CS4550: Computer Networks II network layer basics 1 circuit switches, control signals

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CS4550Computer Networks IInetwork layer
basics 1circuit switches, control signals

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network layer basics

• circuit switching basics(1)
• switching techniques
• control signaling
• spanning tree shortest path algorithms(2)
• basic routing techniques (3)
• congestion control techniques (3)

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basic switch design
switching matrix
control unit
...
network interface
...
...
...
trunks
digital/analog devices
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basic switch design

• control unit contains program (OS) which receives
  services requests
• signaling - passing of messages between CU, other
  parts of switch, network, customer
• trunks - high capacity media connections to other
  switches
• blocking/nonblocking switches

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telephone calls

• 4 formal phases
• 1. call request
• 2. call confirm
• 3. info transfer
• 4. call clear
• see procedure for tel call in text
• exercise describe the basic procedure of a
  control unit in handling a call

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basic switching techniques

• space division switching data follows distinct
  physical paths through switch
• crossbar switch
• multi-stage switch
• time division switching data routed through
  switch using TDM
• TDM bus switching
• TSI time slot interchange
• TMS (another technique, not covered here)

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crossbar switch
inputs
outputs
at each crosspoint is a gate, enabled in 1 of 2
directions. CU enables/disables the gate settings
of switching elements in X out
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multi-stage switch example

• combine small crossbars in stages for increased
  utilization, redundancy

NInputs k crossbars N outputs
1
1
n
n
N N n n
nk
nk
nk
N N n n
nk
nk
nk
9
9
of switching elements 2kN k(N/n)2
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TDM bus switching
inputs
outputs
each I/O line connected via a gate the bus
receives bits from inputs, transmits to outputs
whenever the gate is opened by CU
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TSI time slot interchange
. . .
. . .
...
...
...
In out
buffer
counter
Mapping table
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control signaling purpose

• messages passed between parts of NW, between
  user NW, to maintain NW, for billing info, to
  create/terminate connections, etc.
• functions
• -communicate with user (dial tone, busy signal)
• -send DA (number dialed) to proper switches
• -send call info (channel seized, released, busy)
• -billing info
• -equipment status (down, busy, up, etc.)

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control signaling

• in channel same channel as user
• in band
• out of band
• common channel separate control channel from
  user
• associated mode
• nonassociated mode

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control signals in channel

• used throughout early telco networks still in
  wide use
• use same communication channel as the customer
• with 4K Hz analog channel, used the unused
  portions of the spectrum
• advantages?
• disadvantages?

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control signals in channel
guard bands
4KHz
3KHz
in band more bandwidth, less time for use out
of band more time, less bandwidth, more
difficult electronics and protocols
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control signals common channel

• use different channels than user
• associated mode - control channels on same
  physical transmission links
• nonassociated mode - control channels on their
  own separate physical link/network
• analogy highway patrol cars versus helicopter
  traffic control