1. Data Link Issues Relates to Lab 2.
This module covers data link layer issues, such as local area networks (LANs) and point-to-point links, CSMA, and Ethernet,

2. Data Link Layer The main tasks of the data link layer are:
Transfer data from the network layer of one machine to the network layer of another machine using hop by hop transmission over single links (single segments).
Convert the raw bit stream of the physical layer into groups of bits (“frames”) and vice versa
Datagram
Frame
Electrical and
Optical signals

3. Hop-by-Hop Transport Argon Neon Data Link Hop Single Segment
The actual path
followed by data
Data Link Hop
Single Segment

4. Data Link Layer
Datagram transferred by different link protocols over different links:
e.g., Ethernet (802.3) on first link, frame relay on intermediate links, WiFi (802.11) on last link
Each link protocol provides different services
e.g., may or may not provide error control over link

5. Data Link Layer Services
flow control
pacing between adjacent sending and receiving nodes
error detection
errors caused by signal attenuation, noise
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
transmission - half-duplex and full-duplex
half duplex - nodes at both ends of link can transmit, but not at same time
full duplex – nodes can transmit in both directions at the same time
channel access

6. Data Link Layer Implementation
in each and every host
data link layer implemented in “adaptor” (aka network interface card NIC) or on a chip
Ethernet card, card; Ethernet chipset
Implements:
Data Link layer,
AND
Physical layer
attaches into host’s system buses
combination of hardware, software, firmware
controller
physical
transmission

7. Types of “Single Segment” Networks
There are two types of communication networks:
Broadcast Networks: All stations share a single communication channel
Point-to-Point Networks: Pairs of hosts (or routers) are directly connected
Examples of single segment networks:
broadcast: Ethernet, WiFi,
point to point link: Frame Relay
Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point

8. Local Area Network (LAN)
Local area networks (LANs) typically connect devices within a building or a campus
Almost all LANs are broadcast networks
Typical topologies of LANs are bus or ring or star
LANs use an algorithm to gain access to shared channel to transmit
Star

9. LANs: Data Link Layer - MAC and LLC
In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel
The protocol that determines who can transmit on a broadcast channel is called the Medium Access Control (MAC) protocol
The MAC protocol is implemented in the MAC sublayer which is the lower sublayer of the data link layer
The MAC is physical layer/topology
dependent
The higher portion of the data link layer is often called the Logical Link Control (LLC)

10. Logic Link Controls Frame Structure Flow Control Error Control
Fields: types, lengths
Flow Control
Pacing control:
none
stop and go, window control
Error Control
None
Error detection/correction
Error recovery

11. MAC Protocols multiple access protocol three broad classes:
an algorithm that determines how nodes share channel, i.e., determine when node can transmit
Centralized – a master that controls how nodes share the channel
Distributed – no one in charge, nodes cooperate for access
three broad classes:
channel partitioning
divide channel into smaller “pieces” (time slots, frequency, code)
allocate a piece to each node a priori for exclusive use
random access
channel use not allocated - all nodes can use any piece (or full channel)
can result in collisions if nodes transmit at the same time
often implements a mechanism to “recover” from collisions
round robin “taking turns”
nodes take turns using pieces or full channel, but nodes with more to send can take longer/larger turns – causing variable delays

12. Random Access when node has packet to send
transmit at full channel data rate R (generally no pieces)
NO a priori allocation of channel among nodes
two or more transmitting nodes on channel ➜ “collision”
random access MAC protocol specifies:
how to detect collisions
how to recover from collisions (e.g., via delayed retransmissions)
examples of random access MAC protocols:
slotted ALOHA
ALOHA
CSMA, CSMA/CD, CSMA/CA

13. Carrier Sense multiple access (CSMA)
CSMA: listen before transmit, i.e., don’t interrupt others, wait until there is a pause:
if channel sensed idle: transmit entire frame
if channel sensed busy, defer transmission
When channel sensed idle: collisions can still occur:
propagation delay means two nodes may not hear each other’s transmission and the collision occurs in mid transmission
When channel sensed busy: collisions can still occur:
deferred nodes all detect pause at the same time after a transmission is completed and will attempt to transmit
Collision: entire packet transmission time wasted
distance & propagation delay play role in in determining collision probability

14. CSMA with Collision Detection (CSMA/CD)
CSMA/CD: carrier sensing, deferral as in CSMA
collisions detected within short time (propagation delay)
colliding transmissions aborted and a jam signal transmitted, reducing channel wastage
collision detection:
easy in wired LANs: measure signal strengths, compare transmitted and received signals
difficult in wireless LANs: received signal strength overwhelmed by local transmission strength – detection not functional --> don’t use /CD. Use Collision Avoidance (CA). Small reservation packets used to request channel usage. Possible because of centralized architecture (Access Point (AP).

15. CSMA with Collision Detection (CSMA/CD)
NIC receives datagram from network layer, creates frame
If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits for pause, then checks if channel idle. If YES transmits, if NO, repeats - waits for next pause.
If NIC transmits entire frame without detecting another transmission, NIC is done with frame!
If NIC detects another transmission while transmitting, aborts and sends jam signal
After aborting, NIC enters binary (exponential) back-off:
after mth collision, NIC chooses a value K at random from {0,1,2, …, 2m-1}.
NIC waits K·512 bit times, returns to Step 2
longer back-off interval with more collisions

16. CSMA with Collision Detection (CSMA/CD)
spatial layout of nodes B and D

17. A LAN Data Link Layer Example: Ethernet II
LLC very simple:
Frame structure (next slide)
connectionless: no handshaking between sending and receiving NICs
unreliable: uses error detection only no recovery
detected error: frame dropped
receiving NIC doesnt send acks or nacks to sending NIC
data in dropped frames recovered only if initial sender uses a reliable higher layer (e.g., TCP), otherwise dropped data lost
MAC protocol:
CSMA/CD with binary backoff when collisions occur

18. Ethernet II frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble:
7 bytes with pattern followed by one byte with pattern
used to synchronize receiver and sender clock rates
dest.
address
source
data (payload)
CRC
preamble
type

19. Ethernet II frame structure (more)
addresses: 6 byte source/destination MAC addresses
if adapter receives frame with matching destination address, or with broadcast address (e.g. ARP packet), it passes data in frame to upper layer (e.g., IP or ARP)
otherwise, adapter discards frame
type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) or ARP
CRC: cyclic redundancy check at receiver
error detected: frame is dropped
dest.
address
source
data (payload)
CRC
preamble
type

20. Ethernet II standards: link & physical layers
many different Ethernet standards
common MAC protocol and LLC frame format
different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps
different physical layer media: fiber, cable
application
transport
network
link
physical
MAC protocol
and LLC frame format
copper (twisted
pair) physical layer
100BASE-TX
fiber physical layer
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX

21. Ethernet II Frame vs IEEE 802.3
Note:
all fields in bytes
IEEE has embedded 8 byte Link Control header 802.2
Data field is padded if payload is less than < 38bytes
Ethernet II:
FCS is CRC
Data field is padded if payload is less than < 46bytes

22. Ethernet Star Configurations: Hubs vs. Switches
An Ethernet switch is a packet switch for Ethernet frames
Buffering of frames prevents collisions
Each port is isolated and builds its own collision domain
An Ethernet Hub does not perform buffering
Collisions occur if two frames arrive at the same time (WiFi access points are virtually hubs)
Hub
Switch