Chapter 5: The Data Link Layer

1
Chapter 5 The Data Link Layer

• Our goals
• understand principles behind data link layer
  services
• error detection, correction
• sharing a broadcast channel multiple access
• link layer addressing
• reliable data transfer, flow control done!
• instantiation and implementation of various link
  layer technologies

2
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

3
Link Layer Introduction

• Some terminology
• hosts and routers are nodes
• communication channels that connect adjacent
  nodes along communication path are links
• wired links
• wireless links
• LANs
• 2-PDU is a frame, encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node to adjacent
node over a link
4
Link layer context

• Datagram transferred by different link protocols
  over different links
• e.g., Ethernet on first link, frame relay on
  intermediate links, 802.11 on last link
• Each link protocol provides different services
• e.g., may or may not provide reliable data
  transfer over link

5
Link Layer Services

• Framing
• encapsulate datagram into frame, adding header,
  trailer
• physical addresses used in frame headers to
  identify source, dest
• different from IP addresses!
• Link access
• Media Access Control (MAC) protocols coordinate
  frame transmissions of nodes that share a
  broadcast link
• Reliable delivery between adjacent nodes
• we learned how to do this already (chapter 3)!
• seldom used on low bit-error links (fiber, coax,
  twisted pair)
• often used for wireless links high error rates

6
Link Layer Services (more)

• Flow Control
• pacing between adjacent sending and receiving
  nodes
• Error Detection
• errors caused by signal attenuation, noise.
• sender includes error-detection bits in frame
• receiver detects presence of errors
• signals sender for retransmission or drops frame
• Error Correction
• receiver identifies and corrects bit error(s)
  without resorting to retransmission
• Half-duplex and full-duplex
• with half duplex, nodes at both ends of link can
  transmit, but not at same time

7
Adaptors Communicating
datagram
rcving node
link layer protocol
sending node
adapter
adapter

• receiving side
• error checking, rdt, flow control, random access,
  etc
• extracts datagram, passes to network layer

• link layer implemented in adaptor (aka NIC)
• Ethernet card, 802.11 card
• sending side
• encapsulates datagram in a frame
• adds error checking bits, rdt, flow control,
  random access, etc.

8
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

9
Error Detection

• EDC Error Detection and Correction bits
  (redundancy)
• D Data protected by error checking, may
  include header fields
• Error detection not 100 reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and
  correction

10
Parity Check
Two Dimensional Parity Bit Detect and correct
single bit errors

• Single Parity Bit
• suppose d bits of data
• even/odd parity add one additional bit such that
  the total number of 1s in the d1 bits is
  even/odd
• can detect an odd number of bit errors can not
  detect an even number of bit errors

Example of even parity 0111000110101011 1
o
o
d data bits
parity bit
11
Internet checksum (RFC 1071)
Goal detect errors (e.g., flipped bits) in
transmitted segment (used at transport layer)

• Receiver
• compute 1s complement sum of received data,
  including the checksum field
• check if the result contains all 1 bits
• NO - error detected
• YES - no error detected but may be errors
  nonetheless.

• Sender
• treat data as sequence of 16-bit integers
• compute 1s complement sum of data
• add carry-out (17th bit) back into the lowest
  significant bit
• put 1's complement of the sum into checksum field

12
Cyclic Redundancy Check (CRC)

• often used in link layer
• view data bits, D, as a binary number
• sender, receiver agree on an r1 bit pattern
  (generator), G, leftmost bit of G must be 1
• goal choose r CRC bits, R, such that
• ltD,Rgt exactly divisible by G (modulo 2)
• receiver knows G, divides ltD,Rgt by G. If
  non-zero remainder error detected!

