MAC Layer Survey

1
MAC Layer Survey
2
The Medium Access Sublayer

• MAC Medium Access Control
• As opposed to high access control or low
  access control?
• NO!
• It means controlling access to the medium.
• provides arbitration mechanism for shared medium
• What are arbitration mechanisms in this
  classroom?
• Raising your hand
• Verbal cues, e.g. go ahead, Fred.
• Eye contact
• Background review of
• Broadcast vs. Point-to-Point media
• Topologies Bus, Ring
• Then Static vs. Dynamic channel allocation
• Finally ALOHA, and CSMA/CD (Ethernet)

3
Broadcast Networks vs. Point to Point
networks

• built from cables connecting single machines.
• often used to connect LANs into an internetwork
  (internet)

• typically use a shared cable
• typically are LAN technologies
• examples
• Ethernet
• Cable Modems

• a special case Token Ring
• wiring is point to point,
• but treated like broadcast

4
The Channel Allocation Problem (continued)

• Statistical multiplexing is preferred to FDM and
  TDM.
• Arrange all the packets in a giant central queue,
  and share the bandwidth on a first come first
  served basis.
• Dont pre-allocate bandwidth for users that may
  just waste it.
• For details, need to review some basic queuing
  theory, and know what an exponential and poisson
  distribution are.

5
Dynamic channel allocation

• Five characteristics of channel allocation
  models
• 1) Station model stations independent
• 2) Single channel assumption
• 3) Collisions
• 4) Continuous (free-for-all, pure Aloha) vs
  Discrete Time (time is slotted)
• 5) Carrier sense (polite, listen first) vs. no
  Carrier Sense (just blurt out)

6
Aloha

• 1970s Norman Abramson et. al, University of
  Hawaii
• Send packets on shared channel if collisions
  occur,wait random amount of time, and try again.
• Original purpose
• host to terminal communication (very bursty)
• ground-based radio channel want to share it.
• However, basic ideas have application to any
  environmentwhere there is a shared medium, and
  uncoordinated users
• a bus-topology LAN
• the up-link part of a Satellite channel

7
Two variationsof Aloha
8
Efficiency of pure aloha...

• When does frame X suffer no collisions? When
  there are no frames that overlap with frame
  Xi.e., only one frame starts in an interval of
  2t. (Frames that start after t0 2t are ok).

9
Channel allocationmethods...
FDM
TDM
Pure ALOHA
Slotted ALOHA
1-persistent CSMA
Nonpersistent CSMA
P-persistent CSMA
CSMA/CD

• then an actual productthat applies CSMA/CD

10
Carrier Sense Multiple Access

• Listen the channel before you transmit!
• If the channel is busy dont transmit if its
  idle, transmit.
• Most basic form 1-persistent CSMA
• If busy, listen persistently until channel is
  idle, then transmit immediately with probability
  1.
• Problem with 1-persistent CSMA
• Q If any two stations B,C become ready during
  As transmission, what happens
• A guaranteed collision!

11
Carrier Sense Multiple Access

• Listen the channel before you transmit!
• Most basic form 1-persistent CSMA

• Sense the channel first

if idle, transmit immediately.

• then transmit immediately with probability 1.

12
Problem with 1-persistent CSMA
Q If any two stations B,C become ready during
As transmission, what happens?
A Collision is guaranteed!

• To address this problem, researchers have
  proposed
• non-persistent CSMA
• p-persistent CSMA (for slotted channels).

13
Non-persistent and p-persistent CSMA
p-persistent CSMA (for slotted channels). when
idle, transmit with probability p w/ prob (1-p),
wait for next slot.
Do we expect fewer collisions with these
methods? What is the tradeoff?
14
Performance of CSMA protocols vs. Aloha

• Tanenbaum doesnt say where this analysis comes
  from, or what the assumptions are. But it does
  make the tradeoff clear
• less aggressive transmission means higher
  throughput, because there are fewer collisions,
• but also higher delay!

15
Collision detection

• All of the previous protocol (ALOHA, CSMA) assume
  collision a total loss.
• CSMA with Collision Detection (CSMA/CD) makes a
  different assumption
• we can detect a collision,and abort immediately,
  so we dont waste an entire slot.
• So, use 1-persistent CSMA, but if there is a
  collision, immediately stop, and wait a random
  period of time before trying again.

