When data transmission began, not many choices were available to the user. Direct circuits from the LEC were the best bet, but they provided no more bandwidth than that of an analog phone line. This low bandwidth, mixed with negligible security levels, caused rapid growth in transmission technology. New alternatives include packet switching, frame relay, ATM, Ethernet, VPNs, and more.
IXCs and LECs offer a wide variety of alternatives. Some alternatives are identical in both wide area and metropolitan networks.
Point-to-Point Circuits
Point-to-point circuits are devoted to the exclusive use of the customer.
·
Dedicated
or private lines
·
Premise-to-premise
or multidrop
Public switched telephone network is used for a great deal of data communications. There are many advantages and disadvantages:
Advantages
·
Low
cost
·
Convenience
·
Good
back-up method
Disadvantages
·
Call
set-up time
·
Limited
bandwidth (better with ISDN)
·
Low
security
·
Offered
by IXC and LEC
·
Data,
dial-up video, group 4 facsimile
·
Once
available, ISDN will take over
·
Good
where usage is limited
In a multidrop circuit, the terminals share a common transmission medium. Advantages and disadvantages are:
Advantages
·
Reduction
in circuit costs (terminals often idle, would waste connection if it weren’t
shared. e.g., Bank terminals)
Disadvantages
·
Amount
of non-information bits in system
·
Low
security, must encrypt
In a premise-to-premise circuit, the full bandwidth of the circuit is dedicated to the connection between terminal and host. Advantages and disadvantages are:
Advantages
·
Large
amount of traffic (uses multiplexer to divide bandwidth)
·
High
security
Disadvantages
·
Large
cost
Packet switching is available
over both private and public value-added networks. Devices are connected to
the network through a packet assembler-disassembler. Large companies may have
their own, smaller companies can use public packet network (SprintNet). Advantages
are:
·
Economic
·
Robustness
(“immune to overload”)
·
Not
fixed bandwidth
·
Good
for short, bursty traffic and low volume to distant areas
Frame relay has become an enormously popular network. It is usually the first choice for multiple-location data networks. Advantages and disadvantages follow:
Advantages
·
T1
bandwidth
·
Simplicity
·
Easy
expansion
·
Non-tariffed,
price negotiable
Disadvantages
·
No
error correction
·
No
delivery guarantee
·
Gigabit
Ethernet and IP protocol slowed the growth of ATM
·
Good
QoS
Classes of Traffic
·
Class A constant-bit rate (voice, video – where timing is critical)
·
Class B time-critical, variable-bit-rate (video)
·
Class C not time-critical, variable-bit-rate (data)
·
Class D non time critical, variable-rate connectionless
AAL (ATM Adaptation Layer)
·
CBR-
constant bit rate
·
VBR-
variable bit rate
·
ABR-
available bit rate
·
UBR-
unspecified bit rate
· From IXP or ISP, various providers
Classifications
·
Over
a private network or over the Internet
·
Site-to-site
or remote access
Applications
·
Limited
use for voice
Tunnel
·
Encryption,
authentication, and other security issues
Platforms
·
Hardware
based in proprietary platform
·
Software
based in a server
·
Or
both
Security
·
Authentication
·
Blocking,
filtering
·
Encryption
·
Firewall
This section discusses the carriers’
service offerings. Some criteria apply regardless of the type of network.
·
Disaster
recovery
·
Customer
support and problem resolution
·
Network
monitoring and problem resolution
·
Performance
reporting
·
Provisioning
·
Referencing
While digital circuits are replacing
analog circuits, many data networks still employ analog private lines furnished
by the LECs and IXCs.
·
Analog
private lines
·
Digital
private lines (T1 from LEC, digital radio or fiber from IXC)
·
T1
service (twisted pair, fiber, microwave radio)
·
T3
service (28 T1s, cost effective of you have 8-10 T1s)
Quality-of-service considerations
are identified as:
·
Point-to-point
circuit quality
·
Amplitude
distortion
·
Bit
error rate (BER)
·
Block
error rate
·
Envelope
delay
·
Error-free
seconds
·
Improving
Circuit Performance
·
Obtaining
digital circuits
·
Line
conditioning
·
Disaster
protection (usually solid and stable)
·
Congestion
control (FECN, BECN = Forward/Backward Explicit Congestion Notification)
·
Management
information
·
Span
of coverage
·
Local
Management Interface (LMI)
·
Access
alternatives (56kb/s or T1)
·
Switched
Virtual Circuit (SVC)
·
CoS
·
Easy
to set-up and administer
·
Site-to-site
or remote access