Wireless LAN
MSTP
Dr. Paul Chen
paulpchen@hotmail.com
March, 2004
Dr. Paul Chen
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IEEE802.11 Wireless LAN
• 802.11b was defined in an attempt to increase the original
bandwidth of 802.11 (1-2 Mbps) in the range of 2.4GHz
operational frequency using direct sequence spread spectrum
(DSSS). Its bandwidth capacity is set to be between 5.5Mbps
and 11Mbps.
• 802.11a
As the need for bandwidth grew, 802.11a was specified in
1999 as a Physical Layer (PHY) standard to operate at 5GHz
frequency band with possible data rates between 6-54 Mbps.
Those products are slowly becoming available to a wider
public. The additional advantage of 802.11a over 802.11b is
that there is also much less interference with radio at its 5GHz
frequency in comparison to 802.11b and 802.11g.
March, 2004
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IEEE802.11 Wireless LAN (continued)
• 802.11c
To help define the standard of development of access points (APs)
for wireless technologies to bridge the information flow, 802.11c was
established and its work has already been concluded.
• 802.11d
As the operation, especially in the 5GHz range, may differ from
country to country (or domain to domain), the 802.11d protocol was
established. It also better defined interoperability issues.
• 802.11e
With the expansion of wireless device technologies and the featurerich applications already in development for video and audio (voice),
it was apparent that the 802.11 PHYs were not quite optimized to
fulfill such tasks. This lead to the development of 802.11e which
refines a 802.11 medium access layer (MAC) to prioritize traffic to
improve quality of service (QoS) for support of video and audio
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IEEE802.11 Wireless LAN (continued)
• 802.11f
This protocol specification addresses the roaming need for
transmission for a user from one access point (AP) to another
and ensures the continuity of transmission; it would ultimately
provide inter-access point protocol.
• 802.11g
The "g" technology specification is still in the works and is the
most recent redefinition for 802.11. Its goal is defined as
extension to 20+ Mbps rate by adding one more channel to
the current three in the operation spectrum of 2.4GHz, which
would compete with 802.11a rates.
• 802.11h
802.11h is an extension of 802.11a to satisfy regulations in
Europe for the spectrum band of 5GHz by providing dynamic
channel selection (DCS) and transmit power control (TPC).
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IEEE802.11 Wireless LAN (continued)
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802.11i
802.11j
802.11k
802.11m
802.11n
March, 2004
Security Enhancements
Extending 802.11a to Japanese brand
Radio Resource Measurement
Maintenance
High Throughput (100 Mbps)
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The Problems of RF
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Fixed resources / shared media
Coverage extends outside the physical building
RF problems are statistical in nature
The air space is constantly changing
- signal strength
- interference
- signal to noise ratio
- coverage area
- throughput
- load
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Taking A System Level Approach for Wireless Networks
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Mobility
RF Physical layer security
Layer 2 security (802.1x, WPA, 802.1i)
Layer 3 security (VPN)
Client security
Physical layer management / visibility
Client management
Planning / deployment
Location
March, 2004
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802 Architecture and Relation to Bluetooth
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Functions of 802.15.1 lower layer protocols
• RF layer
The air interface is based on antenna power range starting from 0
dBm up to 20 dBm. Bluetooth operates in the 2.4 GHz band and the
link range is anywhere from 10 centimeters to 10 meters.
• Baseband layer
The Baseband layer establishes the Bluetooth physical link between
devices forming a piconet -- a network of devices connected in an
ad hoc fashion using Bluetooth technology. A piconet is formed
when two Bluetooth devices connect, and can support up to eight
devices. In a piconet one device acts as the master and the other
devices act as slaves.
• Link manager
The link manager sets up the link between Bluetooth devices. Other
functions of the link manager include security, negotiation of
Baseband packet sizes, power mode and duty cycle control of the
Bluetooth device, and the connection states of a Bluetooth device in
a piconet.
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Functions of 802.15.1 lower layer protocols (continued)
• Logical Link Control and Adaptation Protocol (L2CAP)
This layer provides the upper layer protocols with connectionless
and connection-oriented services. The services provided by this
layer include protocol multiplexing capability, segmentation and
reassembly of packets, and group abstractions.
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Differences Between WPAN (802.15) and WLAN (802.11)
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IEEE 802.1w RSTP Overview
• RSTP significantly reduces the time to reconfigure the active
topology of the network when changes
occur to the physical topology or its configuration parameters. RSTP
selects one switch as the root of a
spanning tree-connected active topology and assigns port roles to
individual ports of the switch,
depending on whether that port is part of the active topology.
