Chapter 11: Approaches to Networking
Download
Report
Transcript Chapter 11: Approaches to Networking
Chapter 11:
Approaches to Networking
Business Data Communications, 4e
LANs, WANs, and MANs
Ownership
WANs can be either public or private
LANs are usually privately owned
Capacity
LANs are usually higher capacity, to carry greater internal
communications load
Coverage
LANs are typically limited to a single location
WANs interconnect locations
MANs occupy a middle ground
Comparison of Networking Options
Types of WANs
Circuit-switched
Packet-switched
Circuit-Switching
Definition: Communication in which a dedicated
communications path is established between two
devices through one or more intermediate switching
nodes
Dominant in both voice and data communications
today
e.g. PSTN is a circuit-switched network
Relatively inefficient (100% dedication even without
100% utilization)
Circuit-Switching Stages
Circuit establishment
Transfer of information
point-to-point from endpoints to node
internal switching/multiplexing among nodes
Circuit disconnect
Circuit Establishment
Station requests connection from node
Node determines best route, sends message to next
link
Each subsequent node continues the establishment
of a path
Once nodes have established connection, test
message is sent to determine if receiver is ready/able
to accept message
Information Transfer
Point-to-point transfer from source to node
Internal switching and multiplexed transfer
from node to node
Point-to-point transfer from node to receiver
Usually a full-duplex connection throughout
Circuit Disconnect
When transfer is complete, one station
initiates termination
Signals must be propagated to all nodes used
in transit in order to free up resources
Public Switched Telephone Network
(PSTN)
Subscribers
Local loop
Connects subscriber to
local telco exchange
Exchanges
Telco switching centers
Also known as end
office
>19,000 in US
Trunks
Connections between
exchanges
Carry multiple voice
circuits using FDM or
synchronous TDM
Managed by IXCs
(inter-exchange carriers)
Digital Circuit-Switching Node
Circuit Switching Node:
Digital Switch
Provides transparent signal path between any
pair of attached devices
Typically full-duplex
Circuit-Switching Node:
Network Interface
Provides hardware and functions to connect
digital devices to switch
Analog devices can be connected if interface
includes CODEC functions
Typically full-duplex
Circuit-Switching Node:
Control Unit
Establishes on-demand connections
Maintains connection while needed
Breaks down connection on completion
Blocking/Nonblocking Networks
Blocking: network is unable to connect two
stations because all possible paths are already
in use
Nonblocking: permits all possible connection
requests because any two stations can be
connected
Switching Techniques
Space-Division Switching
Developed for analog
environment, but has been
carried over into digital
communication
Requires separate physical
paths for each signal connection
Uses metallic or semiconductor
“gates”
Time-Division Switching
Used in digital transmission
Utilizes multiplexing to place
all signals onto a common
transmission path
Bus must have higher data
rate than individual I/O lines
Routing in Circuit-Switched
Networks
Requires balancing efficiency and resiliency
Traditional circuit-switched model is
hierarchical, sometimes supplemented with
peer-to-peer trunks
Newer circuit-switched networks are
dynamically routed: all nodes are peer-topeer, making routing more complex
Alternate Routing
Possible routes between two end offices are
predefined
Originating switch selects the best route for
each call
Routing paths can be fixed (1 route) or
dynamic (multiple routes, selected based on
current and historical traffic)
Control Signaling
Manage the establishment, maintenance, and
termination of signal paths
Includes signaling from subscriber to network, and
signals within network
In-channel signaling uses the same channel for
control signals and calls
Common-channel signaling uses independent
channels for controls (SS7)
ISDN
1st generation: narrowband ISDN
Basic Rate Interface (BRI)
two 64Kbps bearer channels + 16Kbps data channel
(2B+D) = 144 Kbps
circuit-switched
2nd generation: broadband ISDN (B-ISDN)
Primary Rate Interface (PRI)
twenty-three 64Kbps bearer channels + 64 data channel
(23B+D) = 1.536 Mbps
packet-switched network
development effort led to ATM/cell relay
Past Criticism of ISDN
“Innovations Subscribers Don’t Need” , “It Still
Doesn’t Network” , “It Still Does Nothing”
Why so much criticism?
overhyping of services before delivery
high price of equipment
delay in implementing infrastructure
incompatibility between providers' equipment.
