Download Figure 17

Advanced Networking
Lecture for September 17
X.25, Frame and ATM
Many of these figures were adapted from Tanenbaum
(Computer Networks) and from Forouzan (Data
Communications and Networking)
Shared medium drawbacks
• Shared-medium networks do not scale
– Simple hub sends incoming frames to all output
ports – (a layer 1 hub)
– As more nodes are added, congestion becomes
a problem >> it’s a shared medium
B
A
10BaseT
G
10Base2
C
F
D
E
Layer 2 Switching
• Layer 2 switch is used to interconnect LAN
segments
– Usually Ethernet
• Types of layer 2 switches
–
–
–
–
–
Simple Bridge
Multiport Bridge
Transparent Bridge
Remote Bridge
Possibly others??
Hub and Spoke (cont)
• Filtering alleviates this problem
– Why not just send the frames to the destination port and
not the others?
– This requires processing the destination fields in the
frame
• Drawbacks:
– Requires the switching fabric to be much faster than
before
• 10 users all transmitting at 100Mbps >>> 1Gbps
– Hub must be able to “Learn” the proper destination
• i.e. How does the hub know to selectively forward frames?
Bridges
• Bridges forward at Layer2 based on
destination MAC address in Ethernet frame
– When Ethernet frame comes in, sent out only
on port corresponding to MAC address in table
Port #
MAC address
0
1
2
2b:6:8:f:1e:5b
f:1e:5b:2b:6:8
2b:6:8:f:1e:51
Problem: How is this
table built???
Learning Bridges
• Bridges are Hubs that filter and forward
selected layer 2 frames.
Bridging Switch
B
A
I
H
G
C
F
D
E
N
J
M
K
L
Learning Bridge (cont)
• Old bridges required these tables to be built
by hand.
• The learning bridge builds and maintains a
map of the physical port and the MAC
address.
• It does this by watching source MAC
address of frames and from what physical
port they come.
Problem: What if we have
multiple bridges connecting
networks???
Spanning Tree Bridges
Two parallel transparent bridges.
These can create multiple copies of the frame. Can also cause
forwarding loops.
Spanning tree (cont)
• Often multiple bridges are used for redundancy.
• Spanning tree algorithm is used to eliminate
forwarding loops and multiple copies of frames.
• Routes and bridges with lower numbers become
primary elements of the spanning tree.
• Other routes and bridges are used in the event of a
failure
• Exact algorithm is detailed but straightforward.
• Algorithm also used at layer 3 for multicast IP and
other applications.
Spanning Tree Bridges (2)
(a) Interconnected LANs. (b) A spanning tree
covering the LANs. The dotted lines are not part of
the spanning tree.
Bridges from 802.x to 802.y
Operation of a LAN bridge from 802.11 to 802.3.
MAC layers are different for .11 and .3, LLC the same
Bridges from 802.x to 802.y (2)
The IEEE 802 frame formats.
Remote Bridges
• Remote bridges can be used to interconnect
distant LANs.
Repeaters, Hubs, Bridges,
Switches, Routers and Gateways
(a) Which device is in which layer.
(b) Frames, packets, and headers.
Figure 17-1
X.25
X.25
• Was the first popular packet switched network
• Allowed for the setup of data connections at
speeds between 300bps to about 56kbps.
• Uses the “Virtual Circuit” concept.
– Connection oriented packet switch.
• Still used for low bandwidth transactions
– credit cards / Point-of-Sale (POS) transactions.
– Telemetry networks.
Figure 17-2
X.25 Layers in Relation to the OSI Layers
Figure 17-3
Format of a Frame
Figure 17-6
Frame Layer and Packet Layer Domains
Figure 17-7
Virtual Circuits
in X.25
Virtual circuits
allow “meshy”
network with fewer
physical links.
Figure 17-8
LCNs in X.25
LCN: Logical Channel Number,
identifies the VC at different sections
of the network.
Figure 17-10
PLP Packet Format
Note the LCN is in the Layer 3 portion of the
protocol.
GFI: General Format Identifier-- defines which
device should acknowledge the packet
PTI: Packet Type Identifier – The type of packet
Figure 17-11
Categories of PLP Packets
RR: Receive Ready
RNR: Receive not ready
REJ: Reject
Figure 17-12
Data Packets in the PLP Layer
Figure 17-13
RR, RNR, and REJ Packets
Figure 17-14
Other Control Packets
Figure 17-15
Control Packet Formats
Figure 17-17
Triple-X Protocols
Key points on X.25
•
•
•
•
•
Was developed as a packet switching protocol.
