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Transcript of WAN
*The 53 byte ATM cell contains a 5 byte ATM header followed by 48 bytes of ATM payload.
*Small, fixed-length cells are well suited for carrying voice and video traffic because this traffic is intolerant of delay. *Video and voice traffic do not have to wait for a larger data packet to be transmitted.
*The 53 byte ATM cell is less efficient than the bigger frames and packets of Frame Relay and X.25.
*Furthermore, the ATM cell has at least 5 bytes of overhead for each 48-byte payload.
*A typical ATM line needs almost 20% greater bandwidth than Frame Relay to carry the same volume of network layer data.
WAN Technologies Physical Layer Standards WANs - Data Link Encapsulation
WAN Devices channel service unit (CSU) data service unit (DSU).
Time Division Multiplexing (TDM)
Two or more “channels” of information are transmitted over the same link by allocating a different time interval for the transmission of each channel, i.e. the channels take turns to use the link.
Some kind of periodic synchronizing signal or distinguishing identifier is required so that the receiver can tell which channel is which.
TDM becomes inefficient when traffic is intermittent because the time slot is still allocated even when the channel has no data to transmit
Wide Area Networks (WANs) OSI
Reference Model WAN Standards
& Organizations * Span a large geographical area
* Crossing of public right-of-ways
* Rely in part on common carrier circuits
* Typically consist of a number of
interconnected switching nodes
WAN standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation.
WAN standards are defined and managed by a number of recognized authorities.
Leased Lines *A point-to-point link provides a pre-established WAN communications path from the customer premises through the provider network to a remote destination.
*Point-to-point lines are usually leased from a carrier and are called leased lines.
*Leased lines are available in different capacities.
*Leased lines provide direct point-to-point connections between enterprise LANs and connect individual branches to a packet-switched network.
WAN Link Options
The physical layer protocols describe
how to provide electrical, mechanical, operational ,and functional connections to the services provided by a communications service provider.
The data link layer protocols define how data is encapsulated for
transmission to remote sites, and the mechanisms for transferring
the resulting frames.
A variety of different technologies are used, such as ISDN, Frame Relay or Asynchronous Transfer Mode (ATM).
These protocols use the same basic framing mechanism, high-level data
link control (HDLC), an ISO standard, or one of its sub-sets or variants. When a subscriber makes a telephone call, the dialed number is used to set switches in the exchanges along the route of the call so that there is a continuous circuit from the originating caller to that of the called party.
The internal path taken by the circuit between exchanges is shared by a number of conversations.
Time division multiplexing (TDM) is used to give each conversation a share of the connection in turn.
TDM assures that a fixed capacity connection is made available to the subscriber.
Packet-switched describes the type of network in which relatively small units of data called packets are routed through a network based on the destination address contained within each packet. Packet Switching allows the same data path to be shared among many users in the network. This type of communication between sender and receiver is known as connectionless (rather than dedicated). Most traffic over the Internet uses packet switching and the Internet is basically a connectionless network. Frame Relay
*Frame Relay is probably the simplest data communications protocol ever conceived. Designed to run over virtually error- free circuits
*Frame Relay abolishes the Network Layer of the OSI model, claims the routing and multiplexing functions for itself, and leaves everything else to the higher layers.
*Frame Relay implements no error or flow control.
* It simply discards incorrect data.
*The simplified handling of frames leads to reduced latency, and measures taken to
avoid frame build-up at intermediate
switches help reduce jitter.
*Most Frame Relay connections are PVCs
rather than SVCs Switched virtual circuits (SVCs) permanent virtual circuit
*Frame Relay provides permanent shared
medium bandwidth connectivity that
carries both voice and data traffic.
*Cell switching operates in a similar way to packet switching but uses small fixed
length cells for data transport.
*This technology is found within cell based integrated networks such as Asynchronous Transfer Mode (ATM) networks.
*Cell switching can handle multiple data
types, i.e. voice, video and data
*Cell switching is typically a high bandwidth and
high speed (up to 155 Mbps) Cell switching is essentially
an attempt to combine
the best of circuit switching
(guaranteed delivery) and
packet switching (efficiency). *Communications
providers saw a need for
a permanent shared network
technology that offered very low
latency and jitter at much higher
bandwidths. *Their solution was Asynchronous
Transfer Mode (ATM). ATM has data
rates beyond 155 Mbps. *As with the other shared
technologies, such as X.25
and Frame Relay..
ATM is a technology
that is capable of
transferring voice, video,
and data through private
and public networks.
ATM cell switched It is built on a
Dedicated Physical Circuits
*Dedicated circuits are permanent
circuits dedicated to a single
*The connection is always active.
The subscriber purchases dedicated
time slots, or channels, that provide a specific amount of bandwidth that is always available for the subscriber
*The channels in a dedicated circuit
are created using time division multiplexing (TDM),
*In addition to providing
guaranteed bandwidth at all times,
*dedicated circuits provide the
most secure and reliable WAN
*Many enterprise WANs
will have connections to the
*This provides an alternative for
*Since the Internet probably exists
everywhere that the enterprise
has LANs, there are two principal
ways that this traffic can be carried.
*Each LAN can have a connection to its local ISP, or there can be a single connection from one of the core routers to an ISP.
*The advantage is that traffic is carried on the Internet rather than on the enterprise network, possibly leading to smaller WAN links.
The disadvantage of
links, is that the
whole enterprise WAN
is open to Internet
It is also difficult to
monitor and secure the
many connection points.
A single connection point is more easily monitored and secured, even though the enterprise WAN will be carrying some traffic that would otherwise have been carried on the Internet.