Didn’t live up to early promises
ISDN Principles
Support of voice and nonvoice using limited set of
standard facilities
Support for switched and nonswitched applications
Reliance on 64kbps connections
Intelligence in the networks
Layered protocol architecture (can be mapped onto
OSI model)
Variety of configurations
ISDN User Interface
“Pipe” to user’s premises has fixed capacity
Standard physical interface can be used for
voice, data, etc
Use of the pipe can be a variable mix of voice
and data, up to the capacity
User can be charged based on use rather than
time
ISDN Network Architecture
Physical path from user to office
subscriber loop, aka local loop
full-duplex
primarily twisted pair, but fiber use growing
Central office connecting subscriber loops
B channels: 64kbps
D channels: 16 or 64kbps
H channels: 384, 1536, or 1920 kbps
ISDN B Channel
Basic user channel (aka “bearer channel”)
Can carry digital voice, data, or mixture
Mixed data must have same destination
Four kinds of connections possible
Circuit-switched
Packet-switched
Frame mode
Semipermanent
ISDN D Channel
Carries signaling information using commonchannel signaling
call management
billing data
Allows B channels to be used more
efficiently
Can be used for packet switching
ISDN H Channel
Only available over primary interface
High speed rates
Used in ATM
ISDN Basic Access
Basic Rate Interface (BRI)
Two full-duplex 64kbps B channels
One full-duplex 16kbps D channel
Framing, synchronization, and overhead bring total
data rate to 192kbps
Can be supported by existing twisted pair local loops
2B+D most common, but 1B+D available
ISDN Primary Access
Primary Rate Interface (PRI)
Used when greater capacity required
No international agreement on rates
US, Canada, Japan: 1.544mbps (= to T1)
Europe: 2.048mbps
Typically 23 64kbps B + 1 64kbps D
Fractional use of nB+D possible
Can be used to support H channels
Packet-Switching Networks
Includes X.25, ISDN, ATM and frame-relay
technologies
Data is broken into packets, each of which can be
routed separately
Advantages: better line efficiency, signals can always
be routed, prioritization option
Disadvantages: transmission delay in nodes, variable
delays can cause jitter, extra overhead for packet
addresses
Packet-Switching Techniques
Datagram
each packet treated independently and referred to as a
datagram
packets may take different routes, arrive out of sequence
Virtual Circuit
preplanned route established for all packets
similar to circuit switching, but the circuit is not dedicated
Packet-Switched Routing
Adaptive routing changes based on network
conditions
Factors influencing routing are failure and
congestion
Nodes must exchange information on network status
Tradeoff between quality and amount of overhead
Packet-Switched Congestion Control
When line utilization is >80%, queue length
grows too quickly
Congestion control limits queue length to
avoid througput problems
Status information exchanged among nodes
Control signals regulate data flow using
interface protocols (usually X.25)
X.25 Interface Standard
ITU-T standard for interface between host and
packet-switched network
Physical level handles physical connection between
host and link to the node
Technically X.21, but other standards can be substituted,
including RS-232
Link level provides for reliable data transfer
Uses LAPB, which is a subset of HDLC
Packet level provides virtual circuits between
subscribers
Virtual-Circuit Service
External virtual circuit: logical connection between
two stations on the network
Internal virtual circuit: specific preplanned route
through the network
X.25 usually has a 1:1 relationship between external
and internal circuits
In some cases, X.25 can be implemented as a
packet-switched network
WANs for Voice
Requires very small and nonvariable delays for
natural conversation--difficult to provide this with
packet-switching
As a result, the preferred method for voice
transmission is circuit-switching
Most businesses use public telephone networks, but
a few organizations have implemented private voice
networks
WANs for Data
Public packet-switched networks (X.25)
Private packet-switched networks
Leased lines between sites (non-switched)
Public circuit-switched networks
Private circuit-switched networks (interconnected
digital PBXs)
ISDN (integrated X.25 and traditional circuitswitching)
WAN Considerations
Nature of traffic
stream generally works best with dedicated circuits
bursty better suited to packet-switching
Strategic and growth control--limited with public
networks
Reliability--greater with packet-switching
Security--greater with private networks