Chương 4 :
IEEE 802.x STANDARD AND
VIRTUAL LAN
Khoa Điện – Điện tử - ĐHBK TP.HCM
3-1
Local Area Network
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A Local area Network is the data communication
system that allows a number of independent devices
to communicate directly with each other in a limited
geographical area.
LANs are dominated by four architecture:
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Ethernet
Token Bus
Token Ring
Fiber distributed data interface
Token Bus, Token Ring and Ethernet are standards
of IEEE and a part of project 802.
Project 802
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The computer society of the IEEE started a project, called
802 to set up standards to enable intercommunication
between equipment from a variety of manufacturers.
Project 802 does not seek to replace any part of the OSI
model.
The IEEE has subdivided the data link layer into sub layers:
n Logical link control(LLC)
n Medium access control(MAC)
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The LLC is non architecture specific, that is the same for all
IEEE defined LANs.
The MAC sublayer on the other hand, contains a number of
distinct modules, each carries proprietary information
specific to the LAN product being used.
The strength of project 802 is modularity.
By subdividing the function necessary for LAN management,
the designers were able to standardize those that can be
generalized and to isolate those that must remain specific.
n
Each sub division is identified by a number:
n 802.1 (Internetworking)
n 802.2 (LLC)
n 802.3 (MAC)
n 802.4 (Token Bus)
n 802.5 (Token Ring)
LAN compared with OSI model
Project 802
IEEE 802.1
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IEEE 802.1 is the section of Project 802 devoted to
internetworking issues in LANs and MANs.
It seeks to resolve the incompatibilities between network
architectures without requiring modifications in existing
addressing access and error recovery mechanisms, among
others.
Logical Link Control
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The IEEE project 802 model takes the structure of
an HDLC frame and divides it into two sets
functions.
One set contains the end user portions of the frame:
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The logical address
Control Information
Data
These functions are handled by the IEEE 802.2
logical link control protocol.
IEEE 802.2 logical link control is the upper sub
layer of the data link layer.
Medium Access Control
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The second set of functions, the MAC sub layer, resolves
the contention for the shared media.
It contains the synchronization, flag, flow, and error control
specifications necessary to move information from one
place to another as well as the physical address of the next
station to receive and route packet.
MAC is the lower sub layer of the data link layer.
Protocol Data Unit
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The data unit in the LLC level is called the PDU.
The PDU contains four fields familiar from HDLC: a
destination services access point (DSAP), a source service
access point (SSAP), a control field, and an information
field.
DSAP and SAP
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The DSAP and SSAP are addresses used by the LLC to
identify the protocol stacks on the receiving and sending
machines that are generating and using the data.
The first bit of the DSAP indicates whether the frame in
intended for an individual or a group.
The first bit of the SSAP indicates whether the
communication is a command or response PDU.
Control
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The control field of the PDU is identical to the control field
in HDLC.
As in HDLC, PDU frames can be I-frames, S-frames, or Uframes and carry all of the codes and information that the
corresponding HDLC frames carry.
Virtual LANs
VLAN introduction
VLANs logically segment switched networks based on the
functions, project teams, or applications of the
organization regardless of the physical location or
connections to the network.
All workstations and servers used by a particular
workgroup share the same VLAN, regardless of the
physical connection or location.
VLAN introduction
A workstation in a VLAN group is restricted to
communicating with file servers in the same VLAN
group.
VLAN introduction
VLANs function by logically segmenting the network
into different broadcast domains so that packets are
only switched between ports that are designated for
the same VLAN.
Routers in VLAN topologies
provide broadcast filtering,
security, and traffic flow
management.
VLAN introduction
VLANs address scalability, security, and network
management.
Switches may not bridge any traffic between VLANs, as
this would violate the integrity of the VLAN broadcast
domain.
Traffic should only be routed between VLANs.
Broadcast domains with VLANs and
routers
A VLAN is a broadcast domain created by one or
more switches.
Broadcast domains with VLANs and
routers
Layer 3 routing allows the router to send packets to
the three different broadcast domains.
Broadcast domains with VLANs and
routers
Implementing VLANs on a switch causes the
following to occur:
n The switch maintains a separate bridging table for
each VLAN.
n If the frame comes in on a port in VLAN 1, the
switch searches the bridging table for VLAN 1.
n When the frame is received, the switch adds the
source address to the bridging table if it is
currently unknown.
n The destination is checked so a forwarding
decision can be made.
n For learning and forwarding the search is made
against the address table for that VLAN only.
VLAN operation
Each switch port could be assigned to a different VLAN.
Ports assigned to the same VLAN share broadcasts.
Ports that do not belong to that VLAN do not share these
broadcasts.
VLAN operation
Users attached to the same shared segment, share
the bandwidth of that segment.
Each additional user attached to the shared medium
means less bandwidth and deterioration of network
performance.
VLANs offer more bandwidth to users than a shared
network.
The default VLAN for every port in the switch is the
management VLAN.
The management VLAN is always VLAN 1 and may
not be deleted. All other ports on the switch may be
reassigned to alternate VLANs.
VLAN operation
Dynamic VLANs allow for membership based on the
MAC address of the device connected to the switch port.
As a device enters the network, it queries a database
within the switch for a VLAN membership.
VLAN operation
In port-based or port-centric VLAN membership, the port
is assigned to a specific VLAN membership independent
of the user or system attached to the port.
All users of the same port must
be in the same VLAN.
VLAN operation
Network administrators are responsible for configuring
VLANs both manually and statically.
Benefits of VLANs
The key benefit of VLANs is that they permit the network
administrator to organize the LAN logically instead of
physically.
VLAN types
There are three basic VLAN memberships for
determining and controlling how a packet gets
assigned: n Port-based VLANs
n MAC address based
n Protocol based VLANs
The frame headers are encapsulated or modified to
reflect a VLAN ID before the frame is sent over the
link between switches.
Before forwarding to the destination device, the frame
header is changed back to the original format.
VLAN types
Port-based VLANs
MAC address based VLANs
Protocol based VLANs
Membership by Port
Membership by MAC-Addresses
VLAN types
The number of VLANs in a switch vary depending on
several factors:
n Traffic patterns
n Types of applications
n Network management needs
n Group commonality
VLAN types
An important consideration in defining the size of the
switch and the number of VLANs is the IP addressing
scheme.
Because a one-to-one correspondence between
VLANs and IP subnets is strongly recommended,
there can be no more than 254 devices in any one
VLAN.
It is further recommended that VLANs should not
extend outside of the Layer 2 domain of the
distribution switch.
VLAN types
There are two major methods of frame tagging, InterSwitch Link (ISL) and 802.1Q.
ISL used to be the most common, but is now being
replaced by 802.1Q frame tagging.