• Ethernet and 802.3 use CSMA/CD.

16
Detecting collisions on a Bus network...

• If ? is the maximum propagation time on a bus
  network, then it can take up to twice that long
  (2?) for a collision to be detected.
• This turns out to be a an important design
  parameter, as we will see.

17
If you need 2? to detect collisions...

• then you need to make sure that a frame isnt
  shorter then 2?
• Otherwise, a station might send an entire frame,
  and not realize that it had collided with
  something!
• Thus, the minimum frame size in classic 10Mbps
  Ethernet 64bytes.
• What about 100Mbps Ethernet?
• If minimum frame size is the same, and bit rate
  is 10 times higher,and you still need 2? to
  detect collisions, then what must havechanged?
• the maximum cable length has to be 10 times
  shorter.

18
Ethernet and 802.3

• Ethernet came first, invented at Xerox PARC (Palo
  Alto Research Center).(Bob Metcalfe was key
  figure later went on to be a founder of 3COM.)
• Xerox, Digital, and Intel were the partners in
  original commercial Ethernet spec.
• Later, IEEE make 802.3 a standard
• lots of variations at various bit rates using
  various media,
• changed packet type field to a length field.
  (packet type values are all illegal lengths, so
  its easy to distinguish)
• For most purposes of our discussion, well
  consider them one and the same, although purists
  would insist that they are different standards.

19
802.3 Collision Detection is analog process

• Station must read voltage on wire to see if it is
  the same aswhat the station is sending out.
• Collision of two 0 volt signals would be hard
  to detect!
• This is a motivation for Manchester
  EncodingVoltage is either 0.85 or -0.85, so if
  you add two signals together, with very high
  probability you will get an illegal voltage
  within a few bits.

20
Ethernet

• dominant LAN technology
• cheap 20 for 100Mbs!
• first widely used LAN technology
• Simpler, cheaper than token LANs and ATM
• Kept up with speed race 10, 100, 1000 Mbps

Metcalfes Ethernet sketch
21
Ethernet Frame Structure

• Sending adapter encapsulates IP datagram (or
  other network layer protocol packet) in Ethernet
  frame
• Preamble
• 7 bytes with pattern 10101010 followed by one
  byte with pattern 10101011
• used to synchronize receiver, sender clock rates

22
Ethernet Frame Structure (more)

• Addresses 6 bytes, frame is received by all
  adapters on a LAN and dropped if address does not
  match
• Type indicates the higher layer protocol, mostly
  IP but others may be supported such as Novell IPX
  and AppleTalk)
• CRC checked at receiver, if error is detected,
  the frame is simply dropped

23
Ethernet uses CSMA/CD

• A sense channel, if idle
• then
• transmit and monitor the channel
• If detect another transmission
• then
• abort and send jam signal
• update collisions
• delay as required by exponential backoff
  algorithm
• goto A
• else done with the frame set collisions to
  zero
• else wait until ongoing transmission is over and
  goto A

24
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision 48 bits
• Exponential Backoff
• Goal adapt retransmission attemtps to estimated
  current load
• heavy load random wait will be longer
• first collision choose K from 0,1 delay is K
  x 512 bit transmission times
• after second collision choose K from 0,1,2,3
• after ten or more collisions, choose K from
  0,1,2,3,4,,1023

25
Ethernet Technologies 10Base2

• 10 10Mbps 2 under 200 meters max cable length
• thin coaxial cable in a bus topology
• repeaters used to connect up to multiple segments
• repeater repeats bits it hears on one interface
  to its other interfaces physical layer device
  only!

26
10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Hub to which nodes are connected by twisted pair,
  thus star topology

27
LAN Addresses and ARP

• 32-bit IP address
• network-layer address
• used to get datagram to destination network
  (recall IP network definition)
• LAN (or MAC or physical) address
• used to get datagram from one interface to
  another physically-connected interface (same
  network)
• 48 bit MAC address (for most LANs) burned in the
  adapter ROM

28
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
29
LAN Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address
• MAC flat address gt portability
• can move LAN card from one LAN to another
• IP hierarchical address NOT portable
• depends on network to which one attaches

