COMP514 – Advanced Communications

Lecture 1: Introduction, ATM

Matthew Luckiemluckie@cs.waikato.ac.nz

Introduction to Course

• This is a course on modern Carrier Networks– i.e., how a telco might organise its IP network

from the customer to the core.– ATM, SDH, DSL, DSLAM, BRAS, Wifi, PPP,

DHCP, QoS, RADIUS, RED, GRE, L2TP, Ethernet, VLAN, BFD, VoIP, Multiplay, MPLS, LDP, RSVP, BGP

Introduction to Course

• Lectures– Tues 2-3 G.1.15– Thurs 2-3 G.1.15

• Required Textbook– Broadband Network Architectures: Designing

and Deploying Triple-Play Services– Chris Hellberg, Dylan Greene, Truman Boyes– Prentice Hall, 2007

Introduction to Course

• Lecturers– Donald Neal– Erin Gamble– Matthew Luckie

• mluckie@cs.waikato.ac.nz• G.1.28

Introduction to Course

• Assessment– Two assignments

• 15% each, 30% of final grade• Assignment 1: RADIUS. Due Fri, 7 Sept, 5pm• Assignment 2: MPLS. Due Fri, 12 Oct, 5pm

– Mid-semester test• 20%• In class, Thurs 16 Aug, 2pm.

– Final test• 50%• Date to be advised

Introduction to Course

• Volunteer for Class Rep?

Modern carrier networks: motivation

• In the beginning, a Telco provided an analog phone service

• Cable networks provided television service over different set of cables

• Then, Telco's started providing digital networking over different set of equipment

Modern carrier networks: motivation

• It would be nice if a different transmission network wasn’t required for each service

• However, not all traffic is equal– Cable TV, Voice: real-time– Data: more tolerant of delay

Modern carrier networks

• Multiple ways to solve this problem– Have multiple networks– Asynchronous Transfer Mode (ATM)– MPLS

• This lecture looks at the ATM solution– Cell networking

Cell networking: motivation

Small Packet Caught Behind Big PacketSource: Craig Partridge, Gigabit Networking, Figure 3.2

Cell networking: motivation

Serialisation with CellsSource: Craig Partridge, Gigabit Networking, Figure 3.3

Cell networking

cellspacket reassembled packet

Cells and PacketsSource: Craig Partridge, Gigabit Networking, Figure 3.1

ATM Networks

• Organised in a hierarchy

• Connection-oriented

• Extremely low error-rate medium

• Support low-cost attachments

• Developed in early 1990s

ATM Cell Format

• 53 bytes: 5 byte header, 48 byte data

• 48 bytes is a poor compromise– Compromise between 64-byte payload and

32-byte payload– Too large for voice, too small for data– Partially-filled cells == unproductive work

dataheader

ATM Hierarchy

• ATM networks are designed to be interconnected– Customer/Provider:

• User-Network Interface (UNI)• Protects telco’s ATM network from misbehaving

customer equipment

– Provider/Provider• Network-Network Interface (NNI)• Providers trust each other to be well behaved

ATM Header: NNI8 567 4 123

CRC

Virtual Channel Identifier (VCI)

CLPPayload Type

Virtual Path Identifier (VPI)

ATM Header: NNI8 567 4 123

CRC

Virtual Channel Identifier (VCI)

CLPPayload Type

Virtual Path Identifier (VPI)• VPI + VCI uniquelyidentify an ATMconnection

• Two level routinghierarchy

• A backbone ATMswitch routes on VPI

ATM Header: NNI8 567 4 123

CRC

Virtual Channel Identifier (VCI)

CLPPayload Type

Virtual Path Identifier (VPI)• 3 bits of payload type

• Distinguishes betweenoperations traffic anduser traffic

• If the first bit is not set,the packet is user-traffic

ATM Header: NNI8 567 4 123

CRC

Virtual Channel Identifier (VCI)

CLPPayload Type

Virtual Path Identifier (VPI)• CLP: Cell Loss Priority

• Single bit

• If ATM switch iscongested and has todrop packets, it shouldfirst drop packets withthis bit set

ATM Header: NNI8 567 4 123

CRC

Virtual Channel Identifier (VCI)

CLPPayload Type

Virtual Path Identifier (VPI)• CRC: 1 byte CRCcomputed over the5 byte header

ATM Adaptation Layer (AAL)

• The ATM committee decided there was a need to define the way a packet was divided into cells– AAL 1: constant bit rate applications– AAL 2: variable bit rate applications– AAL 3: connection-oriented data applications– AAL 4: connection-less data applications

AAL 3/4

AAL 3/4 SAR FormatSource: Craig Partridge, Gigabit Networking, Figure 4.7

TSeqNo MID

Header (16 bits)

CRCLength

Trailer (16 bits)

Data (44 bytes)

Type (T) values:10: Beginning of Message00: Continuation of Message01: End of Message11: Single Segment Message

AAL 5

• Developed by computing industry

• Goal was for a more efficient AAL for data communications

AAL 5

AAL 5 SAR and Convergence FormatsSource: Craig Partridge, Gigabit Networking, Figure 4.9

Data (48 bytes)header

1-bit end of datagram field in ATM header

Data + Pad (40 bytes)

8-byte trailer

CRC-32UU

CP

I

Length

Conclusion

• Main contribution of cell networking is to prevent the medium being blocked by a large packet– Not as important as it once was– 1500 byte packet at 10Mbps = 1.2ms– 1500 byte packet at 10Gbps = 1.2us

• QoS based on VPI/VCI still interesting

Homework

• Read chapter 4 of ‘Gigabit Networking’ by Craig Partridge