13
CRC Example
Example D101110, G1001, r3

• Want
• D.2r XOR R nG
• equivalently
• D.2r nG XOR R
• equivalently
• if we divide D.2r by G, remainder is R

D.2r G
R remainder
In CRC calculations, addition and subtraction
are equivalent to bitwise exclusive-or (XOR)
Sender transmit 101110 011
14
Common generators

• CRC-8 100000111
• CRC-12 1100000001101
• CRC-16 1100000000000101
• CRC-32 100000100110000010001110110110111
• Can detect burst errors of less than r1 bits
• Can detect any odd number of bit errors

15
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

16
Multiple Access Links

• Two types of links
• point-to-point
• single sender, single receiver
• link between two routers, or between a dial-up
  modem and an ISP router
• broadcast (shared wire or medium)
• multiple sending and receiving nodes connected to
  single shared channel
• Ethernet, 802.11 wireless LAN

17
Multiple Access protocols

• single shared broadcast channel
• two or more simultaneous transmissions by nodes
  collision
• only one node can send successfully at a time
• multiple access protocol
• determine how nodes share channel, i.e.,
  determine when node can transmit

18
Ideal Multiple Access Protocol

• Broadcast channel of rate R bps
• 1. When only one node wants to transmit, it can
  send at rate R.
• 2. When M nodes want to transmit, each can send
  at average rate R/M
• 3. Fully decentralized no special node to
  coordinate transmissions
• 4. Simple

19
Multiple Access Protocols a taxonomy

• Three broad classes
• Channel Partitioning Protocols
• divide channel into smaller pieces (time slots,
  frequency, code)
• allocate piece to node for exclusive use
• Random Access Protocols
• channel not divided, allow collisions
• recover from collisions
• Taking-turns Protocols
• tightly coordinate shared access to avoid
  collisions

20
Channel Partitioning TDMA

• TDMA time division multiple access
• divide time into time frames and divide each time
  frame into fixed length time slots (length pkt
  trans time)
• each station gets one slot in each frame
• unused slots go idle
• example 6-station LAN, 1,3,4 have pkt, slots
  2,5,6 idle

21
Channel Partitioning TDMA

• Pros
• No collisions
• Fair each node gets a dedicated transmission
  rate of R/N bps
• Cons
• inefficient at low load delay in channel access,
  1/N bandwidth allocated even if only 1 active
  node!

22
Channel Partitioning FDMA

• FDMA frequency division multiple access
• channel spectrum divided into frequency bands
• each station assigned a frequency band
• unused transmission time in frequency bands go
  idle
• example 6-station LAN, 1,3,4 have pkt, frequency
  bands 2,5,6 idle

time
1

• Pros
• no collisions, fair
• Cons
• inefficient at low load 1/N bandwidth allocated
  even if only 1 active node!

2
3
frequency bands
4
5
6
23
Channel Partitioning CDMA

• CDMA Code Division Multiple Access
• used mostly in wireless broadcast channels
  (cellular, satellite, etc)
• all users share same frequency, but each user
  assigned unique code to encode data
• encoded signal original bit X M-bit code
• decoding
• original bit normalized inner-product of
  encoded signal S and code C
• i.e., original bit

24
CDMA Encode/Decode
25
CDMA two-sender interference

• allows multiple users to
• transmit simultaneously
• with no interference if
• codes are orthogonal
• two codes are orthogonal if their normalized
  inner-product is 0.
• e.g. (1,1,1,-1,1,-1,-1,-1) and (1,-1,1,1,1,-1,1,1)
  are orthogonal

26
Random Access Protocols

• When node has packet to send
• transmit at full channel data rate R.
• no a priori coordination among nodes
• two or more transmitting nodes -gt collision,
• random access protocol specifies
• how to recover from collisions (e.g., via delayed
  retransmissions)
• Examples of random access protocols
• slotted ALOHA
• ALOHA
• CSMA, CSMA/CD, CSMA/CA

27
Slotted ALOHA

• Assumptions
• all frames same size
• time is divided into equal sized slots, a slot
  equals the time to transmit one frame
• nodes start to transmit frames only at beginning
  of slots
• nodes are synchronized
• if two or more nodes transmit in a slot, all
  nodes detect collision before the slot ends

• Operation
• when node has a fresh frame to send, it transmits
  in next slot
• no collision, node can send new frame in next
  slot if it has one
• if collision, node retransmits frame in each
  subsequent slot with probability p until success

28
Slotted ALOHA
C collision slot, E empty slot, S
successful slot

• Pros
• single active node can continuously transmit at
  full rate of channel
• highly decentralized each node independently
  decides when to retransmit, only slots in nodes
  need to be in sync
• Simple
• Cons
• collisions waste slots
• idle slots

