Chapter 18 Virtual Circuit Switching : Frame Relay and ATM

18.1 Virtual Circuit Switching Global Addressing Virtual Circuit Identifier Three Phases Data Transfer Phase Setup Phase Teardown Phase

Virtual circuit wide area network

Global Addressing In virtual circuit networks, global addressing is used only to create a virtual circuit identifier.

Virtual Circuit Identifier (VCI) Each switch can use its own unique set of VCIs

Three Phases

Three Phases In the setup phase, the source and destination use their global addresses to help switches make table entries for the connection. In the teardown phase, the source and destination inform the switches to erase the corresponding entry. Data transfer phase occurs between these two phases.

Data Transfer Phase To transfer a frame from a source to its destination, all switches need to have a table entry for this virtual circuit.

Data Transfer Phase (cont’d) Source-to-destination data Transfer

Setup Phase How does a switch create a switch create an entry for a virtual circuit? Permanent virtual circuit (PVC) approach Switched virtual circuit (SVC) approach

Permanent Virtual Circuit (PVC) Simple way Like a leased line The corresponding table entry is recorded for all switches by the administrator (of course remotely and electrically) In the duplex communication, two virtual circuit are established.

Switched Virtual Circuit (SVC) Creating temporary and short connection that exists only when data are being transferred between source and destination. Setup request VCI (77) lets the destination know that the frames come from A, and not other sources

SVC (cont’d) SVC setup acknowledgment

Teardown Phase In this phase, source A, after sending all frames to B, sends a special frame called a teardown request. Destination B responds with a teardown confirmation frame. All switches erase the corresponding entry from their tables.

X.25 vs. Frame Relay Feature X.25 Frame Relay Connection establishment At the network layer None Hop-by-hop flow control and error control At the data link layer End-to-end flow control Data rate Fixed Bursty Multiplxing Congestion control Not necessary Necessary

X.25 vs. Frame Relay

Features of Frame Relay Operating in higher speed such as 1.544 Mbps, 45Mbps Operating in just the physical and data link layers Allowing bursty data Allowing a frame size of 9000 bytes, which can accommodate all local area network fame sizes less expensive Error detection at the data link layer only no flow control or error control

Architecture

Virtual Circuit A virtual circuit in FR is identified by a number called a data link connection identifier (DLCI) Using PVC

Virtual Circuit SVC SVC 연결에 필요한 DLCI

FR Switch 교환기 (Switch) 프레임 경로를 지정해주는 Table 이용

FR Layers Frame Relay operates only at the physical and data link layers Frame Relay does not provide flow or error control; they must be provided by the upper-layer protocols.

Frame Relay frame EA 0 : meaning that another address byte is to follow DE 1 : not to discard this frame

Frame Relay frame

Congestion Control 혼잡 회피(Congestion avoidance) 혼잡이 일어났음을 발신지와 목적지에게 알리기 위해 2개의 비트 사용 BECN(Backward Explicit Congestion Notification) 트래픽 혼잡이 일어났다는 것을 송신자에게 경고하는데 사용

Congestion Control FECN(Forward Explicit Congestion Notification) 트래픽 혼잡이 일어났다는 것을 수신자에게 경고하는데 사용

Congestion Control 4 cases of congestion

FRAD

ATM Asynchronous Transfer Mode (ATM) is the cell relay protocol by the ATM Forum and adopted by the ITU-T

Design Requirements 고속 전송률 매체(광섬유) 사용을 최적화할 수 있는 전송시스템 필요 기존 시스템 (패킷 네트워크) 과 연결할 수 있는 시스템 필요 비용이 적게 들게 설계 다른 것과 호환성을 갖도록 기존의 전기통신 구조 체계 지원 정확성과 예측 가능한 배달을 보증하기 위한 연결 중심(connection-oriented) 하드웨어에 많은 기능을 부여하여 소프트웨어 기능 처리로 인한 감속 제거

Multiplexing using different frame sizes

Cell Networks A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-sized block of information.

Multiplexing Using Cells

Asynchronous TDM

Architecture of ATM Network

Virtual Connection

Example of VPs and VCs Note that a virtual connection is defined by a pair of numbers: the VPI and the VCI.

Connection Identifiers

Virtual connection identifiers in UNIs and NNIs

An ATM Cell

Routing with a Switch

ATM Layers

ATM layers in endpoint devices and switches

ATM Layer

ATM Headers

AAL1 - 고정 비트 전송률 정보 전달 응용 지원 CS : Convergence Sublayer - 기존 디지털 전화 네트워크의 ATM 연결 허용 CS : Convergence Sublayer SAR : Segmentation and Reassembly

AAL2 - 가변 비트 전송률 응용 지원 - 농구 경기 중계와 같은 고속 데이터 전송률 지원

AAL3/4 AAL3 – 연결중심 데이터 서비스 지원 AAL4 – 비연결형 서비스 지원

AAL5 - 단순하고 효과적인 응용층(SEAL : simple and efficient adaptation layer) 지원 - 점대점 연결을 이용하는 ATM 백본과 LAN - 다중화 기능이 없으므로 순서화나 오류정정 메커니즘이 불필요

Questions !