• RSTP provides rapid connectivity following the failure of a switch,
switch port, or a LAN. A new root
port and the designated port on the other side of the bridge
transition to forwarding using an explicit
handshake between them. RSTP allows switch port configuration so
that the ports can transition to
forwarding directly when the switch reinitializes.
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IEEE 802.1w RSTP Overview (continued)
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RSTP as specified in 802.1w supersedes STP specified in 802.1D, but
remains compatible with STP.
• RSTP selectively sends 802.1D-configured BPDUs and topology
change notification (TCN) BPDUs
on a per-port basis.
• When a port initializes, the migration-delay timer starts and RSTP
BPDUs are transmitted. While the migration-delay timer is active,
the bridge processes all BPDUs received on that port.
• If the bridge receives an 802.1D BPDU after a port’s migration-delay
timer expires, the bridge assumes it is connected to an 802.1D
bridge and starts using only 802.1D BPDUs.
• When RSTP uses 802.1D BPDUs on a port and receives an RSTP
BPDU after the migration-delay expires, RSTP restarts the
migration-delay timer and begins using RSTP BPDUs on that port.
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RSTP Port Roles
• Root—A forwarding port elected for the spanning tree topology.
• Designated—A forwarding port elected for every switched LAN
segment.
• Alternate—An alternate path to the root bridge to that provided by
the current root port.
• Backup—A backup for the path provided by a designated port
toward the leaves of the spanning tree. Backup ports can exist only
where two ports are connected together in a loopback by a point-topoint link or bridge with two or more connections to a shared LAN
segment.
• Disabled—A port that has no role within the operation of spanning
tree.
• Port roles are assigned as follows:
- A root port or designated port role includes the port in the active
topology.
- An alternate port or backup port role excludes the port from the
active topology.
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RSTP Port States
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The port state controls the forwarding and learning processes and provides
the values of discarding, learning, and forwarding.
Comparison Between STP and RSTP Port States
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RSTP Port States (continued)
• In a stable topology, RSTP ensures that every root port and
designated port transition to forwarding, and ensures that all
alternate ports and backup ports are always in the discarding
state.
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IEEE 802.1s MST (Multiple Spanning Tree Protocol)
• MST extends the IEEE 802.1w rapid spanning tree (RST) algorithm
to multiple spanning trees. This extension provides both rapid
convergence and load balancing in a VLAN environment.
• MST is backward compatible with 802.1D STP.
• MST allows you to build multiple spanning trees over trunks.
You can group and associate VLANs to spanning tree
instances. Each instance can have a topology independent of
other spanning tree instances.
• This new architecture provides multiple forwarding paths for
data traffic and enables load balancing.
• Network fault tolerance is improved because a failure in one
instance (forwarding path) does not affect other instances.
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IEEE 802.1s MST (continued)
• A spanning tree instance can exist only on bridges that have
compatible VLAN instance assignments. You must configure a set of
bridges with the same MST configuration information, which allows
them to participate in a specific set of spanning tree instances.
• Interconnected bridges that have the same MST configuration are
referred to as an MST region.
• MST runs a variant of spanning tree called internal spanning
tree (IST). IST augments the common spanning tree (CST)
information with internal information about the MST region.
The MST region appears as a single bridge to adjacent single
spanning tree (SST) and MST regions.
• CST (802.1Q) is a single spanning tree for all the VLANs.
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IEEE 802.1s MST (continued)
• A bridge running MST provides interoperability with single
spanning tree bridges as follows:
– MST bridges run IST, which augments the common
spanning tree (CST) information with internal information
about the MST region.
– IST connects all the MST bridges in the region and appears
as a subtree in the CST that includes the whole bridged
domain. The MST region appears as a virtual bridge to
adjacent SST bridges and MST regions.
– The common and internal spanning tree (CIST) is the
collection of ISTs in each MST region, the CST that
interconnects the MST regions, and the SST bridges. CIST is
the same as an IST inside an MST region and the same as
CST outside an MST region. The STP, RSTP, and MSTP
together elect a single bridge as the root of the CIST.
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IEEE 802.1s MST (continued)
• MST establishes and maintains additional spanning trees
within each MST region. These spanning trees are referred to
as MST instances (MSTIs). The IST is numbered 0, and the
MSTIs are numbered 1,2,3, and so on. Any MSTI is local to
the MST region that is independent of MSTIs in another
region, even if the MST regions are interconnected. MST
instances combine with the IST at the boundary of MST
regions to become the CST as follows:
• – Spanning tree information for an MSTI is contained in an
MSTP record (M-record).
M-records are always encapsulated within MST BPDUs (MST
BPDUs). The original spanning trees computed by MSTP are
called M-trees. M-trees are active only within the MST region.
M-trees merge with the IST at the boundary of the MST region
and form the CST.
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Network with Interconnected SST and MST Regions
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STP Default Configuration
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