29
Slotted Aloha efficiency

• For max efficiency with N nodes, find p (1/N)
  that maximizes Np(1-p)N-1
• For many nodes, take limit of Np(1-p)N-1 as N
  goes to infinity, gives 1/e .37

Efficiency is the long-run fraction of
successful slots when there are many nodes, each
with many frames to send

• Suppose N nodes with many frames to send, each
  transmits in slot with probability p
• prob that a given node has success in a slot
  p(1-p)N-1
• prob that any node has a success Np(1-p)N-1

At best channel used for useful transmissions
37 of time!
30
Pure (unslotted) ALOHA

• unslotted Aloha simpler, no synchronization
• when frame first arrives transmit immediately
• If collision, retransmit with prob. P, or wait
  for a frame transmission time with prob. 1-p
• collision probability increases
• frame sent at t0 collides with other frames sent
  in t0-1,t01

31
Pure Aloha efficiency

• P(success by given node) P(node transmits) .
• P(no
  other node transmits in t0-1,t0 .
• P(no
  other node transmits in t0,t0 1
• p .
  (1-p)N-1 . (1-p)N-1
• p .
  (1-p)2(N-1)
• choosing optimum
  p and then letting n -gt infty ...
• 
  1/(2e) .18

Even worse ! Price paid for a fully decentralized
protocol.
32
CSMA (Carrier Sense Multiple Access)

• CSMA listen before transmit
• If channel sensed idle transmit entire frame
• If channel sensed busy, wait a random amount of
  time and sense the channel again
• Human analogy dont interrupt others!

33
CSMA collisions
spatial layout of nodes
collisions can still occur propagation delay
means two nodes may not hear each others
transmission
collision entire packet transmission time wasted
note role of propagation delay in determining
collision probability The longer is propagation
delay, the larger is the chance that a
carrier-sensing node is not able to sense a
transmission that has already begun at another
node
34
CSMA/CD (Collision Detection)

• CSMA/CD carrier sensing, deferral as in CSMA
• Listen to channel while transmitting
• If collision detected, stop transmitting,
  reducing channel wastage

35
Taking Turns protocols

• channel partitioning protocols
• share channel efficiently and fairly at high load
• inefficient at low load 1/N bandwidth allocated
  even if only 1 active node
• Random access protocols
• efficient at low load single node can fully
  utilize channel
• high load collision overhead
• taking turns protocols
• look for best of both worlds!

36
Taking Turns protocols

• Polling
• master node invites each node to transmit in
  turn
• concerns
• polling delay
• single point of failure (master node)

• Token passing
• decentralized no master node
• token passed from one node to next sequentially.
• when receive a token
• send up to a maximum number of frames if has
  frames to send
• forward token to next node
• concerns
• latency
• single point of failure (token)

37
Summary of multiple access protocols

• What do you do with a shared media?
• Channel Partitioning, by time, frequency or code
• Time Division, Frequency Division, Code Division
• Random access
• ALOHA, Slotted-ALOHA, CSMA, CSMA/CD
• Taking-turns
• polling from a master node, token passing

38
LAN technologies

• addressing
• Ethernet
• hubs, bridges, switches
• 802.11

39
LAN Addresses

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

40
LAN Addresses
Each adapter on LAN has unique LAN address
41
LAN Address (more)

• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address space
  (to assure uniqueness)
• MAC flat address gt portability
• does not change no matter where the adaptor goes
• IP hierarchical address NOT portable
• depends on IP network to which node is attached
• Analogy
• (a) MAC address like Social Security
  Number
• (b) IP address like postal address

42
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 dest, source address
datagram source, dest address
As IP addr
Bs IP addr
Bs MAC addr
As MAC addr
IP payload
datagram
frame

• When adaptor in B receives the frame, pass
  datagram to IP
• Other adaptors discard the frame

