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 and broadcast environments.

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 physical 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
Transfer of an IP datagram via a link protocol over a “physical” link (optical, air, coax, twisted pair….)
There are many different link protocols. Each designed for a specific type of physical layer and corresponding access method. E.g.:
Ethernet (802.3), Frame Relay, WiFi (802.11)
Link protocols provide different services to IP depending on the type of link and access method
E.g.: optical cable very reliable, does not require error control. Whereas air is unreliable - uses error control.
Frame Relay over long haul network may require flow control as end device has sole access to the medium. Ethernet does not, the broadcast media access controls user flows.

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 device: network and host
Data link layer is implemented in an “adaptor” - network interface card (NIC) or on a chip. E.g.:
Ethernet card
Ethernet chipset
Attaches into a device’s system bus
combination of hardware, software, firmware
NIC or chip implements:
Data Link layer,
AND
Physical layer
controller
physical
transmission

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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