Local Area Networks Area Networks new.pdf · Differentiate LAN physical and logical topologies ......
Transcript of Local Area Networks Area Networks new.pdf · Differentiate LAN physical and logical topologies ......
Local Area Networks
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Objectives
Describe how different forms of LANs originated and how they evolved
Differentiate LAN physical and logical topologies
Identify LAN addressing issues and the role of MAC addresses
Describe the role of LAN segmentation and its impact on performance
Compare and contrast Ethernet, Token Ring, and FDDI LAN models
Describe the role of VLANs and LANE configurations in networking schemes
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Overview
LAN decisions (configuration, speed, O/S, access, etc.) are made by businesses (LAN owners)
WAN links are owned by public carriers
“Despite the traditional classification
of LANs by span, a more relevant
classification is link ownership.”
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Overview
Two basic LAN classifications Dedicated-server (server-centric or client-server)
Servers function only as servers with specialized functions (printing, database, websites, etc.)
One server must be a file-server
Used by vast majority of businesses!
Peer-to-peer Each station is a functional equal of every other station
Any computer can access files from any other computer
Any computer can be a server (i.e., take on special functions)
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LAN Hardware and Software
“LAN hardware and software are the concern
of the two lowest layers of the
Open Systems Interconnection Model (OSI)
and TCP/IP model architectures:
The two lower layers handle all the protocols and
specifications needed to run the LAN
Higher layers get involved only when interconnecting LANs
Layer 1, the physical layer, and
Layer 2, the data link layer.”
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LAN Hardware and Software
Network Interface Card (NIC) Hardware/firmware combination containing almost all
of the LAN protocols
Contains port(s) to accommodate medium (e.g., CAT 6 copper, fiber, etc.)
Can provide device LAN address
Required for each node on the LAN; a node is a device Directly connected to the LAN
Directly addressable by the LAN
A device must have a NIC to be a LAN node
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LAN Hardware and Software
Medium Access Control (MAC) address Physical address—different for each NIC
Defined (and assigned) by IEEE
Hard-coded by manufacturer
Flat addresses contain no location or sequencing
48 bits long First 24 bits—IEEE Organizationally Unique Identifier (OUI)
Second 24 bits—manufacturer ID [224 = 16,777,216 addresses]
Stored in read-only memory (ROM) on the NIC
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LAN Hardware and Software
Network operating system (NOS)
Mediates between
LAN workstations
LAN resources
LAN processes
Computer operating system (OS)
Mediates individual workstation
resources
Full-blown NOS:
MS Windows Server
Novell Netware
Partial NOS:
(newer) Windows
Mac
UNIX
Linux
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LAN Hardware and Software
NOS functions Contains redirector that determines whether actions
are local (for workstation) or network
Incorporates LAN protocols
Enables LAN software to use LAN hardware
Controls server operations
Manages network storage, disk access, and memory
Provides LAN management tools for administrators
The Channel Allocation Problem
• Static Channel Allocation in LANs and MANs
• Dynamic Channel Allocation in LANs and MANs
The traditional (phone company) way of allocating a single channel is Frequency
Division Multiplexing. (See Figure) FDM works fine for limited and fixed number
of users.
Now divide this channel into N subchannels, each with capacity C/N.
Inefficient to divide into fixed number of chunks. May not all be used, or may
need more. Doesn't handle burstiness.
Static Channel Allocation in LANs and MANs
Dynamic Channel Allocation in LANs and MANs
1. Station Model.
2. Single Channel Assumption.
3. Collision Assumption.
4. (a) Continuous Time.(b) Slotted Time.
5. (a) Carrier Sense.(b) No Carrier Sense.
Possible underlying assumptions include:
Station Model -
Assumes that each of N "stations" (packet generators) independently
produce frames. The probability of producing a packet in the interval dt
is dt where is the constant arrival rate. That station generates no
new frame until that previous one is transmitted.
Single Channel Assumption -
There's only one channel; all stations are equivalent and can send and
receive on that channel.
Collision Assumption -
If two frames overlap in any way time-wise, then that's a collision. Any
collision is an error, and both frames must be retransmitted. Collisions
are the only possible error.
Dynamic Channel Allocation in LANs and MANs (2)
Continuous Time -
There's no "big clock in the sky" governing transmission.