43
ARP Address Resolution Protocol (RFC826)

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

44
ARP protocol

• A wants to send a datagram to B on the same LAN
• Suppose Bs MAC address is not in As ARP table
• A broadcasts (using broadcast address
  FF-FF-FF-FF-FF-FF) an ARP request packet,
  containing As IP address, As MAC address and
  B's IP address
• All adaptors on LAN receive ARP request
• Every node if As IP-to-MAC mapping is in table,
  update table
• B replies to A (unicast) with its MAC address,
  also adds A into its ARP table if its not there
• A receives the ARP reply, adds B's IP and MAC
  addresses to its ARP table.
• Soft state IP-to-MAC address pair in ARP table
  times out (goes away) unless refreshed
• ARP is plug-and-play nodes create their ARP
  tables without intervention from network
  administrator
• ARP resolves IP addresses only for nodes on the
  same LAN

45
Routing to another LAN

• walkthrough send datagram from A to B via R
• assume A knows Bs IP
  address
• Two ARP tables in router R, one for each IP
  network (LAN)

A
R
B
46

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

A
R
B
47
Ethernet (IEEE 802.3)

• dominant LAN technology
• first widely deployed LAN technology
• simpler, cheaper than token ring, FDDI, and ATM
• Lesson learned KISS (Keep It Simple, Stupid)
• kept up with speed race 10, 100, 1000 Mbps

Metcalfes Ethernet sketch
48
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

49
Ethernet Frame Structure (more)

• Addresses 6 bytes each
• if adapter receives frame with matching
  destination address, or with broadcast address
  (e.g. ARP request), it passes data in frame to
  network layer
• otherwise, adapter discards frame
• Type (2 bytes) indicates the higher layer
  protocol, mostly IP but others may be supported
  (such as Novell IPX and AppleTalk)
• Data (46-1500 bytes) MTU is 1500 bytes, MIN
  frame size 46 18 64 bytes 512 bits
• CRC (4 bytes) checked at receiver, if error is
  detected, the frame is simply dropped

50
Unreliable, connectionless service

• Connectionless No handshaking between sending
  and receiving adapter.
• Unreliable receiving adapter doesnt send ACKs
  or NAKs to sending adapter
• stream of datagrams passed to network layer can
  have gaps
• gaps will be filled if app is using TCP
• otherwise, app will see the gaps

51
Ethernet uses CSMA/CD

• No slots
• adapter doesnt transmit if it senses that some
  other adapter is transmitting, that is, carrier
  sense
• transmitting adapter aborts when it senses that
  another adapter is transmitting, that is,
  collision detection

• Before attempting a retransmission, adapter waits
  a random time

52
Ethernet CSMA/CD algorithm

• 1. Adaptor gets datagram from network layer and
  creates frame
• 2. If adapter senses channel idle, it starts to
  transmit frame. If it senses channel busy, waits
  until channel idle (plus 96 bit times) and then
  transmits
• 3. If adapter transmits entire frame without
  detecting another transmission, the adapter is
  done with frame!

• 4. If adapter detects another transmission while
  transmitting, aborts and sends a 48-bit jam
  signal
• 5. After aborting, adapter enters exponential
  backoff after the mth collision, adapter chooses
  a K at random from 0,1,2,,2m-1 where m
  min(m,10). Adapter waits K512 bit times and
  returns to Step 2

53
Ethernets CSMA/CD (more)

• Jam Signal make sure all other transmitters are
  aware of collision
• Bit time 0.1 microsec for 10 Mbps Ethernetfor
  K1023, wait time is about 50 msec

• Exponential Backoff
• Goal adapt retransmission attempts 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

54
Why 64 bytes min frame length?

• 10Base5 Ethernet 10Mbps, max segment 500m, max 4
  repeaters, max network diameter 2500m
• Repeater physical layer device that amplifies
  and retransmits bits it hears on one interface to
  its other interfaces, used to connect multiple
  segments
• Round trip time (worst case collision detection
  time) about 50 microsec
• All frames must take more than 50 microsec to
  send so that transmission is still taking place
  when the noise burst gets back to sender
• With 10Mbps bandwidth, 1 bit time 0.1 microsec
• ? minimum frame size at least 500 bits,
  choose 512 bits to add some margin of safety
• As network speed goes up, the minimum frame
  length must go up or the maximum cable length
  must come down
• E.g. for a 2500m 1Gbps LAN, minimum frame size
  should be 6400 bytes

55
CSMA/CD efficiency

• Tprop max prop. time between 2 nodes in LAN
• ttrans time to transmit max-size frame
• Efficiency goes to 1 as tprop goes to 0
• Efficiency goes to 1 as ttrans goes to infinity
• Much better than ALOHA, but still decentralized,
  simple, and cheap

