MAC

Computer Networks
Chapter 4
Medium Access Sublayer

Chap. 4- MAC 2
Chapter Overview
Here we want to know how to handle broadcast networks. As compared to
point to point networks, a major issue is handling arbitration when there is
competition for the network.
This is the bottom sublayer of the Data Link Layer. This Chapter is especially
relevant for LANs.
4.1 The Channel Allocation Problem
How to allocate a single channel among multiple users.
4.2 Multiple Access Protocols
How to handle contention for the use of a channel.
4.3 IEEE Standards for LANs
How do the protocols of the last sections apply to real systems. Here we talk
about the actual standards in use.
4.4 Bridges
Ways of connecting networks together.
4.5 High Speed LANs
Directions in high speed networks.

Chap. 4- MAC 3
CHANNEL
ALLOCATION
PROBLEM:
Overview
Or, how to allocate a single channel among
competing users.
4.1 The Channel Allocation
Problem
4.4 Bridges
4.5 High Speed LANs

Chap. 4- MAC 4
CHANNEL
ALLOCATION
PROBLEM:
STATIC 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.
Inefficient to divide into fixed number of chunks. May not all be used, or may
need more. Doesn't handle burstiness.
T = mean time delay
C = capacity (bps)
λ = arrival rate
1/ µ = mean length
1
T = ----------
µC - λ

Chap. 4- MAC 5
CHANNEL
ALLOCATION
PROBLEM:
STATIC CHANNEL
ALLOCATION IN LANs AND
MANS
Now divide this channel into N subchannels, each with capacity
C/N. Input rate on each of the N channels is A/N. Now
1 N
T(fdm) = ----------------- = ------------ = NT
µ(C/N) - λ /N µC - λ

Chap. 4- MAC 6
CHANNEL
ALLOCATION
PROBLEM:
DYNAMIC CHANNEL
ALLOCATION
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 λdt where λ 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.

Chap. 4- MAC 7
CHANNEL
ALLOCATION
PROBLEM:
DYNAMIC CHANNEL
ALLOCATION
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. LANs have this, satellite networks don't.

Chap. 4- MAC 8
Multiple Access
Protocols
Overview
How to handle contention for the use of a channel.4.1 The Channel Allocation
Problem
4.4 Bridges
4.5 High Speed LANs

Chap. 4- MAC 9
Multiple Access
Protocols
MULTIPLE ACCESS
PROTOCOLS
Collisions work well for low utilization (they're not likely to happen.) Arbitration, which we'll
talk about later, works better at high utilization.
ALOHA:
Developed in Hawaii in the 1970s.
PURE ALOHA:
Every station transmits whenever it wants to. Colliding frames are destroyed. The sender
knows if its frame got destroyed, and if so waits a random time and then retransmits.
1. ANY overlap is a collision.
2. Best efficiency if frames are same size.
3. BUT, what is the efficiency? Talk about stochastic.

Chap. 4- MAC 10
Multiple Access
Protocols
MULTIPLE ACCESS
PROTOCOLS
• S = frames to be transmitted. In units of frames per frame time so that 0 < S < 1.
(What is the meaning of frame time as used here??)
• G = S + frames retransmitted due to previous collisions.
• P0 = probability that a frame does NOT suffer collision.
• S = P0 x G
Use the Figure to determine collision vulnerability.
Pure Aloha
Continued

Chap. 4- MAC 11
Multiple Access
Protocols
MULTIPLE ACCESS
PROTOCOLS
Probability that k frames are generated during a given frame time (Poisson distribution):
G k
e-G
Pr[k] = --------------
k!
Probability of no traffic initiated during the vulnerable period:
P0 = e-2G
so
Throughput per frame time is:
S = G e -2G
See Figure 4.3
Pure Aloha
Continued

Chap. 4- MAC 12
Multiple Access
Protocols
MULTIPLE ACCESS
PROTOCOLS
SLOTTED ALOHA:
Doubles efficiency by dividing time into "ticks". Sends occur only at the start of a
tick. Vulnerable period is 1/2 of pure Aloha case, so
S = G e-G
See throughput on the last page. Best throughput is at G = 1 when
S = 0.37; empty slots = Pr[0] = 0.37; collisions = G - S = 0.26

Chap. 4- MAC 13
Multiple Access
Protocols
CARRIER SENSE MULTIPLE
ACCESS PROTOCOLS:
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.)
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?)