Time is not in discrete chunks.
Slotted Time -
Alternatively, frame transmissions always begin at the start
of a time slot. Any station can transmit in any slot (with a
possible collision.)
Carrier Sense -
Stations can tell a channel is busy before they try it. NOTE -
this doesn't stop collisions.
Dynamic Channel Allocation in LANs and MANs (3)
This is where the sender listens before ejecting something on the wire.
Collision occurs when a station hears something other than what it sent.
PERSISTENT AND NONPERSISTENT CSMA:
1-persistent CSMA
Station listens. If channel idle, it transmits. If collision, wait a random
time and try again. If channel busy, wait until idle.
If station wants to send AND channel == idle then do send.
Success here depends on transmission time - how long after the
channel is sensed as idle will it stay idle (there might in fact be someone
else's request on the way.)
Carrier Sense Multiple Access Protocols
Nonpersistent CSMA (equivalent to 0-persistent CSMA)
Same as above EXCEPT, when channel is found to be busy,
don't keep monitoring to find THE instant when it becomes
free. Instead, wait a random time and then sense again.
Leads to
1) better utilization and
2) longer delays than 1 - persistent. (why?)
Carrier Sense Multiple Access Protocols(2)
Carrier Sense Multiple Access Protocols(3)
p-persistent CSMA [For slotted channels.]
If ready to send AND channel == idle
then send with probability p,
and
with probability q = 1 - p defers to the next
slot.
Interpret the chart for these shown in the
Figure.
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for
various random access protocols.
CSMA WITH COLLISIONS DETECTION:
CSMA/CD - used with LANs.
When a station detects a collision, it stops sending, even if
in mid-frame. Waits a random time and then tries again.
What is contention interval -- how long must station wait
after it sends until it knows it got control of the channel?
It's twice the time to travel to the furthest station.
Carrier Sense Multiple Access Protocols(4)
Ethernet MAC Sublayer Protocol
Collision detection can take as long as 2 .
CSMA with Collision Detection
CSMA/CD can be in one of three states:
contention, transmission, or idle.
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Ethernet: The Once and Future King
LAN protocols are designed for best effort delivery Data frames have a “good chance” of surviving
Receiver determines whether a frame has errors
Higher-layer protocols might provide more precise error detection and recovery
LANs do not guarantee error-free delivery!
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Ethernet: The Once and Future King
Ethernet—802.3 Was not the first [Arcnet was first in 1977]
Is currently the most widely installed
Is considered a contention protocol (stations contend for access)
Uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Station desiring access must listen
If a transmission is detected—carrier sensed—station waits
If no transmission is detected—bus is idle—station transmits
Simultaneous transmissions cause collisions
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Ethernet: The Once and Future King
Fig 9.1
CSMA/CD
If a collision
is detected
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Ethernet: The Once and Future King
The Ethernet frame Max frame size = 1,518 bytes [data = 1,500 bytes]
Min frame size = 64 bytes [data = 46 bytes]
5 data fields Destination address
Source address
Network protocol or data length (if < 1,518)
Data PDU (higher layer data)
Frame check sequence (error detection)
2 synchronization fields Preamble for frame synchronization
Start frame delimiter indicating frame start for receiver
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Ethernet: The Once and Future King
The Ethernet frame Max frame size = 1,518 bytes (data = 1,500 bytes)
Min frame size = 64 bytes (data = 46 bytes)
DataSynchronization
Fig 9.2
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Ethernet: The Once and Future King
Ethernet collision window (“slot time”) Length of time for frame to travel
from one end of the LAN to the other
Requires frame limits to work (64 byte min frame size)
For 10 Mbps
Key factors
Bit rate—time for a station to transmit a complete frame
Propagation speed—time for 1 bit to travel to the end of the bus
512 bit times = 512 bits/8 bits per byte = 64 bytes
Max length = 500 m
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Improving Traditional Ethernet
Bus and hub comparison
Fig 9.3
bus hub
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Improving Traditional Ethernet
Bus and star cabling comparison (8 nodes)
Fig 9.4
bus
star
Node
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Improving Traditional Ethernet
Ethernet (with media type indicators)
Advantages Reliability improved—bus disruptions don’t take down LAN
Management improved—simple network management protocol (SNMP) installed on hub
Maintenance improved—easier to add workstations
Disadvantages Physical stars require more cabling
Hub becomes single point of failure
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Ethernet: The Once and Future King
Thicknet
10BASE5 10 Mbps data rate
Baseband signaling over thick coaxial copper
Max segment length: 500 m
Up to 100 nodes
Up to 4 repeaters
Physical bus
Connected by medium attachment unit (MAU)
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Improving Traditional Ethernet
Thinnet
10Base2 10 Mbps data rate
Baseband signaling over pencil-thin coaxial copper
Max segment length: 185 m
Up to 30 nodes
Up to 4 repeaters
Physical bus
Connected by NICs (MAU function moved to NICs)
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Improving Traditional Ethernet
Ethernet (with media type indicators)
10BASE-T 10 Mbps data rate
Baseband signaling over twisted pair copper
Max segment length: 185 m
Node limits dictated by ports available on hubs
Hubs could be repeaters (“active hubs”)
Physical star operating as a logical bus
Connected by hubs
Ethernet Cabling
The most common kinds of Ethernet cabling.