56
Ethernet Technologies 10Base2

• 10 10Mbps 2 200 meters (actual is 185m) max
  distance between any two nodes without repeaters
• thin coaxial cable in a bus topology
• max 30 nodes in a segment
• max 4 repeaters, max network diameter 925m
• has become a legacy technology

57
10BaseT and 100BaseT

• 10/100 Mbps rate latter called fast ethernet
• T stands for Twisted Pair
• Nodes connect to a hub star topology 100 m
  max distance between nodes and hub
• Hubs are essentially physical-layer repeaters
• bits coming in one link go out all other links
• no frame buffering
• no CSMA/CD at hub adapters speak CSMA/CD
• provides network management functionality

58
Gigabit Ethernet (IEEE 802.3z)

• 1Gbps data rate
• use standard Ethernet frame format
• star topology, allows for point-to-point links
  (use switches) and shared broadcast channels (use
  hubs)
• Full-Duplex at 1 Gbps for point-to-point links
• in shared mode, CSMA/CD is used

59
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

60
Interconnecting LAN segments

• Hubs
• Bridges
• Switches
• Remark switches are essentially high performance
  multi-interface bridges.
• What we say about bridges also holds for switches!

61
Interconnecting with hubs

• Hubs are physical layer devices operate on bits
• Backbone hub interconnects LAN segments
• Extends max distance between nodes
• But individual segment collision domains become
  one large collision domain
• if a node in CS and a node in EE transmit at same
  time collision
• Cant interconnect 10BaseT 100BaseT

62
Bridges

• Link layer device operate on frames
• stores and forwards Ethernet frames
• examines frame header and forwards frame based on
  destination MAC address
• when frame is to be forwarded on a segment, uses
  CSMA/CD to access the segment
• can interconnect different LAN technologies
• plug-and-play, self-learning
• bridges do not need to be configured

63
Bridges traffic isolation

• Bridge installation breaks LAN into LAN segments
• bridges filter packets
• same-LAN-segment frames not usually forwarded
  onto other LAN segments
• segments become separate collision domains

LAN
64
Forwarding

• How do determine to which LAN segment to forward
  frame?
• Looks like a routing problem...

65
Self learning

• A bridge has a bridge table
• entry in bridge table
• (Node LAN Address, Bridge Interface, Time Stamp)
• stale entries in table dropped (TTL typically 60
  min)
• bridges learn which hosts can be reached through
  which interfaces
• bridge table initially empty
• when frame received, bridge learns location of
  sender
• records senders LAN address, arriving interface,
  and current time in bridge table
• delete an address in table if no frames are
  received with that address as the source address
  after some period of time

66
Filtering/Forwarding

• When bridge receives a frame
• index bridge table using destination MAC address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else flood

forward on all but the interface on which the
frame arrived
67
Bridge example

• Suppose C sends a frame to D and D replies back
  with a frame to C.

• Bridge receives frame from C
• notes in bridge table that C is on interface 1
• because D is not in table, bridge sends frame
  into interfaces 2 and 3
• frame received by D

68
Bridge Learning example
C 1

• D generates frame for C, sends
• bridge receives frame
• notes in bridge table that D is on interface 2
• bridge knows C is on interface 1, so forwards
  frame to interface 1

69
Spanning Tree

• for increased reliability, desirable to have
  redundant, alternative paths from source to dest
• with multiple paths, cycles result - bridges may
  multiply and forward frame forever
• solution bridges determine a spanning tree by
  disabling subset of interfaces

70
Some bridge features

• isolates collision domains resulting in higher
  total max throughput
• limitless number of nodes and geographical
  coverage
• can connect different Ethernet types
• transparent (plug-and-play) no configuration
  necessary

71
Interconnection without backbone

• Not recommended for two reasons
• - single point of failure at Computer Science hub
• - all traffic between EE and SE must pass through
  CS segment

72
Backbone configuration
Recommended ! With a backbone, each pair of LAN
segments can communicate without passing through
a third-party LAN segment
73
Bridges vs. Routers

