Lecture Notes for Telecommunications

17 November 2004 - Metropolitan Area Networks

Metropolitan-Area Networks

• A metropolitan-area network (or MAN) provides high speed (> 1 mbit/sec) switched connectivity across metropolitan areas (around 75 miles or less).

• It is designed to carray a diversity of data (voice, video, and data).

• Generally, a MAN is not a big LAN, although it can be used as one (a distributed LAN).

MAN Structure

• The MAN structure is determined by the physical layers and data-link layers.

  • The physical layer is mostly optical.

  • The data-link layer defines the MAC.

MAN Motivations

• Provide a high-speed interconnect for high-speed LANs.

• Introduce a voice and video friendly standard to the IEEE 802 LAN standard family.

• Provide an interconnect between LANs and WANs.

802.6 MANs

• The IEEE 802.6 standards group is working on MAN standards.

• A MAN is characterized by

  • two-way exchange of digita bit streams,

  • nodes sharing the communications media,

  • 50 km (75 m) maximum inter-node distances, and

  • a minimum 1 kbit/sec transmission rate, with maximums given by the underlying transport media.

MAN MACs

• The media access control (MAC) for a MAN must

  • Be applicable over a shared medium.

  • Exploit fast transport media.

  • Be aware of the distances between nodes.

  • Provide for bandwidth and delay guarantees.

Distance Effects

• Networks of long distances have long propagation times.

• Network utilization varies with propagation time as

  Utilzation = 1/(1 + a)

  a = (DataRate/PacketLength)*NetworkPropagation

  • Data rate is fixed by technology.

  • Propagation is fixed by network length (the speed of light is constant).

  • Packet length is fixed by the MAC.

Optical MANs

• Optical MANs are based on ring topologies.

  • Rings can be either token-based or slotted.

Token-Based Rings

• Adjacent stations are connected bidirectionally to for a pair of oppositely circulating rings.

• A special frame, the token, circulates from station to station around the ring.

• A station wishing to transmit must first receive the token, and then transmit the data.

  • The sender reinjects the token into the ring when the data has been sent.

• Receivers watch the frames go by and extract the ones addressed to them.

  • The other frames are resent on the ring.

• The fiber distributed data interface (FDDI) is a well-known token ring network.

Circuit-Switched Token Rings

• Circuits (a.k.a. voice traffic) has guarantees that are hard to keep in a token ring.

  • Token holding effects the distance part of utilization.

• The trick is to provide slotted frames and allocate the slots by reservation, not by token holding.

  • Enough slotted frames are circulated to ensure enough storage capacity for 125 usec of data with no delay.

Slotted Rings

• Make every frame on the ring be slotted.

• Non-isochronous frames support delay insensitive data streams.

  • The frame header determines frame type.

  • A non-isochronous frame is either empty, used, or full.

  • A sender waits for an empty frame, sets it to full, fills it, and sends it.

  • The receiver reads the full frame, sets it to used, and retransmits it.

  • The sender receives the used frame, sets it to empty, and retransmits it.

• An isochronous frame supports delay sensitive data streams.

  • The sender negotiates with a controller or the other stations to reserve some slots in the isochronous frame.

  • At each pass of an isochronous frame, the sender writes whatever data available into its reserved slots.

  • When the data stream ends, the associated slots are released.

• • Monitor stations are responsible for maintaining ring operation.

    • Recognizing and regenerating lost tokens.

    • Formatting and sending slotted frames in sufficient quantity to support isochronous traffic.

    • Recognizing and emptying old used frames.

  • Potentially any station can serve as a ring monitor station.

    • Actual monitors can be selected either statically or dynamically.

  MANs and BISDN

  • ATM is the link between MANs and BISDN.

  • AT&T in the mid 80s developed a series of proposals linking the two.

  • Once the ATM standards were settled in the late 80s, the MAN standards could be aligned to insure compatibility between ATM networks and MANs.

  • This unification served to link MANs with (ATM-based) WANs.

    • ATM switches can also link several MANs.

  MAN Topology

  • The usual MAN topology is a ring of rings.

  • The central ring is called the core ring; around it are collector rings.

  • The collector rings reach to campuses and businesses and provide hubbed connectivity.

  • The core ring interconnects collector rings, and interconnects to high-speed backbones.

  SONET in the MAN

  • SONET is a voice-oriented protocol, not well suited for data traffic.

    • There's also problems with oeo conversions.

  • The alternative is WDM systems.

This page last modified on 18 November 2004.