In Jumpstart, the forwarding of a burst or a control message can be done by using either the next hop information as in the IP network or labels as in the MPLS scheme. Explain how
the scheme using MPLS labels can be implemented.
11
Access Networks
An access network is a packet-switching network that provides high-speed Internet con- nectivity to homes. It is anticipated that access networks will also provide additional
services, such as voice over IP or ATM, and video on demand. Access networks have different features and requirements than LANs, MANs, and WANs. Currently, there are
two different access networks in place; one is provided over the telephone line and the other over the TV cable. New access networks, such as the ATM passive optical net-
work APON
, and Ethernet-based and wireless-based access networks are also beginning to emerge.
Telephone operators provide high-speed access to the Internet over the telephone line in addition to basic telephone services. Video on demand and voice over IP or ATM will
also be provided in the future. A family of modems known as x-type digital subscriber line xDSL
has been developed to provide high-speed access to the Internet over the telephone line. Of the xDSL modems, the asymmetric DSL ADSL is the most popular one.
Cable operators provide access to the Internet over their cable network in addition to the distribution of TV channels. In addition, voice over IP and video on demand services
over the cable have been introduced recently. The cable-based access network uses the data-over-cable service interface specification DOCSIS
. APON is a cost-effective alternative to the telephone-based and cable-based access
networks. An APON uses an optical distribution network, which consists of optical fibers and passive splitters. It can be used to provide high-speed Internet connection, voice over
IP, voice over ATM, and video on demand services. In this chapter, we describe the ADSL-based access network, the cable-based access
network, and the APON. The ADSL-based access network and the APON have been designed to support ATM and consequently they are connection-oriented networks. The
cable-based access network supports the IP network. Although the cable-based access net- work is not connection-oriented, it has been included in this chapter due to its importance
in the access network market.