Chap. 4- MAC 14
Multiple Access
Protocols
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?)
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.

Chap. 4- MAC 15
Multiple Access
Protocols
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.

Chap. 4- MAC 16
Multiple Access
Protocols
COLLISION-FREE
PROTOCOLS
How long is a packet (or how long a wire is needed to contain a packet) of length 1500
bytes on a 100 Mbps ethernet?
As cables become longer and faster, the above methods become less efficient. So, . . . .
Bit map protocol -
A "contention slot", subdivided into bits, allows each station to announce that it wants to
send. After the announcement, all stations can send in priority order, and there will be
no fighting over the channel. Called "reservation protocol".
What are pros and cons of this method? Analyze at low and high loads.
Binary Countdown -
In the contention slot, each station places its ID. They all get or’d on top of each other.
A particular station knows if it won because no wanting-to-send station had a higher
number than it did in the slot. ( For instance, 101101 OR 110011 : The 101101
station knows it lost by the time it sends its second bit - it sees a “1” on the wire when it
just sent out a “0”, so it knows the game is up.

Chap. 4- MAC 17
Multiple Access
Protocols
LIMITED-CONTENTION
PROTOCOLS:
Collision techniques work well for low utilization (they're not likely to happen.)
Arbitration, which we'll talk about later, works better at high utilization. This
method provides best of these techniques.
Divide the stations up into groups. Stipulate that only members of group 0 can
arbitrate for slot 0, members of group 1 for slot 1, etc. Works because it cuts
down on the contention felt by any particular station.
Want a method that will have many members per group at low contention, and
few (or one) member at high contention. Can use a binary search to do this.

Chap. 4- MAC 18
Multiple Access
Protocols
Wireless LAN Protocols
IEEE 802.11
Wide Range of uses:
• Infrared signals within a building
• Mobile computing
• Network of low flying satellites
Physical Properties:
The spec allows running over three possible media
• Radio using frequency hopping
• Radio using direct sequencing
• Infrared over short distances ( 10 meters )
The explanation of this requires a detour into “spread spectrum radio”

Chap. 4- MAC 19
Multiple Access
Protocols
IEEE 802.11
Spread Spectrum Radio
• Purpose is to spread signal over a wider frequency range so as to
minimize interference ( military uses this for anti-jamming ).
• So the signal can be in a very noisy environment and still get through.
Frequency Hopping:
• Transmit the signal over a pseudo-random sequence of frequencies.
1st one frequency, then a second, etc. The sender and receiver are
using the same random number generator so they can stay in sync.
• The spec calls for using 79 different 1 MHz wide bandwidths.
Direct Sequence:
• Each bit of data is replaced by multiple bits in the signal.
• Transmitter sends the exclusive or of the data, PLUS n random bits.
• Again, both the sender and receiver know the random sequence.

Chap. 4- MAC 20
Multiple Access
Protocols
IEEE 802.11
Direct Sequence Example:
A. Data Stream 1 0 1 0
B. Random Sequence 0100101101011001
C. XOR of the two: 1011101110101001
A.
B.
C.

Chap. 4- MAC 21
Multiple Access
Protocols
IEEE 802.11
Collision Avoidance:
• Similar to Ethernet, but not quite the same.
• But more complicated because all nodes don’t see each other.
Example of the problem:
• A and C send a signal to B.
• But A and C aren’t aware of each other’s signals.
• Signals collide at B.
• But A and C don’t know they collided so don’t
go into collision avoidance.
A and C are “hidden nodes”
Example of another problem:
• B is sending to A.
• C can hear this signal from B
• C assumes it can NOT transmit
• But C could in fact transmit to D
This is called the “exposed node problem”
A. D.B. C.
C can “see” this range

