WAN TechnologiesChapter 14

Switched Networks

Circuit Switch Virtual Circuit Switch Packet Switch

Switched Networks

Switched Networks

Circuit Switched Network

• Circuit from A to M could be dynamic• Circuit from A to M could be permanent• Once set up, looks like a Point-to-Point circuit

Switched Virtual Circuits

• A form of switched circuits

• May be dynamic or permanent

• For any session data path is fixed

• Two very popular Switched Virtual Circuit technologies: Frame Relay Asynchronous Transfer Mode (ATM)

• These are Data Link technologies

Frame Relay

• High speed wide area network service

• Designed to accept and deliver frames

• Frames up to 8k bytes

• Actual data transported over traditional telephony circuits (T1, T3)

• Often used to interconnect LANs over wide geographical areas

Frame Relay

Frame Relay

Virtual Circuit Identifier (VCI)

In Frame Relay, VCI is called Data Link Connection Identifier (DLCI)

Asynchronous Transfer Mode (ATM)

• Designed by the Telecommunications Industry• Designed to support all existing and future

telecommunications services Voice, high speed data, video

• Connection oriented to ensure predictable delivery – quality of service

• Designed to support all forms of networking• Designed to include a LAN architecture• Defines different levels of service – (QoS)

CBR – Continuous Bit Rate VBR – Variable Bit Rate ABR – Available Bit Rate UBR – Unspecified Bit Rate

ATM

• Basic data unit is fixed frame called a cell • A cell carries 48 bytes of data plus 5 bytes of

identification• A composite circuit between 2 ATM switches

delivers a fixed rate of cells• Any virtual circuit is made up of a certain number

of equally spaced cells per second. Ex. A voice circuit (64Kbps) would be 166 equally

spaced cells A T1 would be 2020 equally spaced cells

ATM CELL

ATM

• ATM was intended to be the universal communications architecture

• Why isn’t it dominant today? Cost – end user equipment is expensive Cell Overhead – 10 per cent Complexity of Quality of Service (QoS) Overtaken but enhancements to Ethernet

• Gigabit Ethernet and beyond• MultiProtocol Label Switching (MPLS)

Switched Virtual Networks

• These architectures present the view of a point to point circuit to the end client

• Thus, various WAN protocols can be run over then Point-to-Point Protocol (PPP) HDLC

• At upper layers, any protocol stack can also be used, including the Internet Protocol