Network topology II May 3, 2018

+ Examples optical networks: point-to-point links, optical-domain multiplexing, optical-domain switching and photonic computing.
Synchronous optic network (SONET) solve problems in TDM. T1 line (1.544 Mbps) is not an exact multiple of 64 kbps and req. high speed freq. justification.
SONET uses pointer processing to justify all low-speed signals with the same local clock and multiplexs with byte interleaving. Trade-off between simple switch design and guard-time inefficiency as well as more overhead bits.
WDM uses parallel implementation to exploit huge optical bandwidth.

+ System characteristics: WDM vs. WDoMA
Phys. vs. logical -- with wavelength tuning, logical ring can be formed.
Fixed tuning vs. dynamics tuning -- Dynamic tuning (DT) adjusts wavelength according to traffic. DT at receiver only req. a signaling channel. DT at transmitter only req access control to avoid sending to the same receiver at the same time. DT at both Rx and Tx may cause blocking.
Single hop vs. multihop -- in single hop, a wavelength channel exists between any two nodes [equation] req. N-1 receivers and one transmitter.
Multihop reduces hardware with lower throughput.
Packet switching vs. circuit switching -- circuit switching req. periodically and deterministically tuning. Packet switching tunes based on packet.

+ Routing config for fixed tuning WDMA (sec 8.2):
Hypercube -- number of nodes [equation] for p Rx/Tx pair; address [equation]; max. number of hops = p & average hops = p N / [2(N-1)].
deBrujn -- [equation] with maximum number of hops H; address [equation] with [equation] assuming value of 0 to p-1; average hops [equation].
Perfect shuttle -- [equation]; address [equation] with col. addr [equation] and row addr [equation]; max hops = 2k-1 & average hops [equation]
Trade off between numbers of Rx/Tx and hops
Nonuniform flow of traffic [equation] need dynamic tuning.

+ Freq. Reuse : Number of wavelengths [equation]. Need freq. reuse to reduce [equation].
- Multiple shared media
- Terminate wavelength channel at its destination and reuse it for a diff signal with ADM
- wavelength converters (WC) (sec 8.2.3) -- 4 wavelengths for traffic within each group and 1 wavelength for intergroup traffic. WCs are used to achieve desirable permutation.

+ Subcarrier multiplexing (SCM) (sec 4.2): apply microwave carriers to transmit multiple channels over fiber. It performs MUX/DEMUX in electrical domain.
Microwave - subcarrier; optical - main carrier.
Used with analog & digital signals (in TDM & WDM).

+ Fiber distributed data interface (FDDI): Using [equation] core / [equation] cladding fiber and operating at 100 Mbps with 500 nodes.
max. access delay [equation] where N is the number of nodes, T= target token rotation time, [equation] is the physical ring delay.
Variable delay makes FDDI unsuitable for const. bit rate applications. FDDI-II addresses this problem.

+ SONET (sec 6.1): Fiber optics network in telephone companies.
Provide a common international rate structure.
Move beyond DS3 (44.7 Mbps) level.
It is equiv to Syn. Digital Hierarchy (SDH). Features -- scalable, high speed (51.84 Mbps to 2.488 Gbps), syn. operation, extensive operation (including maintenance and administration). Any tributary can be inserted, support asyn. interfaces.

+ Architectural layers: Photonic layer -- converting electrical STS-N frames to optical OC-N frames.
Section layer -- including termination points between either 2 repeaters or a repeater and a terminal network element. Functions include framing, scrambling, error monitoring section level overhead.
Line layer -- information from one network to another.
Path layer -- source and destination logical connection.

+ Frame format: Basic unit is syn. transport signal level 1 (STS-1), bit rate of 51.84 Mbps and repeat very 125 [equation], 51.84 Mbps support interconnectivity between T3 (44.73 Mbps) and E-3 (34.36 Mbps), support for sub-STS-1 signals.
VT (Virtual Tributary) is a structure to maintain a consistent payload structure which providing for transport of a variety of lower rate services. The lower speed signals are handled by ADM.
STS-1 has two areas -- transport overhead (TOH) and syn. payload envelope (SPE).
Higher rate STS-N is obtained by syn. multiplexing N STS-1 inputs.
Byte interleaving is a SONET multiplexing process.
OC-N is optical equiv. of STS-N electrical signal. Other than scrambling OC-N is derived by bit-for-bit conversion from STS-N. The standard goes up to STS-235 (13.219 Gbps) and even OC-768.
Advantages -- integrated network for all types of traffic, syn. network (allow single stage MUX/DEMUX), optical mid-span between vendors, high optical bandwidth, builtin management, protection & fault tolerance, supports various services.
Disadvantages: syn. restriction, lack control & bandwidth management.

+ Multiprotocol label switching (MPLS) (sec 6.3.3): An alternative to asyn. transfer mode (ATM) under Internet Protocol (IP) for multiplex Internet services over individual circuits in DWDM. It offers simpler mechanism for packet-oriented traffic.
Extension to existing IP and latest evolution of routing & forwarding.
Enhanced scalability, speed, service provisioning.
Features -- map IP addr to simple fixed length protocol-specific id called ``label'', use a single forwarding paradigm (label swapping), use different technologies and link-layer technologies to realize label swapping, provide flexibility in delivery of new routing services, support quality of service.

Last Modified: April 26, 2018
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