Chap. 4- MAC 22
Multiple Access
Protocols
IEEE 802.11
Collision Avoidance:
Problem is solved by using a protocol Multiple Access with Collision
Avoidance ( MACA).
Sender and receiver exchange control frames so the transaction goes like this:
• Sender (A) does Request to Send (RTS) to receiver(B)
• B sends back Clear to Send (CTS)
• A sends packet.
• B sends an ACK after receiving the frame.
Logic:
• If a node hears the CTS, it knows it is
near the receiver - so don’t transmit.
• If a node hears the RTS but not the CTS,
it’s not near the receiver so it can
start its own transaction.
RTS and CTS contain length of packet
to be sent so others know how long to wait.
A. D.B. C.
C can “see” this range

Chap. 4- MAC 23
Multiple Access
Protocols
IEEE 802.11
Distribution System:
Some nodes roam (A - H in the figure.)
Some nodes are wired together -- Access Points (AP in the figure). These
“AP’s” are called the distribution system.
A.
D.
B.
C.
Distribution System
AP-1
AP-2 AP-3
H.
E.
F.
G.
The three regions shown
are like cells in a cell
phone system.
Two nodes ( A & B) could
communicate with each
other directly, but in
practice they go thru the
AP’s.
The path from A to E is:
A > AP-1 > AP-3 > E.
Nodes

Chap. 4- MAC 24
Multiple Access
Protocols
IEEE 802.11
Distribution System:
Protocol for how a node finds an access point:
A.
D.
B.
C.
Distribution System
AP-1
AP-2 AP-3
H.
E.
F.
G.
1. The node sends a Probe
frame.
2. All AP’s within range respond
with a “Probe Response”
frame.
3. The nodes selects one of the
AP’s & sends that AP an
Association Request frame.
4. That AP replies with an
Association Response
frame.
AP’s also send a Beacon frame
that advertises they are
available.

Chap. 4- MAC 25
Multiple Access
Protocols
IEEE 802.11
Frame Format:
Contains the following fields:
Control - is the frame carrying data or is it RTS or CTS or is it forwarding data.
Payload - up to 2312 bytes of data
CRC - checksum of the packet.
Addr(I) - It’s possible that the packet needs to be sent across the distribution
system in which case we keep track of the original sender and the original
receiver, but we also want to know intermediate senders and receivers.
Control Duration Addr1 Addr2 Addr3 SeqCtrl Addr4 Payload CRC
16 16 48 48 48 16 48 0-2313 32

Chap. 4- MAC 26
IEEE Standard 802
For LANs and
MANs
Overview
How do the protocols of the last sections
apply to real systems. Here we
talk about the actual standards in
use.
802.2 Describes the upper part of the data
link layer, the LLC (Logical Link
Control).
Descriptions of the physical and lower part of
the DLL are:
802.3 Is CSMA/CS LAN
802.4 Is Token Bus
802.5 Token Ring
Problem
4.4 Bridges
4.5 High Speed LANs

Chap. 4- MAC 27
IEEE Standard 802
IEEE STANDARD 802.3: ETHERNET
This is a 1-persistent CSMA/CD LAN. Originated in Aloha.
WIRES:
Name Cable Max Segment Nodes/seg. Advantages
10 Base 5 Thick Coax 500 m 100 Good for Backbones
10 Base 2 Thin Coax 200 m 30 Cheapest System
10 Base T Twisted Pair 100 m 1024 Easy Maintenance
10 Base F Fiber Optics 2000 m 1024 Best between buildings

Chap. 4- MAC 28
IEEE Standard 802
IEEE STANDARD 802.3: ETHERNET
Wiring
Repeaters - Multiple
cables can be
connected. From
software point, a
repeater is
transparent.

Chap. 4- MAC 29
IEEE Standard 802
Manchester Encoding
Life would be easy if:
binary 0 = 0 volts
binary 1 = 5 volts
But there's no way to distinguish a 0 from nothing-happening. Need to know
when is middle of bit WITHOUT a clock.

Chap. 4- MAC 30
IEEE Standard 802
802.3 MAC SUBLAYER PROTOCOL
Preamble == 7 bytes of 10101010
Start == 1 byte of 10101011
Dest == 6 bytes of mac address
multicast == sending to a group of stations.
broadcast == (dest. = all 1's) to all stations on network
Source == 6 bytes of mac address
Length == number of bytes of data
Data == comes down from network layer
Pad == ensures 64 bytes from dest addr thru checksum.
The pad ensures transmission takes enough time so it's still being sent when
the first bit reaches the destination. The frame needs to still be going out
when the noise burst from another stations collision detection gets back to
the sender.
checksum == 4 bytes of CRC.
Packet Definition

Chap. 4- MAC 31
IEEE Standard 802
Packet Definition
Why you need minimum Packet Size.

