Asynchronous Transfer Mode (ATM) - A Detailed Overview, Slides of Data Communication Systems and Computer Networks

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Data Communication

Computer Networks

Lecture # 15

● Asynchronous Transfer Mode

Riaz Hussain [email protected] docsity.com 1

Lecture 14 Review

• Frame Relay is a _______ technology.
• At what layers of OSI model Frame Relay works?
• How come the Frame Relay achieved better data rates than X.25?
• How do you compare Frame Relay with T-Lines?
• What is bursty traffic?
• What made it possible to remove the error checking and flow control mechanism from data link layers?
• Explain packet transfer mechanism in Frame Relay?What is DLCI?
• How does Frame Relay switch manage switching?
• What is the purpose of “DE” field in Frame Relay header?
• What is the purpose of FECN/BECN?
• Define “Access Rate”, “Committed Burst Size”, “Committed Information Rate”, “Excess Burst Size”
• What is FRAD?
• What protocols are involved with Frame Relay?

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Outline

• Introduction
• Overview
• Packet Size
• Virtual Circuits In ATM
• ATM Cells
• Switching In ATM
• ATM Layers
  • Physical
  • ATM
  • AAL (AAL1, AAL2, AAL3/4, AAL5)
• Service Classes and Quality Of Service

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Asynchronous Transfer Mode (ATM)

• Like ISDN, X.25 and Frame Relay ATM is also a long distance communication technology
• As ISDN and X.25 were the drivers behind Frame Relay, B-ISDN was the starting point for ATM
• Ambitious technology ever dreamt of
  • One stop solution for voice and non-voice transmission requirements
• ATM has emerged out of B-ISDN, which is based on N- ISDN
• N-ISDN uses synchronous TDM technique. However ATM is ASYNCHRONOUS

A3 A2 A

B

C

B3 B

C

M

U

X

C2 B3 A3 C1 B2 A2 B1 A

Synchronous TDM: Empty Slot For Line 3 is was

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• Packet Network ATM: Overview
• Supports multiple logical connections over a single physical channel (just like X.25, Frame Relay)
• Information flows through the logical connections in the form of cells: Fixed Size Packet
• Also called cell-relay: Same concept as Frame Relay, but cells
• Does NOT provide any flow control or error control at the data link layer
• Backward compatible: ATM can serve as a LAN or a WAN backbone without requiring any major hardware replacement - Designers have provided mapping mechanisms that allow frame and packet formats of other protocols to be transformed into ATM cells
• Modern WAN switching technique for
  • Voice docsity.com

ATM: Overview (cntd)

• Designed with an ambitious goal of providing
  • A universal service
  • Which would eventually replace all the existing diverse network technologies Till that happens, ATM must support them
• Uses switches: Multiple hosts connect to switch and and a switch can have connection to other switches ---- Scalable
• Can support extremely high data rates: 155Mbps
  • Would need fiber optic
• Connection between a host and a switch consists of pair of optical fiber cables ---- full duplex
• Must be able to fulfil the conflicting and varying needs of different applications efficiently - Audio: Communication must be very fast, minor errors not an issue - Computer Data: High accuracy, even if speed suffers docsity.com

Packet Size? (cntd)

• Now look at the other side of the coin
  • Let's make a packet of size 8Kb
    • How much time it will require to fill one packet, using » voice signal every 125 micro seconds » 8 bit PCM » 125 milli seconds this much delay is unacceptable for voice
• Variable Length Packet:
• To avoid the problem of large packet sizes, some networks use variable length packets
• Voice traffic can be sent in smaller packets
• Other data in bigger packets
• Drawback:
• Complexity (varying formats and their interpretations) docsity.com

Packet Size? (cntd2)

• ATM Solution:
  • To achieve higher data rates and to avoid transmission delays, ATM uses fixed packet size of 53 octets --- cell
  • Each cell contains 5 octets of header and 48 octets of data
  • Even if the input is received in the form of large packets or variable length packets, all these packets are converted into equal size cells (53 Octets) - No injustice with smaller packets
  • Using TDM these packets are delivered in turn
  • Different applications can specify their requirements using QoS

Large Packet

Y X

M

U

X

Large Packet

Z

... Y^ X

Large Packet

Y X

M

U

X

cket

Z

...Y^ X

ge Pa Lar

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Virtual Circuits In ATM (cntd)

• Route is determined using the routing algorithm, taking into account - Congestion condition - Data rates available on various links in the route
• During the connection establishment, the switch updates the table, which tells the next hop the cell should be routed to
• After the connection is established, the data is transferred
  • No need for every cell to contain full source/destination address
• When the call terminates the table maintained by the switch deletes the entry of the virtual circuit

A

B

C

D

E

F

G

H

I

ATM^4

N/W

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Virtual Circuits In ATM (cntd2)

• Data received by ATM is
  • split into 48 octet chunk
  • 5 octet header is attached to the the packet to form a cell
• Virtual Path Identifier (VPI): Each cell header contains a field
  • 12 bits unique number to uniquely identify the path
  • Generated at the time of connection establishment
• Virtual Channel Identifier (VCI): (16 bits)
  • Identifies individual circuit on the path
  • Required because there could be multiple circuits, connections or channels multiplexed on the same path

VPI = 2

VCI = 1

VCI = 2

VCI = 3

VPI = 3

VCI = 1

VCI = 2

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ATM Cells

• ATM works with two types of cell formats --- Slightly different - UNI: User-Network Interface - User access device connects to ATM switch - NNI: Network-Network Interface - Switch connects with other ATM switches

ATM

Switch

ATM

Switch

ATM

Switch

UNI NNI NNI UNI

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ATM Cells (cntd)

-

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ATM Switches: (VP)

• Two types of ATM switches
  • VP Switch
    • Routes packet based on VPI

Incoming Outgoing Interface VPI Interface VPI 2 78 10 37 2 121 12 147 2 .... .... .... 7 .. .. ..

ATM

Switch

VPI: 78

VPI: 37

VPI:

VPI:

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ATM Switches: (VPC)

• Two types of ATM switches
  • VPC Switch
    • Routes packet based on Path and Circuit both

Incoming Outgoing Interface VPI/VCI Interface VPII 2 78/10 10 37/ 2 121/11 12 147/ 6 137/6 12 46 7 .. .. ..

ATM

Switch

VPI/VCI:121/

VPI/VCI:78/

VPI/VCI:147/

VPI/VCI:37/

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