This is a 1-persistent CSMA/CD LAN. Originated in Aloha.
WIRES:
Ethernet Cabling (2)
Three kinds of Ethernet cabling.
(a) 10Base5, (b) 10Base2, (c) 10Base-T.
Ethernet Cabling (3)
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
Repeaters - Multiple cables can be connected. From software point, a
repeater is transparent.
Ethernet Cabling (4)
(a) Binary encoding, (b) Manchester encoding,
(c) Differential Manchester encoding.
After a collision, station waits 0 or 1 slot. If it collides
again while doing this send, it picks a time of 0,1,2,3
slots. If again it collides the wait is 0 to 23 -1 times.
Max time is 210 -1 (or equal to 10 collisions.) After 16
collisions, an error is reported.
Slot is determined by the worst case times; 500 meters
* 4 repeaters = 512 bit times = 51.2 microseconds.
Algorithm adapts to number of stations.
Binary Exponential Backoff Algorithm
Uses 10Base-T to each of the hosts. And a high speed backplane
between the connectors. Works because the assumption is that
many requests can be routed within the switch. Relieves congestion
on the hub.
Routing -
Local (on-switch) destinations are sent there directly. Off-switch are
sent to the backplane.
Collision Detection -
The connections on the switch
form their own LAN and do
collision handling as we've just
seen. The switch buffers the
transmission and ensures no
collisions occur.
Switched Ethernet
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Improving Traditional Ethernet
Replacing the hub with a switch
How it works Switch connects workstations in pairs
Will not connect transmitting stations to a busy one
LAN no longer operates as a bus—no contention!
Advantages No collisions—each station has own link to switch
Compatibility with CSMA/CD is maintained
Multiple workstations can transmit simultaneously
Simple to upgrade—replace hub with switch
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Improving Traditional Ethernet
Fast Ethernet
100BASE-TX 100 Mbps data rate
Baseband signaling, cat 5 UTP
Max segment length: 100 m (span limit: 250m)
Node limits dictated by ports available on hubs
Hubs could be repeaters (“active hubs”)
Physical star operating as a logical bus
Connected by switches
100BASE-FX is
multimode fiber-optic version
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Improving Traditional Ethernet
Fast Ethernet (100BASE-TX)
Advantages Speed boost (10 Mbps to 100 Mbps)
Backward compatible—10/100 Mbps on same LAN
Easy device upgrade
Upgrade switch
With CAT 5 UTP or STP, swap NICs
Disadvantages Maximum segment length is 100 m [total span limit: 250 m]
Switch is single point of failure
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Improving Traditional Ethernet
Gigabit Ethernet
1000BASE-T
1000 Mbps data rate
Baseband signaling over cat 5 UTP
Max segment length: 100 m (span limit: 100 m)
Min frame size: 512 bytes (up from 64 byte)
Connected by switches
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Improving Traditional Ethernet
Gigabit Ethernet (other classifications)
1000BASE-X 1000BASE-CX
Copper over twinax or quad cabling
Max span: 25 m
1000BASE-LX Fiber-optic (1,300 nm signals)
Max span: 550 m (multimode)
Max span:3,000 m (single-mode)
1000BASE-SX Fiber-optic (850 nm signals)
Max span: 550 m (multimode)
Max span 3,000 m (single-mode)
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Improving Traditional Ethernet
10 Gigabit Ethernet
10GBASE-X 10 Gbps data rate
Full duplex signaling over fiber-optic media
7 versions 10GBASE-SR (short-range) and –SW (short-wavelength)
10GBASE-LR (long-range) and –LW (long-wavelength)
10GBASE-ER (extended-range) and –EW (extra-long wavelength)
10GBASE-LX4 (carries signals on 4 light wavelengths)
IEEE Standard 802.4:Token BusNeed a mechanism to handle real-time, deterministic requirements.
