Wide Area Network Flashcards
Characteristics of Value-Added WAN’s
Unlimited distance:
. Interconnection is by means of public carriers such as ISP’
A WAN is high speed and relatively expensive and complex in design
Only the interface and network services are of concern to the user:
. The internals of the “network cloud” are not an issue
Value-added WAN’s add features beyond those of dedicated, point-to-point links
Transparent LAN services (TLS) hide the complexities of the WAN from LAN administrator
Packet/Frame/Cell-Switched WAN Links
Individual data units may be called:
. Packets
. Frames
. Cells
The principal distinction is that packets and frames are of variables Length:
. They usually require software processing
. This limits the data processing rate
. X.25, frame relay and TLS are examples where these are used
In contrast, cells are fixed length:
. A 5-byte header and a 48-byte data content
. They can be processed in hardware
. This results in much higher data rates
. ATM is an example
Switched and Permanent Virtual Circuits
Some packet/frame/cell WAN alternatives may be available in one or each of two forms:
. Switched Virtual circuits – SVC
. Permanent Virtual Circuits – PVC
Switched circuits are like dial-up links
Permanent circuits are always connected:
. Dedicated circuit
. Lease lines
Not all WAN technologies support both:
. Some have a preferred approach in terms of usage
. X.25 VCs are usually SVCs
. Frame-relay VCs are normally PVCs
. ATM VCs may be either PVCs or SVCs
. TLS is more like a “best efforts” services
Objectives and Services for Value-Added WANs
. To provide an appropriate topology
. To Provide a path (route) across the network
. To divide (segment data as required and to reassemble the segment
. To limit the network traffic to that which can be handled effectively (congestion control)
X.25 Interface
X.25 is a WAN interface ITU-T standard:
. connected to a public packet-switching network
. With physical, data link and network layer Form the OSI reference model.
. Procurer to IP addressing
. Last used in the WAN in 2015
. By Financial card companies
. Still utilised by the aviation industry
Frame Relay as an Alternative to X.25
Connection oriented public switched service provided by the telecommunication company
A Layer 2 protocol defined by the ITU and ANSI in 1984
Frame relay is a high-performance alternative to X.25:
. Needed for applications, such as graphics and image transfers
. Especially for LAN-to-LAN communications in which high throughput is required
Frame relay provides higher throughput by means of:
. Larger frame size (1500 Bytes+)
. Higher interface data rates
. Reduced processing requirements
Frame relay is a variation of High-Level Data Link Control (HDLC):
. it detects and discards frames with errors, it does not retransmit them
. With frame relay you would need to run another protocol e.g. TCP
Frame relay:
. Builds on the highly reliable fibre-optics infrastructure
. A good alternative to T and E carriers
Frame Relay Details
Frame relay supports two levels of traffic
Committed information rate (CIR):
.Traffic up to this rate will be accepted
Excess information rate (EIR):
. Traffic between the rate of CIR and EIR may be accepted
. Marked as being “eligible for discard”
. With a reduction in cost
Frame relay traffic conveys congestion information:
. Frame relay users are expected to exert flow control
Advantages of Frame Relay
Advantages:
. A stable protocol – international standard
. Available in many (but not all) countries
. Available from all major vendors
Takes advantage of modern fibre-optic infrastructure:
. Good LAN-to-LAN support
. T and E carriers throughput capabilities
. Less expensive than fully meshed T1/E1 lines for bursty traffic
Disadvantages of Frame Relay
Little support for SVCs
Does not provide improved fault tolerance if requires other protocols to mange errors
Not suitable for sending delay sensitive data such as real; time voice or video or teleconferencing
It involves data overhead and processing overhead with every packet
It is more expensive compare to internet service
