Transcript Error Detection and Correction

Chapter 18
Virtual-Circuit Networks:
Frame Relay and ATM
Kyung Hee
University
1
18-1 FRAME RELAY
Frame Relay is a virtual-circuit wide-area network that was
designed in response to demands for a new type of WAN in
the late 1980s and early 1990s.
Topics discussed in this section:
Architecture
Frame Relay Layers
Extended Address
FRADs
VOFR
LMI
Kyung Hee
University
2
Frame Relay
Frame Relay
Prior to FR, they were using a virtual-circuit switching
network called X.25.
Drawbacks of X.25



X.25 has a low 64 kbps data rate.
X.25 has extensive flow and error control at the data & the network
layer
- Flow & error control at both layers create a large overhead and
slow down transmissions.
X.25 has its own network layer that user’s data are encapsulated in
the network layer packets of X.25.
- Internet has its own network layer.
- If the Internet wants to use X.25, this doubles the overhead.
Kyung Hee
University
3
X.25 vs. Frame Relay
Feature
X.25
Connection establishment At the network layer
Hop-by-hop flow control
and error control
End-to-end flow control
and error control
Data rate
Frame Relay
None
At the data link layer None
At the network layer
None
Fixed
Bursty
Multiplxing
At the network layer
At the data link layer
Congestion control
Not necessary
Necessary
Kyung Hee
University
4
X.25 vs. Frame Relay
Kyung Hee
University
5
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
Kyung Hee
University
6
Architecture (Frame Relay)
Figure 18.1 Frame Relay network
Kyung Hee
University
7
Virtual Circuit (Frame Relay)
 A Virtual Circuit Identifier (VCI) in FR is identified by a
number called a Data Link Connection Identifier (DLCI)
Permanent Virtual Circuits (PVC)
The connection setup is simple. The corresponding table
entry is recorded for all switches by the administrator.
Kyung Hee
University
8
Virtual Circuit (Frame Relay)
 Switched Virtual Circuit (SVC)
 The SVC creates a temporary, short connection that
exists only when data are being transferred between
source and destination.
Kyung Hee
University
9
FR Switch
Switches
Each switch in a Frame Relay network has a table to
route frames.
Kyung Hee
University
10
Frame Relay 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.
Kyung Hee
University
11
Frame Relay frame
Figure 18.3 Frame Relay frame
EA 0 : meaning that another address byte is to follow
DE 1 : to discard this frame if there is congestion
Kyung Hee
University
12
Frame Relay frame
(EXTENDED ADDRESS)
Figure 18.4 Three address formats
Kyung Hee
University
13
FRAD
 FRAD (Frame Relay Assembler/ Disassembler)
 A FRAD assembles and disassembles frames coming
from other protocols to allow them to be carried by Frame
Relay frames.
Figure 18.5 FRAD
Kyung Hee
University
14
Voice Over Frame Relay
 VOFR (Voice Over Frame Relay)
 VOFR sends voice through the network.
 Voice is digitized using PCM and then compressed.
The result is sent as data frames over the network.
 LMI (Local Management Information)
 F/R was originally designed to provide PVC connections.

There was not a provision for controlling or managing intefaces.
LMI is a protocol added recently to the FR protocol to provide
more management features.



A keep alive mechanism to check if data are flowing.
Multicasting mechanism
A mechanism to allow an end system to check the status of a
switch.
Kyung Hee
University
15
Congestion Control (Frame Relay)
 Congestion Avoidance
The FR protocol uses 2 bits in the frame to explicitly warn the
source and the destination of the presence of congestion.
 BECN(Backward Explicit Congestion Notification)
BECN bit warns the sender of congestion in the network.
Kyung Hee
University
16
Congestion Control (Frame Relay)
FECN(Forward Explicit Congestion Notification)
FECN bit is used to warn the receiver of congestion in the
network.
Kyung Hee
University
17
Congestion Control (Frame Relay)
 4 cases of congestion
Kyung Hee
University
18
18-2 ATM
Asynchronous Transfer Mode (ATM) is the cell relay
protocol designed by the ATM Forum and adopted by the
ITU-T.
Topics discussed in this section:
Design Goals
Problems
Architecture
Switching
ATM Layers
Kyung Hee
University
19
ATM - Design Requirements
Foremost is the need for a transmission system to optimize the use
of high-data-rate transmission media, in particular optical fiber.

