WAN Network 2

Physical Layer Standards

•The physical layer protocols describe how to provide electrical, mechanical, operational, and functional connections to the services provided by a communications service provider.

WANs - Data Link Encapsulation

•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.

WAN Link Options

Definitions of Leased Line, Circuit Switching, and Packet Switching

Term Description

Leased line A dedicated, always-on circuit between two endpoints. The service provider just passes a constant rate bit stream; it does not interpret

or make decisions based on the bits sent over the circuit. Generally

is more expensive than packet switching today.

Circuit switching/dial Provides dedicated bandwidth between two points, but only for the duration of the call. Typically used as a cheaper alternative to leased lines, particularly when connectivity is not needed all the time. Also is useful for backup when a leased line or packet-switched service

fails.

Packet switching Provides virtual circuits between pairs of sites, with contracted traffic rates for each VC. Each site’s physical connectivity consists of a leased line from the site to a device in the provider’s network.

Generally cheaper than leased lines.

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.

DSL

•Digital Subscriber Line (DSL) technology is a broadband technology that uses existing twisted-pair telephone lines to transport high-bandwidth data to service subscribers.

•The term xDSL covers a number of similar yet competing forms of DSL technologies.

•DSL technology allows the local loop line to be used for normal telephone voice connection and an always-on connection for instant network connectivity. The two basic types of DSL technologies are asymmetric (ADSL) and symmetric (SDSL).

•All forms of DSL service are categorized as ADSL or SDSL and there are several varieties of each type.

•Asymmetric service provides higher download or downstream bandwidth to the user than upload bandwidth.

•Symmetric service provides the same capacity in both directions.

•Multiple DSL subscriber lines are multiplexed into a single, high capacity link by the use of a DSL Access Multiplexer (DSLAM) at the provider location.

•DSLAMs incorporate TDM technology to aggregate many subscriber lines into a less cumbersome single medium, generally a T3/DS3 connection techniques to achieve data rates up to 8.192 Mbps.

Circuit Switched

  POTS , ISDN

Routers frequently use ISDN to create a backup link when their primary leased line or Frame Relay connection is lost. 

•When a subscriber makes a telephone call (or ISDN), 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 Switching  

Frame Relay, X.25, ATM

•An alternative is to allocate the capacity to the traffic only when it is needed, and share the available capacity between many users.

•With a circuit-switched connection, the data bits put on the circuit are automatically delivered to the far end because the circuit is already established.

•If the circuit is to be shared, there must be some mechanism to label the bits so that the system knows where to deliver them.

•It is difficult to label individual bits, therefore they are gathered into groups called cells, frames, or packets.

•The packet passes from exchange to exchange for delivery through the provider network.

•Networks that implement this system are called packet-switched networks.

MORE 

•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.

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