4. Packet-Switching Networks
Basic technology the same as in the 1970s
One of the few effective technologies for long
distance data communications
Frame relay and ATM are variants of packetswitching
Advantages:
- flexibility, resource sharing, robust, responsive
Disadvantages:
Time delays in distributed network, overhead penalties
Need for routing and congestion control
4
5. Circuit-Switching
Long-haul telecom network designed for voice
Network resources dedicated to one call
Shortcomings when used for data:
Inefficient (high idle time)
Constant data rate
5
6. Packet-Switching
Data transmitted in short blocks, or packets
Packet length < 1000 octets
Each packet contains user data plus control
info (routing)
Store and forward
6
10. Disadvantages relative to Circuit-Switching
Packets incur additional delay with every node
they pass through
Jitter: variation in packet delay
Data overhead in every packet for routing
information, etc
Processing overhead for every packet at every
node traversed
10
12. Switching Technique
Large messages broken up into smaller packets
Datagram
Each packet sent independently of the others
No call setup
More reliable (can route around failed nodes or
congestion)
Virtual circuit
Fixed route established before any packets sent
No need for routing decision for each packet at
each node
12
16. Figure 4.5 The Use of Virtual Circuits
Chapter 4 Frame Relay
16
17. Figure 4.6 User Data and X.25
Protocol Control Information
Chapter 4 Frame Relay
17
18. Frame Relay Networks
Designed to eliminate much of the overhead in X.25
Call control signaling on separate logical connection
from user data
Multiplexing/switching of logical connections at layer
2 (not layer 3)
No hop-by-hop flow control and error control
Throughput an order of magnitude higher than X.25
18
19. Figure 4.7 Comparison of X.25 and
Frame Relay Protocol Stacks
Chapter 4 Frame Relay
19
21. Frame Relay Architecture
X.25 has 3 layers: physical, link, network
Frame Relay has 2 layers: physical and data link (or
LAPF)
LAPF core: minimal data link control
Preservation of order for frames
Small probability of frame loss
LAPF control: additional data link or network layer
end-to-end functions
21
22. LAPF Core
Frame delimiting, alignment and transparency
Frame multiplexing/demultiplexing
Inspection of frame for length constraints
Detection of transmission errors
Congestion control
22
24. User Data Transfer
No control field, which is normally used for:
Identify frame type (data or control)
Sequence numbers
Implication:
Connection setup/teardown carried on separate
channel
Cannot do flow and error control
24
25. Frame Relay Call Control
Frame Relay Call Control
Data transfer involves:
Establish logical connection and DLCI
Exchange data frames
Release logical connection
25
30. Logical Connections
VCC (Virtual Channel Connection): a logical
connection analogous to virtual circuit in X.25
VPC (Virtual Path Connection): a bundle of VCCs
with same endpoints
30
44. Header Error Control
8-bit field calculated based on remaining 32 bits of
header
error detection
in some cases, error correction of single-bit errors in
header
2 modes:
error
detection
Error correction
44
48. Service Categories
Real-time service
Constant
bit rate (CBR)
Real-time variable bit rate (rt-VBR)
Non-real-time service
Non-real-time
variable bit rate (nrt-VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR)
Guaranteed frame rate (GFR)
48
50. ATM Adaptation Layer (ATM)
Support non-ATM protocols
e.g.,
PCM voice, LAPF
AAL Services
Handle
transmission errors
Segmentation/reassembly (SAR)
Handle lost and misinserted cell conditions
Flow control and timing control
50
51. Applications of AAL and ATM
Circuit emulation (e.g., T-1 synchronous TDM
circuits)
VBR voice and video
General data services
IP over ATM
Multiprotocol encapsulation over ATM (MPOA)
LAN emulation (LANE)
51
52. AAL Protocols
AAL layer has 2 sublayers:
Convergence Sublayer (CS)
Supports specific applications using AAL
Segmentation and Reassembly Layer (SAR)
Packages data from CS into cells and unpacks at
other end
52
55. AAL Type 1
Constant-bit-rate source
SAR simply packs bits into cells and unpacks
them at destination
One-octet header contains 3-bit SC field to
provide an 8-cell frame structure
No CS PDU since CS sublayer primarily for
clocking and synchronization
55
56. AAL Type 3/4
May be connectionless or connection oriented
May be message mode or streaming mode
56
62. Emergence of High-Speed LANs
2 Significant trends
Computing
power of PCs continues to grow
rapidly
Network computing
Examples of requirements
Centralized
server farms
Power workgroups
High-speed local backbone
62
63. Classical Ethernet
Bus topology LAN
10 Mbps
CSMA/CD medium access control protocol
2 problems:
A
transmission from any station can be received by
all stations
How to regulate transmission
63
64. Solution to First Problem
Data transmitted in blocks called frames:
User
data
Frame header containing unique address of
destination station
64
66. CSMA/CD
Carrier Sense Multiple Access/ Carrier Detection
If the medium is idle, transmit.
If the medium is busy, continue to listen until the
channel is idle, then transmit immediately.
If a collision is detected during transmission,
immediately cease transmitting.
After a collision, wait a random amount of time, then
attempt to transmit again (repeat from step 1).
66
71. Hubs and Switches
Hub
Transmission from a station received by central hub
and retransmitted on all outgoing lines
Only one transmission at a time
Layer 2 Switch
Incoming frame switched to one outgoing line
Many transmissions at same time
71
73. Bridge
Frame handling done
in software
Analyze and forward
one frame at a time
Store-and-forward
Layer 2 Switch
Frame handling done
in hardware
Multiple data paths
and can handle
multiple frames at a
time
Can do cut-through
73
74. Layer 2 Switches
Flat address space
Broadcast storm
Only one path between any 2 devices
Solution 1: subnetworks connected by routers
Solution 2: layer 3 switching, packetforwarding logic in hardware
74
81. Benefits of 10 Gbps Ethernet over ATM
No expensive, bandwidth consuming conversion
between Ethernet packets and ATM cells
Network is Ethernet, end to end
IP plus Ethernet offers QoS and traffic policing
capabilities approach that of ATM
Wide variety of standard optical interfaces for 10
Gbps Ethernet
81
82. Fibre Channel
2 methods of communication with processor:
I/O
channel
Network communications
Fibre channel combines both
Simplicity
and speed of channel communications
Flexibility and interconnectivity of network
communications
82
84. I/O channel
Hardware based, high-speed, short distance
Direct point-to-point or multipoint communications
link
Data type qualifiers for routing payload
Link-level constructs for individual I/O operations
Protocol specific specifications to support e.g.
SCSI
84
85. Fibre Channel Network-Oriented Facilities
Full multiplexing between multiple destinations
Peer-to-peer connectivity between any pair of ports
Internetworking with other connection technologies
85
86. Fibre Channel Requirements
Full duplex links with 2 fibres/link
100 Mbps – 800 Mbps
Distances up to 10 km
Small connectors
high-capacity
Greater connectivity than existing multidrop channels
Broad availability
Support for multiple cost/performance levels
Support for multiple existing interface command sets
86
89. Wireless LAN Requirements
Throughput
Number of nodes
Connection to backbone
Service area
Battery power consumption
Transmission robustness and security
Collocated network operation
License-free operation
Handoff/roaming
Dynamic configuration
89