A technology is needed to take advantage of large bandwidth and
strength to noise degradation.
The system must interface with existing systems.
Must be implemented inexpensively.
The new system must be able to work with and support the existing
telecommunications hierarchies.
The new system must be connection-oriented to endure accurate and
predictable delivery.
One objective is to move as many of the function to hardware as
possible and eliminate as many software functions as possible.
Kyung Hee
University
20
Problems (ATM)
Problems associated with existing systems.
Frame networks

Different protocols use frames of varying size and intricacy.

As networks become more complex, the information that
must be carried in the header becomes more extensive.
Mixed network traffic

The variety of frame size makes traffic unpredictable.

Internetworking among the different frame networks is slow
and expensive at best, and impossible at worst.
- The sheer size of X creates an unfair delay for frame A.
Kyung Hee
University
21
Cell Networks (Multiplexing Using Cells)
 Cell Networks
A cell is a small data unit of fixed size.
 Cell network uses the cell as the basic unit of data exchange, all data are
loaded into identical cells that can be transmitted with complete predictability
and uniformity.
Because each cell is the same size and all are small, the problems
associated with multiplexing different-sized frames are avoided.
Despite interleaving, a cell network can handle real-time transmissions, such
as a phone call, without the parties being aware of the segmentation or
multiplexing at all.
Kyung Hee
University
22
Asynchronous TDM

AsychronousTDM
ATM uses asynchronous time-division multiplexing – that
is why it is called Asynchronous Transfer Mode – to multiplex
cells coming from different channels.
Kyung Hee
University
23
Architecture of ATM Network
 Architecture
 ATM
is a cell-switched network.
The user access devices, called the endpoints, are
connected through a UNI (User- to-Network Interface) to the
switches inside the network.
The switches are connected through NNI (Networkto-Network interface) .
Kyung Hee
University
24
Virtual Connection of ATM Network
Virtual Connection – Connection between two endpoints is
accomplished through transmission paths (TPs), Virtual paths (VPs), and
Virtual circuits (VCs).
TP (Transmission Path)

TP is the physical connection (cable, satellite, and so on)
between an endpoint and a switch or between two switches.
VP (Virtual Path)

VP provides a connection or a set of connections between
two switches. (A TP is divided into several VPs)
VC (Virtual Circuit)

A VP is logically divided into several VCs.

Think of a VC as the lanes of highway (VP).
Kyung Hee
University
25
Virtual Connection of ATM
Kyung Hee
University
26
Example of VPs and VCs
 Identifiers
 VPI (Virtual Path Identifier)

The VPI defines the specific VP.
 VCI (Virtual Circuit Identifier)

The VCI defines a particular VC inside the VP.
Kyung Hee
University
27
Connection Identifiers
Note that a virtual connection is defined by a pair of numbers:
the VPI and the VCI.
Kyung Hee
University
28
Virtual connection identifiers in UNIs and NNIs
 VPI for UNI and NNI
 The lengths of the VPIs for UNIs(8b) and NNIs(12b) are different.
 The lengths of the VCI is the same in both interface (16bits)
 Dividing a VCI into two parts is to allow hierarchical routing.
 Most of the switches in typical ATM network are routed using VPIs.
 The switches at the boundaries of the network, those that interact
directly with the endpoint devices, use both VPIs and VCIs.
Kyung Hee
University
29
An ATM Cell
 cells
 The basic data unit in an ATM network is called a cell.
 A cell is only 53 bytes long
 5 bytes allocated to the header
 48 bytes carrying the payload.
Kyung Hee
University
30
PVC & SVC
Connection Establishment and Release
 PVC

The VPIs and VCIs are defined for the permanent
connections, and the values are entered for the tables of each
switch.
 SVC

Each time an endpoint wants to make a connection with
another endpoint, a new virtual circuit must be established.