Standard includes Layer 1,2,3
Incorporates SVCs and PVCs
Limited in bandwidth
Not optimized for high quality links
– Too much error checking for “good” networks
• Not optimized for TCP / IP transport
– Already has PLP defined at Layer 3
Chapter 18
Frame
Relay
Figure 18-1
Frame Relay versus Pure
Mesh T-Line Network
Figure 18-2
Fixed-Rate versus Bursty Data
Figure 18-3
X.25 Traffic
Figure 18-4
Frame Relay Traffic
Figure 18-5
Frame Relay Network
Figure 18-6
DLCIs: Data Link Connection Identifier
Figure 18-7
PVC DLCIs
Figure 18-8
SVC Setup and Release
Figure 18-9
SVC DLCIs
Figure 18-10
DLCIs Inside a Network
Note that the overall connection is a
mixture of different interfaces and DLCIs
This allows DLCIs to be re-used on
different interfaces
Figure 18-11
Frame Relay Switch
Figure 18-12
Frame Relay Layers
Figure 18-13
Comparing Layers in
Frame Relay and X.25
Figure 18-14
Frame Relay Frame
Figure 18-15
BECN
BECN assumes the source can reduce congestion by
slowing down the transmission of data.
Figure 18-16
FECN
FECN notifies the receiver that congestion is
occurring. It can then be more patient and not
request so much data.
Figure 18-17
Four Cases of Congestion
Frame Relay bandwidth
management
• Frame customers typically connect at T1 or T3
line rate.
– They can “burst” up to this speed
• They pay for something less
– Normally pay based on CIR: Committed Information
Rate
• When Customers exceed CIR for an extended
period, their traffic is subject to being discarded
• Allows service providers to “oversubscribe” the
network, but prevent individual users from
“hogging” the resources.
Figure 18-18
Leaky
Bucket
Figure 18-19
A Switch Controlling the Output Rate
Figure 18-20
Flowchart for Leaky
Bucket Algorithm
Figure 18-21
Example of Leaky Bucket Algorithm
Figure 18-22
Relationship between Traffic
Control Attributes
Figure 18-23
User Rate in Relation to Bc and Bc + Be
Figure 18-25
FRAD
Chapter 19
ATM
ATM in context
• Development of ATM began prior to the WWW
and TCP/IP explosion- early nineties.
• There was a desire for a packet switched protocol
that was faster than X.25 and Frame and could
support multiple classes of service
– Video, Voice, Data
• ATM was selected as the technology of choice for
BISDN
– The plan was to allow “fast” phone calls all over the
place. These could support differing levels of
bandwidth and QoS.
Figure 19-1
Multiplexing Using Different Packet Sizes
53 byte “cell” allowed for higher levels of QoS
with minimal cell-tax – the ratio of overhead to
payload in the cell.
Figure 19-2
Multiplexing Using Cells
Figure 19-3
ATM Multiplexing
Figure 19-4
Architecture of an ATM Network
UNI – User to
Network Interface
NNI – Network to
Network Interface
Figure 19-5
TP, VPs, and VCs
TP: Transmission path – link between
two switches
VP: Virtual Path – Contains several
VCs
VC: Virtual Circuit
Figure 19-6
Example of VPs and VCs
VPs originally
conceived to
simplify
management of VC
bundles
Figure 19-7
Connection Identifiers
Figure 19-8
Virtual Connection Identifiers
in UNIs and NNIs
Figure 19-9
An ATM Cell
Figure 19-10
SVC
Setup
Figure 19-11
Routing with a VP Switch
Figure 19-12
A Conceptual View of a VP Switch
In theory, a VP switch is simpler because it just
switches based on the VPI. Fewer entries to be
maintained.
Figure 19-13
Routing with a VPC Switch
Figure 19-14
A Conceptual View of a VPC Switch
Figure 19-15
Crossbar Switch
Figure 19-16
Knockout Switch
Output
queuing
reduces lost
cells.
Introduces
some jitter.
Figure 19-17
A multi-stage switch
A Banyan Switch
Self routing switch, minimizes
control hardware required.
Figure 19-18-Part I
Example of Routing in a Banyan Switch (a)
Figure 19-18-Part II
Example of Routing in a Banyan Switch (b)
Figure 19-19
Batcher-Banyan Switch
Cells are reordered at input
port so they don’t block
each other as they go
through the fabric
Figure 19-20
ATM Layers
Figure 19-21
ATM Layers in End-Point Devices and Switches
Figure 19-27
ATM Layer
ATM Adaption Layers
• These allow other protocols to mapped into cells.
• AAL1: Good for Constant Bit Rate (CBR)
• AAL2
– Variable Bit Rate Services – Particularly Video
• AAL3/4
• AAL5
– Ethernet and IP – Data oriented protocols
• Lots of references available on these
Figure 19-28
ATM Header
Figure 19-29
PT Fields
Figure 19-30
Service Classes
QoS
• Constant Bit Rate (CBR) used for carrying
DS1 and DS3 or other traffic requiring a
constant guaranteed QoS
• VBR (Real Time and Non Real Time) used
for data that is bursty but need QoS
– Especially for Video
• UBR/ABR lower QoS for Data similar to
Frame Relay level of QoS
Figure 19-31
Service Classes and Capacity of Network
Figure 19-32
QoS
Lots of parameters for QoS. Homework: Go
look these up and also understand the concept of
CAC.
Figure 19-33
ATM WAN
Figure 19-34
Ethernet Switch and ATM Switch
ATM hasn’t really taken hold in the
Enterprise. Much more expensive than
Ethernet switches.
Figure 19-35
LANE Approach