Download 514-01-ATM

COMP514 – Advanced
Communications
Lecture 1: Introduction, ATM
Matthew Luckie
[email protected]
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
• [email protected]
• 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 Packet
Source: Craig Partridge, Gigabit Networking, Figure 3.2
Cell networking: motivation
Serialisation with Cells
Source: Craig Partridge, Gigabit Networking, Figure 3.3
Cell networking
packet
cells
reassembled packet
Cells and Packets
Source: 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
header
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
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: NNI
8
7
6
5
4
3
2
1
Virtual Path Identifier (VPI)
Virtual Channel Identifier (VCI)
Payload Type CLP
CRC
ATM Header: NNI
8
7
6
5
4
3
2
1
Virtual Path Identifier (VPI)
Virtual Channel Identifier (VCI)
Payload Type CLP
CRC
• VPI + VCI uniquely
identify an ATM
connection
• Two level routing
hierarchy
• A backbone ATM
switch routes on VPI
ATM Header: NNI
8
7
6
5
4
3
2
1
Virtual Path Identifier (VPI)
• 3 bits of payload type
• Distinguishes between
operations traffic and
user traffic
Virtual Channel Identifier (VCI)
Payload Type CLP
CRC
• If the first bit is not set,
the packet is user-traffic
ATM Header: NNI
8
7
6
5
4
3
2
1
Virtual Path Identifier (VPI)
• CLP: Cell Loss Priority
• Single bit
Virtual Channel Identifier (VCI)
Payload Type CLP
CRC
• If ATM switch is
congested and has to
drop packets, it should
first drop packets with
this bit set
ATM Header: NNI
8
7
6
5
4
3
2
1
Virtual Path Identifier (VPI)
Virtual Channel Identifier (VCI)
Payload Type CLP
CRC
• CRC: 1 byte CRC
computed over the
5 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
Header (16 bits)
T
Seq
No
MID
Trailer (16 bits)
Data (44 bytes)
Length
CRC
Type (T) values:
10: Beginning of Message
00: Continuation of Message
01: End of Message
11: Single Segment Message
AAL 3/4 SAR Format
Source: Craig Partridge, Gigabit Networking, Figure 4.7
AAL 5
• Developed by computing industry
• Goal was for a more efficient AAL for data
communications
AAL 5
1-bit end of datagram field in ATM header
header
Data (48 bytes)
Data + Pad (40 bytes)
UU
CPI
8-byte trailer
Length
CRC-32
AAL 5 SAR and Convergence Formats
Source: Craig Partridge, Gigabit Networking, Figure 4.9
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