ATM cannot do the job by itself, but needs the network layer
addresses and the services of another protocol (such as IP).
Kyung Hee
University
31
Routing with a ATM Switch
Switching
A switch routes the cell using both the VPIs and VCIs.
 The routing requires the whole identifier.
Kyung Hee
University
32
ATM Layers
ATM Layers
 AAL (Application Adaptation Layer)
 ATM layer
 Physical layer
Kyung Hee
University
33
ATM layers in endpoint devices and switches
 ATM layers
The switches use only the two bottom layers.
The endpoints use all three layers.
Kyung Hee
University
34
Physical Layer
 Physical Layer
ATM cells can be carried by any physical layer carrier.
ATM layer is divided into two parts.


Physical medium : 매체와 비트 타이밍 기능 정의
Transmission convergence : 셀의 생성 및 전송 확인
SONET



ATM was based on SONET as the physical layer carrier.
The high data rate of SONET reflects the philosophy of ATM.
The boundaries of cells can be clearly defined in using SONET.
Other Physical Technologies



ATM does not limit the physical layer to SONET.
Other technologies, even wireless, may be used, If the problem of
cell boundaries are solved.
One solution is for the receiver to guess the end of the cell and
apply the CRC to the 5-byte header.
Kyung Hee
University
35
ATM Layer
 ATM Layer
 ATM layer provides routing, traffic management,
switching, and multiplexing services.
 Outgoing Processes
 Accepting 48-byte segment from the AAL sublayer
Transforming them into 53-byte cells by the adding of a 5-byte header.
Kyung Hee
University
36
ATM Headers
 ATM Header Format
 Format for UNI Cells : User-to-Network Interface
 Format for NNI Cells : Network-to-Network Interface.
Kyung Hee
University
37
ATM Headers
 Generic flow control (GFC)
 The 4-bit GFC field provides flow control at the UNI level.
 UNI level of flow control is not necessary at the NNI level, therefore, these bits are added to
the VPI.
 Virtual Path Identifier (VPI)
 The VPI is an 8-bit field in a UNI cell and a 12-bit field in an NNI cell.
 Virtual Circuit Identifier (VCI)
 The VCI is a 16-bit field in both frames.
 Payload Type (PT)
 3-bit PT field defines the payload as user data or managerial information.
 Cell loss Priority (CLP)
 The 1-bit CLP field is provided for congestion control.
 A cell with its CLP bit set to 1 must be retained as long as there are cells with CLP of 0.
 Header Error Correction (HEC)
 The HEC is a code computed for the first 4 bytes of the header.
 It is a CRC that is used to correct single-bit errors and a large class of multiple-bit errors.
Kyung Hee
University
38
Application Adaptation Layer (AAL)
Application Adaptation Layer (AAL)
 ATM must accept any type of payload, both data frames
and streams of bits.
 Segmentation and Reassembly (SAR) sublayer

The AAL defines a SAR sublayer, to do so, segmentation is
at the source; reassembly, at the destination.
 Convergence Sublayer (CS)

Before data are segmented by SAR, they must be prepared
to guarantee the integrity of the data. This is done by a CS.
ATM defines four versions of the AAL :

AAL1, AAL2, AAL3/4, and AAL5
Kyung Hee
University
39
AAL1

- AAL1은 비디오/음성같은 고정 비트률의 정보(CBR) 전달에 응용 지원
- ATM을 기존의 음성채널과 T회선 같은 디지털 전화망에 연결할 수 있도록 허용
Kyung Hee
University
40
AAL2
- 가변 비트 스트림(VBR)을 지원,-압축/비압축의 낮은 비트율 오디오와 짧은 트래픽에 사용
- 같은 발신지 또는 여러 발신지로부터의 짧은 프레임을 하나의 셀로 캡슐화 지원
Kyung Hee
University
41
AAL3/4
- AAL3은 연결중심 데이터 서비스를, AAL4는 비연결형 서비스를 지원하므로 쟁점 같아 통합
Kyung Hee
- University
포괄적인 순서화와 오류제어 메커니즘을 제공 – SMDS 서비스 제공(VBR패킷 전송에 사용)
42
AAL5
- 단순하고 효과적인 응용층(SEAL : simple and efficient adaptation layer) 지원
- 다중화 기능이 없으므로 순서화나 오류정정 메커니즘이 불필요
- 한 메시지에 속한 모든 셀은 순차적으로 전송되며, connection-oriented VBR
Kyung 서비스를
Hee
지원
University
43