30
Recall earlier routing discussion

• Starting at A, given IP datagram addressed to B
• look up net. address of B, find B on same net. as
  A
• link layer send datagram to B inside link-layer
  frame

frame source, dest address
datagram source, dest address
As IP addr
Bs IP addr
Bs MAC addr
As MAC addr
IP payload
datagram
frame
31
ARP Address Resolution Protocol

• Each IP node (Host, Router) on LAN has ARP
  module, table
• ARP Table IP/MAC address mappings for some LAN
  nodes
• lt IP address MAC address TTLgt
• lt .. gt
• TTL (Time To Live) time after which address
  mapping will be forgotten (typically 20 min)

32
ARP protocol

• A knows B's IP address, wants to learn physical
  address of B
• A broadcasts ARP query pkt, containing B's IP
  address
• all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
  (B's) physical layer address
• A caches (saves) IP-to-physical address pairs
  until information becomes old (times out)
• soft state information that times out (goes
  away) unless refreshed

33
Routing to another LAN

• walkthrough routing from A to B via R
• In routing table at source Host, find router
  111.111.111.110
• In ARP table at source, find MAC address
  E6-E9-00-17-BB-4B, etc

A
R
B
34

• A creates IP packet with source A, destination B
• A uses ARP to get Rs physical layer address for
  111.111.111.110
• A creates Ethernet frame with R's physical
  address as dest, Ethernet frame contains A-to-B
  IP datagram
• As data link layer sends Ethernet frame
• Rs data link layer receives Ethernet frame
• R removes IP datagram from Ethernet frame, sees
  its destined to B
• R uses ARP to get Bs physical layer address
• R creates frame containing A-to-B IP datagram
  sends to B

A
R
B
35
Bridges, Internetworking

• Before talking about bridges in the context of
  the MAC sub-layer, lets briefly skip ahead, and
  cover the differences among

gateways
routers
bridges
repeaters
36
Repeaters

• Copy individual bits between cable segments, and
  that is all.
• Operate at physical layer only.

37
Bridges

• Operate at Data link layer, and may connect two
  similar LANs,or two dissimilar, but at least
  partially compatible LANs.
• For dissimilar LANs (e.g. 802.x to 802.y, where
  x?y)differences in frame size, priority schemes,
  etc. create headachesand incompatibilities.

38
Routers

• Operate at Network layer. Usually connect two
  networks using same network layer.
• Tanenbaum discusses multiprotocol routers
• Often these connect two networks that have
  multiple network protocols, but dont do
  conversions from one to another.(This is how I
  have seen them used.)
• Tanenbaum implies sometimes they convert from
  one network layer to another.

39
Gateways

• Operate at Transport and/or Application Layer
• Examples
• a gateway between Internet (TCP/IP) SMTP-based
  email and a proprietary email system like IBMs
  PROFS (based on SNA) or Digitals (DECs)
  All-in-one or VAX/VMS Mail (based on DECnet).
• a gateway between SNAs 3270 terminals and
  TCP/IP Telnet style terminal sessions.
• Proxy server

40
Summary Repeaters, Bridges, Routers, Gateways

• The distinction lies mainly in the highest
  layer at which each operates.
• Although this terminology is fairly standard, and
  the preferred common usage, the term gateway is
  sometimes used in the Internet community as a
  synonym for router. (for example, in the name
  of the Exterior Gateway Protocol.)
• Now lets turn our attention back to a possibly
  confusing questions about bridges
• do bridges do routing?

41

• Ethernet Repeaters
• each segment can be 500m
• no more than 4 repeatersbetween any two
  computers
• maximum distance 2500m
• a 10BaseT hub countsas a repeater.

• Ethernet Bridges for collisions/noise/traffic
  each segment looks like a separate Ethernet
• BUT, for forwarding frames to destination
  Ethernet addresses the entire network seems to
  bea single large Ethernet
• The bridges handle routing

42
Bridges can connect more than one LAN

• We can build an extended LAN by connecting up
  smaller LANs through bridges
• Goal
• (LAN3, LAN4, LAN5, LAN6) separate for collisions,
  local traffic
• (LAN3, LAN4, LAN5, LAN6) all one LAN for sending,
  addressing
• Bridges functions as data-link layer routers
• Routing tables are self-configuring. Four
  (equiv.) names for this
• adaptive bridges backwards-learning bridges
• learning bridges transparent bridges

43
How adaptive bridges learn
seg. 2
seg. 1

• The bridge B has this goal only forward frames
  when necessary!
• dont forward local traffic between hosts on seg.
  1 to seg. 2, and vice-versa
• e.g., traffic between u and w should stay local
  to seg. 1
• How is this achieved?
• At first forwarding every incoming frame onto
  every destination lan (flooding)
• As the bridge learns where hosts are, it only
  forwards the frame if necessary.
• How does it learn where hosts live? By
  backward learning
• We figure out the destination seg. for packets
  going to y, by looking at the source seg. for
  packets coming from y.