802.3 could contend forever and this is often not acceptable.
A ring, with stations taking turns is deterministic. Uses logical ring
on linear cable.
Mechanism -
All stations numbered; station knows # of its
neighbors.
A token, required in order to send, is initialized by the
highest number station.
A station, receiving the token, does a send if it has a
request, then sends the token to its logical (not
necessarily physical) neighbor.
Activation -
Stations can come and go on the bus, without breaking
mechanism.
Cabling -
Uses 75 ohm coax. Speeds are 1, 5, 10 Mbps.
IEEE Standard 802.4:Token Bus
A Token bus
Station has 4 possible priorities, 0, 2, 4, 6; station maintains 4 queues
for requests.
Within each station,
Token comes first to priority 6 queue. Sends occur
until nothing to send OR timer expires.
Token goes next to priority 4 queue. Sends occur
until nothing to send OR timer expires.
And so on . . . .
Proper setting of the various timers ensures that high priority
requests happen first.
The Token Bus MAC Sublayer Protocol
The frame format. Fields are:
Preamble - used to synchronize receiver clock.
Start/End Delimiter - contains a non-data (illegal) Manchester Encoding.
Frame control - shows control or data. shows priority of datapackets. flag requiring ACK from receiver. showstype of control frame (more later).
Destination Address - (same as 802.3) - usually 6 bytes.
Source Address - (same as 802.3) - usually 6 bytes.
Data - BIG - 8182 or 8174 bytes (note no length field)
Checksum - (Same as 802.3)
The Token Bus MAC Sublayer Protocol
The 802.4 frame format
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Token Ring
Token ring – 802.5 Patented by Olof Söderblom in the late 60s—licensed to IBM
Practically no new installations
Speeds typically 4/16 Mbps (100 Mbps standard exists)
A round-robin protocol (stations take turns in order)
Most commonly configured as a physical star/logical ring
Each station is connected to a multistation access unit (MAU)
Logically, each station is connected point to point to a predecessor node and a successor node
A small packet (token) controls medium access
A station can transmit data only when it has the token
Only one token is in circulation at any time
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Token Ring
Fig 9.7
Not broadcast but point to point.
All digital rather than analog (such as used by 802.3 for collision detection.)
Chosen by IBM for its LAN; included by IEEE as Token Ring.
Calculate the number of bits on the ring at any one time:
At R Mbps, a bit is emitted every 1/R microseconds (µsecs).
At a speed of 200 m/µsec, each bit occupies 200/R meters ofthe ring. So a 1 Mbps ring, with circumference 1000 metershas only 5 bits on it at any one time.
In addition, there's a 1 bit delay at each station. (Data bit can be modifiedbefore being forwarded.)
Token is 3 bytes. Must be sufficient delay on the ring so that the whole token is
there. Why?? Stations may be powered down, etc. - no guarantee that stations are
adding delay. So may need to add artificial delay.
IEEE Standard 802.5: Token Ring
IEEE Standard 802.5: Token Ring
a) Ring network b ) Listen mode c) Transmit mode
• Arbitration -
Must hold the token in order to transmit.
• Listen mode -
Input just copied to output.
• Transmit mode -
Seize the token and put own data on ring. As sender's data comes back
around, it removes data. At end of transmission, stick token back on.
Receiver can ACK receipt by flipping a bit on end of packet.
Efficiency is excellent: At high usage, with many stations transmitting, they
get token one after the other.
IEEE Standard 802.5: Token Ring
IEEE Standard 802.5: Token Ring
Four stations connected via a wire center
Wires -
Shielded twisted pair/ 1 or 4 Mbps.
Differential Manchester encoding.