Chap. 4- MAC 32
IEEE Standard 802
BINARY EXPONENTIAL BACKOFF ALGORITHM:
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 X 4 repeaters = 512
bit times = 51.2 microseconds.
500 m  Rep  500 m  Rep  500 m  Rep  500 m  Rep
 500 m 
Algorithm adapts to number of stations.

Chap. 4- MAC 33
IEEE Standard 802
802.3 PERFORMANCE
Note that channel efficiency depends on
F -- frame length,
B -- network bandwidth,
L -- cable length
c -- speed of signal propagation
e -- optimal number of contention slots per frame. (512 bits = 64 bytes means a
64 byte frame has value == 1.) BUT, this is not the optimal value.
1
channel efficiency = ---------------------
1 + 2 B L e / c F
Note: Efforts focus on improving both B and L, both of which will decrease efficiency.
Note on traffic patterns; arrivals are not Poisson, but self similar. This means that
fluctuations occur on any observation scale (kind of like fractals.)

Chap. 4- MAC 34
IEEE Standard 802
Switched 802.3 LANs
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.

Chap. 4- MAC 35
IEEE Standard 802
IEEE STANDARD 802.4: TOKEN BUS:
Need 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 -
o All stations numbered; station knows # of its neighbors.
o A token, required in order to send, is initialized by the highest number
station.
o 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 -
o Stations can come and go on the bus, without breaking mechanism.
Cabling -
o Uses 75 ohm coax. Speeds are 1, 5, 10 Mbps.

Chap. 4- MAC 36
IEEE Standard 802
TOKEN BUS MAC SUBLAYER PROTOCOL:
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.

Chap. 4- MAC 37
IEEE Standard 802
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 data packets.
flag requiring ACK from receiver. shows type 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 - why not?)
Checksum - (Same as 802.3)

Chap. 4- MAC 38
IEEE Standard 802
TOKEN BUS:
LOGICAL RING MAINTENANCE
control frames for ring maintenance.
SOLICIT_SUCCESSOR -
Gives sender’s address and the current successor's address. Stations not in the ring,
with address between these two are invited to bid to be inserted.
• No response within given time ==> go on as before.
• One response ==> newcomer is inserted; becomes new successor.
• Two or more responses ==> answers collide so garbled.

Chap. 4- MAC 39
IEEE Standard 802
TOKEN BUS:
LOGICAL RING MAINTENANCE
control frames for ring maintenance.
RESOLVE_CONTENTION -
Causes responding stations to NOT immediately try to be successors, but use binary countdown by 0, 1,
2, or 3 slots. Mechanism also ensures that traffic isn't slowed down by solicitation.(limited to less
frantic times.)
SET_SUCCESSOR -
Used by a leaving station. Sent to the predecessor to say the leaver's successor is now the
predecessor's successor.
WHO_FOLLOWS -
The token sender listens to make sure the successor got and then passed on the token. If doesn't
happen, it sends a WHO_FOLLOWS and failed station's successor sends a SET_SUCCESSOR to
the failed one's predecessor.
SOLICIT_SUCCESSOR_2 -
The token sender can't find the successor and there's no response from WHO_FOLLOWS; This causes
ALL stations to once again bid for a place in the ring - this is like starting from scratch.
CLAIM_TOKEN -
If the token holder crashes, then nothing appears on the ring. All station's timers go off and the
contention algorithm determines who gets to generate the token.

Chap. 4- MAC 40
IEEE Standard 802
IEEE STANDARD 802.5: TOKEN RING
• 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 (usecs).
• At a speed of 200 m/usec, each bit occupies 200/R meters of the ring. So a 1
Mbps ring, with circumference 1000 meters has only 5 bits on it at any one time.
In addition, there's a 1 bit delay at each station. (Data bit can be modified before 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.

Chap. 4- MAC 41
IEEE Standard 802
• 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.