44
Startup/Steady State
LAN 2
LAN 1

• The startup behavior is to forward every frame
  everywhere (flooding)
• The steady state is to forward frames ONLY where
  they belong.

45
Do Bridges Do Routing?Where the confusion can
arise...

• In discussing bridge design, we may talk about
  routing tables in the bridges and how routing
  is done.
• e.g. sending a frame D?A vs. D?C vs. D?H.
• Were talking about routings packets at the Data
  Link Layer through an interconnection of LAN
  segments that make up an extended LAN.
• This is similar in function (in some ways) to
  routing at the network layer, but it is NOT the
  network layer the network layer sits above
  all this.

46
Planning A Bridged Network

• Bridges allow parallelism, and isolate traffic.
• Simultaneously (A?B) (E?D) (F?H) without any
  collision.
• Exploit locality of reference idea is that
  most accesses are to
• local file servers
• local printers
• So put users of a server/printer on the same
  bridged segment
• keep local traffic local

47
Bridging Between BuildingsGoal is to keep local
traffic local!
building 1
building 2
Ethernet
Ethernet
optical fibers
Bad Traffic from both sides goes across the
fiber optic link
fiber modem
fiber modem
Good local trafficstays local
building 1
building 2
bridge
optical fibers between buildings
fiber modem
fiber modem
48
Bridging Across Longer Distances Keep local
traffic off the long-distance link
satellite
LAN 2
LAN 1
bridge
bridge
49
A Cycle Of Bridges

• In transparent bridging the idea is, you just
  plug and play
• the bridges configure themselves!
• but what if you put in a cycle?
• What will happen?

50
Problems with a cycle of bridges

• The fear
• more than one bridge may forward a frame,
  creating wasteful duplicates
• loops may cause frames to be forwarded forever
• Example
• Suppose that frame F is going to an unknown
  destination,
• Both bridge B1 and bridge B2 use flooding (send
  it everywhere)
• Now what happens when B1 sees frame F2, and B2
  sees frame F1?
• They will forward it back to LAN1, creating
  frames F3 and F4
• etc. etc. etc.

51
Easy solution for LANs Create a Spanning Tree
The Resulting Spanning Tree
Interconnected LANs

• Lowest serial number bridge becomes the root of
  the tree (leader election problem). Perlmans
  algorithm used to do computation.
• Note that this does not scale up to WANs it
  would be wasteful to not usethe capacity of the
  non-tree links.
• An example of how LAN routing (data link layer)
  differs from WAN routing (network layer)

52
Backward Learning in Transparent Bridges

• Transparency idea plug and play no
  configuration needed!
• Review of the algorithm
• Build a hash table where key is Ethernet address,
  data is LAN id. (initially empty)
• When packet arrives, put source address in table
  along with LAN it came in on.(this is the
  backward learning part we learn destinations, by
  looking at sources!)
• Look up destination address in table.
• if (found) if (source LANdest LAN) do
  nothing frame is already where it needs to be
  else send out on destination LAN.
• else we dont know where it is, so send out on
  every interface (flooding.)
• Periodically purge entries from table
  (soft-state) in case hosts moveor LAN
  configuration changes.

53
Source Routing Bridges (used in IBM Token Ring,
802.5)

• Source chooses the route the host.
• Route chosen by using discovery packets
• broadcast packet from x saying what is route to
  y?
• bridges record route taken use info to get reply
  back to x.
• x receives a reply for every possible route to y,
  and picks the best one.
• Advantage can use the bandwidth more
  effectively.Transparent bridges only use the
  spanning tree.
• Disadvantages
• Not transparent.(not plug and play).Administrat
  or must configure bridge addresses to be unique
  among all bridges attached to a particular LAN
• Frame explosion problem discovery frames can
  grow exponentially. (only linear growth with
  flooding in spanning tree bridges.)