• both store-and-forward devices
• routers network layer devices (forward packets
  using network layer addresses)
• bridges link layer devices (forward packets
  using LAN addresses)
• routers maintain routing tables, implement
  routing algorithms
• bridges maintain bridge tables, implement
  learning and spanning tree algorithms

74
Routers vs. Bridges

• Bridges and -
• Bridges are plug-and-play
• Bridge operation is simpler requiring less
  packet processing?high packet filtering and
  forwarding rates
• - All traffic confined to spanning tree, even
  when alternative paths are available
• - Bridges do not offer protection from broadcast
  storms

75
Routers vs. Bridges

• Routers and -
• arbitrary topologies can be supported, cycling
  is limited by TTL counters (and good routing
  protocols)?packets can use the best path
• provide protection against broadcast storms
• - require IP address configuration (not plug and
  play)
• - require larger packet processing time
• bridges do well in small (few hundred hosts)
  networks while routers used in large (thousands
  of hosts) networks

76
Ethernet Switches

• Essentially high-performance multi-interface
  bridges
• switches have large number of interfaces
• layer 2 (frame) forwarding, filtering using LAN
  addresses
• automatically build forwarding tables
• often individual hosts star-connected into
  switch
• Switching A-to-A, B-to-B, and C-to-C
  simultaneously
• Full duplex, no collisions!
• combinations of 10/100/1000 Mbps interfaces

77
Ethernet Switches

• cut-through switching frame forwarded from input
  to output port without waiting for assembly of
  entire frame
• slight reduction in latency
• store-and-forward and cut-through switching
  differ only when the output buffer becomes empty
  before the entire packet has arrived

Preamble
DA
SA
type
Data
CRC
Store-and-forward sends after CRC
Cut-though forwards after DA
78
An Example LAN
Dedicated
Shared
79
Summary comparison
80
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

81
IEEE 802.11 Wireless LANs

• 802.11b
• operate at 2.4 GHz, 11 Mbps
• widely deployed
• 802.11a
• 5-6 GHz range
• up to 54 Mbps
• 802.11g
• 2.4 GHz
• up to 54 Mbps
• All have base-station and ad-hoc network versions
• All use CSMA/CA for multiple access

82
Base station approach

• Basic Service Set (BSS) (a.k.a. cell) contains
• wireless hosts
• access point (AP) base station
• Perform polling, allow polled stations to
  transmit without contention
• BSSs combined to form distribution system (DS)

83
Ad Hoc Network approach

• No AP
• No connection to the outside world
• Wireless hosts communicate with each other
• Applications
• laptop meeting in conference room, car

84
IEEE 802.11 multiple access

• Collision if 2 or more nodes transmit at same
  time
• CSMA makes sense
• shouldnt cause a collision if you sense another
  transmission
• get all the bandwidth if youre the only one
  transmitting
• Collision detection doesnt work hidden terminal
  problem and fading problem

signal strength
location
85
IEEE 802.11 MAC Protocol CSMA/CA

• 802.11 CSMA sender
• - if sense channel idle for DIFS sec.
• then transmit entire frame, duration field of
  frame indicates how long the medium will be busy
• -if sense channel busy then wait until channel
  becomes idle for a DIFS period, then computes a
  random backoff time and counts down the time as
  the channel is sensed idle
• 802.11 CSMA receiver
• - if received OK
• return ACK after SIFS sec.
• Other stations adjust NAV (network allocation
  vector) based on the duration field

86
Collision avoidance mechanisms

• Problem
• two nodes, hidden from each other, transmit
  complete frames even though collision has
  occurred
• wasted bandwidth for long duration!
• Solution
• use small reservation packets
• nodes track reservation interval with NAV

87
Collision Avoidance RTS-CTS exchange

• sender transmits short RTS (request to send)
  packet to receiver indicates duration of
  transmission
• stations hearing the RTS set their NAVs
• receiver replies with short CTS (clear to send)
  packet
• notifying (possibly hidden) nodes
• stations hearing CTS set their NAVs

88
Collision Avoidance RTS-CTS exchange

• RTS and CTS are short
• avoid both the hidden terminal problem and the
  fading problem
• collisions less likely, of shorter duration
• IEEE 802.11 allows
• CSMA/CA
• polling from AP