Chap. 4- MAC 42
IEEE Standard 802
Wires -
Shielded twisted pair/ 1 or 4 Mbps.
Differential Manchester encoding.
Reliability -- Star Shaped Ring --

Chap. 4- MAC 43
IEEE Standard 802
TOKEN RING MAC SUBLAYER
PROTOCOL:
Frame Structure Components -
SD, ED 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.

Chap. 4- MAC 44
IEEE Standard 802
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.

Chap. 4- MAC 45
IEEE Standard 802
PROTOCOL:
RING MAINTENANCE:
Monitor station oversees the ring, but on failure any station can become monitor.
CLAIM_TOKEN is a request to become the new monitor.
Monitor oversees:
• .Lost token management - If timer says token not seen in a while, produce new
one.
• .Orphan frames - (Frame on ring, but sender crashes before draining frame.) Sets
"monitor" bit in access control byte. If this bit seen as set the next time around,
then something is wrong.
• .Garbled frame - Monitor drains the frame and issues new token.
• .Delay time - Ensures enough delay so whole token fits on ring.
Broken rings handled by any station who thinks neighbors unreachable. Uses BEACON control
type.
Token management handled by the monitor so not de-centralized. Management easier, but
susceptible to berserk behavior.

Chap. 4- MAC 46
IEEE Standard 802
COMPARISONS OF 802.3, 802.4,
AND 802.5:
In great scheme of
things, differences
are small. All three
have approximately
same technology and
speed.
POSITIVES NEGATIVES
802.3 Large installed base. Has analog requirements.
Simple protocol. Must detect possible weak remote station.
Good configurability.
Passive and cheap cable. Minimum size = 64 bytes.
Low latency (no waiting Non-deterministic/no priorities.
for token.) Short cable length.
Efficiency drops at higher speeds.
802.4 Highly reliable hardware. Lots of analog.
More deterministic except Complex protocol.
when token is lost. Delay at low load waiting for token.
Supports priorities.
Good throughput and Small installed base.
efficiency.
Cable can support
multiple channels.
802.5 Connections are Centralized control means
Point-to-point. critical component.
Simple engineering. Delay at low load waiting for token.
Fully digital.
Use many media.
Priorities possible.
Short & long frames
possible.
Good throughput and
efficiency.

Chap. 4- MAC 47
IEEE Standard 802
IEEE 802.2: Logical Link Control
LLC
For when a reliable error-controlled flow-
controlled data link protocol is required.
Also hides differences inherent in the
802.3/4/5 from the network layer.
Three possible options:
o Unreliable datagram service.
o Acknowledged datagram service.
o Reliable connection-oriented service.
Destination
Address
Source
Address
Control Information
LLC
Protocol
Entity
LLC
Protocol
Entity
Set mode/Information/
Acknowledge/Poll
Set mode/Information/
Acknowledge/Poll

Chap. 4- MAC 48
BRIDGES
Overview
This is one way that networks are
connected together.
Bridges operate in the data link layer, and
so don’t have the intelligence to do
much address resolution.
What we will talk about here -
Translation from one LAN type to another.
Given a MAC address, how does a
packet get to its destination.
Problem
4.4 Bridges
4.5 High Speed LANs

Chap. 4- MAC 49
BRIDGES
The Big Picture
Hub or repeater just electronic amplification.
Bridges operate with active Data Link Layer. Can convert between different
physical/data link types. Way to connect multiple LANs.
Routers operate at Network layer - they read and depend on a specific protocol.
Protocol Converters are able to convert from one network layer type to another.
Detour on why to have multiple LANs -
Organizations have different LANs (802.3/4/5) to meet various needs.
• Cost - may make the cabling less expensive.
• To carry a combined load heavier than any one LAN could do.
• Total distance more than 2.5 Km.
• Bridges can act as firewalls, to partition against errant hardware.
• LANs broadcast everything on the LAN to all stations. May want to
prevent this from happening for some data. A bridge partitions off
these messages.

Chap. 4- MAC 50
BRIDGES
The Big Picture
How they work -

Chap. 4- MAC 51
BRIDGES From 802.x to 802.y
Issues -
• Each LAN type has its own frame format. Bridges take off one type and put on
another. <<< Figure 4.36 >>>
• LANs don't necessarily run at the same speed, so must reject or buffer the data
• Two input LANs feeding one output LAN.
• Each LAN type has it's own maximum data length. So bridges must do framing in
order to translate.
• Network layers may time out because they expect the destination to ACK within a
given time; all this translating slows down the transmission.
• All LAN types don't carry the same information:
o Priority bits.
o Acknowledgment bits.
In essence, the LAN standards are incompatible.