Reliability – Star-Shaped Ring --
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Token Ring
The token ring frame
Three frame types
Token frame
Data frame
Control frame
Data and control frames have the same
format
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Token Ring
The token ring frame
Fig 9.8
data and control frames
have the same format
The Token Ring MAC Sublayer ProtocolFrame Structure Components -
SFD, EFD Delimiters - have illegal encoding so not confused as data.
AC Access control, containing bits for:
The token bit - flip this bit and it’s a data preamble
Monitor bit,
Priority bits,
Reservation bits
Frame control Provides numerous control options.
Source/Destination addresses/checksum
same as 802.3 & 802.4.
The Token Ring MAC Sublayer Protocol
Frame Structure Components -Frame status
A bit - the intended receiver saw the packet
C bit - the receiver copied the packet into its buffers. Serves as
acknowledgment.
Priorities -Token gives priority of that token - a sender must wait for token
of correct priority. The access control byte (of the token or data
frame) has reservation bits. As frame goes by, a requester can
say it wants the token at that priority the next time around.
The Token Ring MAC Sublayer Protocol
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LAN Segmentation
LAN segmentation
Goal Reduce congestion by grouping stations according to traffic
Approach Segments include workstations that often communicate with
On another
Common data source
Common resource
Each segment becomes a LAN in itself
Segments can later be interconnected to share resources
40 stations : 10 Mbps LAN 10 Mbps/40 = 250 Kbps
(2) 20 stations: 10 Mbps LAN `10 Mbps/20 = 500 Kbps
Segmentation in action
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LAN Segmentation
Bridge operation and bridge types What is a bridge?
A traffic monitor between two LANs
A filter to keep local traffic from crossing between LANs
A segment device that keeps local traffic off other LANs
Bridge address tables Track device addresses on both sides
How they are created distinguishes types of bridges
Types of bridges Manual bridge—addresses are manually loaded into a table
Learning bridge—automatically creates its own tables
Bridge can flood both sides of a LAN to learn what devices respond
Bridge can learn device addresses when new source addresses appear in frames
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LAN Segmentation
Using backbones to interconnect LANs
Instead of directly connecting LANs and bridges,
all interLAN links traverse the backbone
Backbones may be
Linked to LANs by bridges
Based on routers
LANs themselves
LAN stations connect to the backbone via
their LAN hubs or switches
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LAN Segmentation
Bridged backbone
Each server has
one port connection to
the backbone bus
One port connection to
its LAN switch
Bridge forwards only to
the bus frames from its
LAN destined for a
nonlocal LANFig 9.11
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LAN Segmentation
Star-wired (collapsed) backbone
Each LAN switch is connected to a
router that sends frames according
to frame destination addresses
Backbone is considered to be
shrunk (collapsed) into the router
itself
Fig 9.11
If the router fails
the backbone fails
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LAN Segmentation
Backbone LAN
Same as star-wired backbone except a LAN takes the place of
a router
Each connected LAN becomes a node on the backbone LAN
Fig 9.12
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LAN Segmentation
FDDI (Fiber Distributed Data Interface)
Token-passing protocol
100 Mbps
Station separation up to 2 Km (1.25 mi)
on single-mode fiber
Originally used as MAN backbone
Superseded by higher speed Ethernet
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LAN Segmentation
FDDI (Fiber Distributed Data Interface)
Fig 9.13
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VLANs
Virtual LAN (VLAN)
802.3ac
Grouped by
Station characteristics
Switch characteristics
Frame protocols
Physical LAN memberships or links
are not changed
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VLANs
Virtual LAN (VLAN) Benefits
Security
Traffic reduction
Flexibility
Cost savings
Caveats
Ease in setup does not presume well-designed
Be wary of too many members on too many physical LANs
Stations with occasional communications should not be members
Problems
Congestion
Network management difficulty
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VLANs
Fig 9.14
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VLANs
Attribute-based VLANs Configured by creating list mappings (access lists)
Switches discern which ports belong to which VLANs
Membership can be assigned Mostly manual
Partly manual
Mostly automatic
Protocol-based VLANs Membership determined on a frame-by-frame basis
Participation based on individual transmissions instead of port assignment
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VLANs
Tagged Ethernet
Enables workstations to belong to several VLANS at same time
First 20 bytes are same as Ethernet frame
Four tag bytes are inserted between the source address and the
type/length field
Fig 9.15