89
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

90
Point to Point Data Link Control

• one sender, one receiver, one link easier than
  broadcast link
• no Media Access Control
• no need for explicit MAC addressing
• e.g., dialup link, link between two routers
• popular point-to-point data link protocols
• PPP (point-to-point protocol)
• HDLC High level data link control

91
PPP Design Requirements RFC 1547

• packet framing encapsulation of network-layer
  packet in data link layer frame
• carry network layer data of any network layer
  protocol (not just IP) at same time
• ability to demultiplex upwards
• bit transparency must carry any bit pattern in
  the data field
• error detection receiver must be able to detect
  bit errors in the received frame
• connection liveness detect, signal link failure
  to network layer
• network layer address negotiation endpoints can
  learn/configure each others network-layer address

92
PPP non-requirements

• no error correction
• no flow control a PPP receiver is able to
  receive frames at the full rate of the physical
  layer
• out of order delivery OK
• no need to support multipoint links

Error recovery, flow control, data re-ordering
all relegated to higher layers!
93
PPP Data Frame

• Flag delimiter
• Address does nothing
• Control does nothing
• Protocol upper layer protocol to which frame
  delivered (e.g. IP, AppleTalk, DECnet, LCP, IPCP,
  etc)

94
PPP Data Frame

• info upper layer data being carried, default
  maximum is 1500 bytes
• check cyclic redundancy check for error
  detection

95
Byte Stuffing

• data transparency requirement data field must
  be allowed to include flag pattern lt01111110gt
• Q is received lt01111110gt data or flag?
• Sender adds (stuffs) a control escape byte lt
  01111101gt before each lt 01111110gt data byte
• Receiver
• 01111110 preceded by 01111101 discard first byte
• 01111110 not preceded by 01111101 flag byte
• If control escape byte appears in data, it must
  be preceded by a stuffed control escape byte

96
Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
97
PPP Link-Control Protocol and Network-Control
Protocols

• Before exchanging network-layer data, data link
  peers must
• configure PPP link (max. frame length,
  authentication protocol, skip address and control
  fields) using Link Control Protocol (LCP)
• learn/configure network
• layer information
• for IP carry IP Control Protocol (IPCP) msgs
  (protocol field 8021) to configure/learn IP
  address
• one network-control protocol for each upper-layer
  network protocol

98
Chapter 5 outline

• 5.1 Introduction and services
• 5.2 Error detection and correction
• 5.3Multiple access protocols
• 5.4 LAN addresses and ARP
• 5.5 Ethernet

• 5.6 Hubs, bridges, and switches
• 5.7 Wireless links and LANs
• 5.8 PPP
• 5.9 ATM
• 5.10 Frame Relay

99
Asynchronous Transfer Mode ATM

• Goal integrated, end-end transport of voice,
  video, data
• meeting QoS requirements of voice, video (versus
  Internet best-effort model)
• packet-switching (fixed length packets, called
  cells) using virtual circuits
• high-speed 155Mbps, 622 Mbps, and higher

100
ATM architecture

• ATM adaptation layer (AAL) only at edge of ATM
  network
• roughly analogous to Internet transport layer
• different types of AALs to support different
  types of services
• ATM layer network layer
• cell (53-byte packet) switching
• physical layer deal with voltages, bit timings,
  and framing on the physical medium

101
ATM network or link layer?

• Vision end-to-end transport ATM from desktop
  to desktop
• ATM is a network technology
• Reality used to connect IP backbone routers
• IP over ATM
• ATM as link layer, connecting IP routers

102
ATM Adaptation Layer (AAL)

• ATM Adaptation Layer (AAL) adapts upper layers
  (IP or native ATM applications) to ATM layer
  below
• AAL present only in endpoints of an ATM network
  (hosts, IP routers), not in switches
• Analogous to transport layer in Internet
• AAL layer segment (header/trailer fields, data)
  fragmented across multiple ATM cells

103
ATM Adaptation Layer (AAL) more

• Different versions of AAL layers, depending on
  ATM service class
• AAL1 for CBR (Constant Bit Rate) services, e.g.
  circuit emulation
• AAL2 for VBR (Variable Bit Rate) services, e.g.,
  MPEG video
• AAL5 for data, e.g, IP datagrams

