DCCN Lecture 2: Medium Sized City Infrastructure

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DCCN Lecture 2Medium Sized City Infrastructure

• Dr S J Rees

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Basic Points

• Optimisation of Public Benefit
• High technology clusters
• Delivery of quality levels of service to industry
  and the consumer
• Generally, municipal areas benefit first
• Suppliers profit driven
• Concentration of people good business!
• Commonly accepted good bandwidth is 2Mbps
• Broadband access is the goal

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Situational Data

• Businesses tend to be clustered together
• Industrial estate/town centre/shopping centre
• Look to concentrate traffic from these centres
  onto a single communications carrier
• Businesses have different data requirements
• Bulk users will (almost) always make their own
  arrangements

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Data Traffic

• Sporadic, bursty and inconsistent
• Do not need to supply 2Mbps per company at the
  backbone level
• Random nature of traffic lowers average
  throughput
• Congestion (periodically) is not a problem if the
  pricing is aggressive
• If frequent congestion, under-designed system!!!

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Carrier Options

• Infrastructure requires considerable investment
• Two serious options for infrastructure and
  backbone (local loop is another issue, not
  considered here)
• ATM with Optical Fibre
• Cell-based Wireless Transmission
• OR adopt public utility supply if available
  (assumption here is that it is not)

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Basics

• Wireless easy to set up (once sites for masts
  found!)
• Laying cable requires road works expensive
• Wireless lower bandwidth limits application
• Optical fibre BW determined by blades (see later)

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Basic ATM Layout
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ATM Cells (Packets)

• 53 octet fixed packet size
• 48-octet payload (information) field, and a
  5-octet header
• Header consists of routing and other necessary
  information
• Cells relating to different traffic types are
  statistically multiplexed random time interval
  between successive packets (hence asynchronous)
• Small packet (cell) size has benefits

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ATM Cell (User-Network Interface)
8 7 6
5 4 3 2 1
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ATM Topology

• ATM LANs utilise a mesh topology, based on
  interconnected switching exchanges
• Key elements are, therefore, the network
  interface and the ATM switch
• ATM switch
• number of ports
• provide a high bit rate switched communication
  path between those ports
• routing task is a matter of identifying the paths
  between switch ports

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ATM Switches

• Cost of the switch is dependent on its
  complexity,
• the number of ports required is an obvious issue
• minimised as a problem by the use of
  concentrators for groups of workstations (or a
  single building)
• Remote concentrator unit (RCU) provides the
  multiplexing and demultiplexing function for the
  link connecting the workstations to the switch
• Key requirement is that the link has sufficient
  bandwidth (bit rate) to handle the aggregated
  traffic from the attached workstations

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ATM Topology

• Communications path is established prior to use
  (this is a connection orientated system)
• All cells relating to the communication
  thereafter follow the same path
• Virtual connections (VCs) through the networks
  are logical rather than physical circuits
• This is not a circuit switched approach

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ATM Services

• Constant bit rate (CBR)
• demands a fixed data rate from the service
  provider
• Variable bit rate (VBR)
• specified as a sustained rate for normal use,
  with a faster burst rate for occasional peak
  traffic use

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ATM Services

• Available bit rate (ABR) guarantees a minimum
  speed
• subscriber may use such additional capacity as is
  available at any given point in time
• Unspecified bit rate service (UBR)
• no guaranteed performance in terms of data rate
• permits cell (packet) discarding in case of
  congestion

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ATM and Legacy IP

• ATM adaptation layer (AAL) permits conversion to
  and from the ATM structure and protocols
• IP carriage?
• IP connectionless protocol
• Conversion and passage (but what about QoS?)
• IP tunnelling

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ATM Summary

• ATM has the features required by public utility
  suppliers
• High speed, massive flexibility, efficiency as a
  carrier network, partitioning of bandwidth, and
  controllability
• Normal implementation is high-performance ATM
  switches connected to high capacity trunk
• (typically optical fibre as the cable medium)
  such as SONET
• The carrier will provide smaller ATM switches as
  the interfaces to user networks