Chap. 4- MAC 52
BRIDGES Bridge Types
In addition to translating packets, bridges also route packets between source
and destination. It’s this function we now turn to.
Transparent Bridges and Source Routing Bridges are two competing and
mutually exclusive ways of routing packets.

Chap. 4- MAC 53
TRANSPARENT BRIDGES:
Also called Spanning Tree Bridges -
Goals:
• "Perfect" transparency. No one needs to do anything. It just works.
• No hardware or software configuration required.
• No switches, no routing tables.
• Stateless (or as stateless as possible.)
Promiscuous mode:
• Accepts all packets from all LANs attached to the bridge.
• If destination is on incoming LAN, discard the packet.
• Otherwise, forward the packet.
• Use table (hashed) in bridge to determine choice of the LAN for forwarding.

Chap. 4- MAC 54
Parallel Redundant Bridges:
Here two or more bridges are used for
reliability.
Problems with infinite flooding.
Solution is to overlay the Backward Learning
policy with a virtual loop-free topology.
A Spanning Tree Bridge does this.
Note how paths are reduced.

Chap. 4- MAC 55
Parallel Redundant Bridges
(cont):
Algorithm is as follows:
• Choose one bridge to be root of tree
(lowest unique serial number wins.)
• Each bridge determines cost of the path from the root bridge to each of its ports. (The root
path cost.) Cost determined by number of segments and the bit rate of those segments.
• Determine the root port - for a bridge, which of its ports has the lowest root path cost.
• Determine the designated bridge - the bridge that will handle requests for a particular LAN
(even though that LAN may have several bridges attached to it.) Selection based on
smallest path cost from the segment to the root bridge.
• Tree includes every LAN but not necessarily every bridge.
• Continues to check for topology changes.

Chap. 4- MAC 56
SOURCE ROUTING BRIDGES:
Used by IBM/rings. Here the sender holds ALL knowledge of how a packet
should be routed.
The sender of a frame:
• Knows if destination is on its own LAN.
• Sets a bit alerting bridges destination NOT on its LAN.
• Places, in header, the exact path frame will follow.

Chap. 4- MAC 57
SOURCE ROUTING
BRIDGES(cont):
In the figure above, the Path from A to D is L1, B1, L2, B2, L3.
The Bridge looks for bit set.
• Scans route - is the incoming LAN number followed by the number of the bridge doing
the looking? If so, forward the frame, otherwise reject it.
• Can be done in software, hybrid, hardware.
• If the source doesn't know the route, it sends a "discovery frame" that goes to every
LAN in the network. The destination replies and each bridge along the way puts its ID
in that reply. The source then knows all that it needs. This discovery produces lots of
excess packets.

Chap. 4- MAC 58
COMPARISON OF 802 BRIDGES

Chap. 4- MAC 59
High Speed LANs
Overview
xxxx4.1 The Channel Allocation
Problem
4.4 Bridges
4.5 High Speed LANs

Chap. 4- MAC 60
SUMMARY
2.1 Theoretical Basis For Data Communication
What every sophomore EE knows !!! How much data can be put on a
wire? What are the limits imposed by a medium?
2.2 Transmission Media
Wires and fibers.
2.3 Wireless Transmission
Radio, microwave, infrared, unguided by a medium.
2.4 The Telephone System
The system invented 100 years ago to carry voice.
2.5 Narrowband ISDN
Mechanisms that can carry voice and data.
Here we want to know how to handle broadcast networks. As compared to
point to point networks, a major issue is handling arbitration when there is
competition for the network.
This is the bottom sublayer of the Data Link Layer. This Chapter is especially
relevant for LANs.
4.1 The Channel Allocation Problem
How to allocate a single channel among multiple users.
How to handle contention for the use of a channel.
How do the protocols of the last sections apply to real systems. Here we talk
about the actual standards in use.
4.4 Bridges
Ways of connecting networks together.
4.5 High Speed LANs
Directions in high speed networks.

MAC

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