User data
CPCS PDU
ATM cell
SAR PDU
104
AAL5 - Simple And Efficient Adaptation Layer
(SEAL)

• AAL5 low overhead AAL used to carry IP datagrams
• PAD ensures CPCS-PDU multiple of 48bytes
• Length field identifies the size of the CPCS-PDU
  payload
• 4 bytes cyclic redundancy check
• CPCS-PDU fragmented into 48-byte segments by SAR
  AAL header and trailer are empty

CPCS-PDU for AAL5
105
ATM Layer

• Service transport cells across ATM network
• analogous to IP network layer
• very different services than IP network layer

Guarantees ?
Network Architecture Internet ATM ATM ATM ATM
Service Model best effort CBR VBR ABR UBR
Congestion feedback no (inferred via
loss) no congestion no congestion yes no
Bandwidth none constant rate guaranteed rate gua
ranteed minimum none
Loss no yes yes no no
Order no yes yes yes yes
Timing no yes yes no no
106
ATM Layer Virtual Circuits

• VC transport cells carried on VC from source to
  dest
• establish virtual circuit before data can flow
• A VC is a path between source and dest, each link
  of the VC has a virtual circuit identifier (VCI)
• each packet carries VC identifier (not
  destination ID)
• every switch on source-dest path maintains
  state (in VC translation table) for each
  passing connection
• link, switch resources (bandwidth, buffers) may
  be allocated to VC to get circuit-like
  performance
• Permanent VCs (PVCs)
• long lasting connections
• typically permanent route between two IP
  routers
• Switched VCs (SVC)
• dynamically set up on per-call basis

107
ATM VCs

• Advantages of ATM VC approach
• QoS performance guarantee for connections mapped
  to VC (bandwidth, delay, delay jitter)
• Drawbacks of ATM VC approach
• one PVC between each source/dest pair does not
  scale ( N(N-1) connections needed )
• SVC introduces call setup latency, processing
  overhead for short lived connections

108
ATM Layer ATM cell

• 53-byte cell 5-byte header and 48-byte payload
• Fixed-length cell and simple header facilitate
  high-speed switching

Cell header
Cell format
109
ATM cell header

• VCI virtual channel ID
• will change from link to link
• PT Payload type (e.g. RM cell, data cell, idle
  cell)
• CLP Cell Loss Priority bit
• set by source, CLP 1 implies low priority cell,
  can be discarded if congestion
• HEC Header Error Control
• error detection and correction bits, protect cell
  header

110
ATM Physical Layer

• Two pieces (sublayers) of physical layer
• Transmission Convergence (TC) Sublayer adapts
  ATM layer above to PMD sublayer below
• Header checksum (8 bits CRC) generation at
  transmit side
• Cell delineation at receive side
• With cell-based (no frames) PMD sublayer,
  transmission of idle cells when no data cells to
  send
• Physical Medium Dependent (PMD) Sublayer depends
  on physical medium being used
• Interfaces to the actual cable
• Move the bits on and off and handles bit timing
• Different for different cables

111
IP-Over-ATM
Internet-over-ATM protocol stack

• Have a permanent virtual circuit between each
  pair of routers

112
IP-Over-ATM

• IP datagrams encapsulated into ATM AAL5 PDUs
• Each router interface that connects to the ATM
  network has both an IP address and an ATM address
• Map from IP address to ATM address use ARP

113
Datagram Journey in IP-over-ATM Network

• at entry router
• examines the destination IP address of the
  datagram
• indexes the routing table, determines the IP
  address of the exit router
• determines the ATM address (using ARP) of the
  exit router
• passes datagram to AAL5 along with the ATM
  address of the exit router
• AAL5 encapsulates datagram in a CPCS-PDU,
  segments CPCS-PDU into cells, passes to ATM layer
• ATM layer indexes a table to determine the VCI
  for the VC that leads to the ATM destination
  address
• ATM network moves cells along VC to destination
• at exit router
• AAL5 reassembles cells into original datagram

114
Chapter 5 Summary

• data link layer services
• error detection/correction
• sharing a broadcast channel multiple access
• link layer addressing, ARP
• link layer technologies Ethernet, hubs, bridges,
  switches, IEEE 802.11 wireless LANs, PPP, ATM