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Link Layer 2
Mozafar Bag-Mohammadi
University of Ilam
Spanning Tree Algorithm
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 Problem: loops
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B3
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B5
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B7
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B1
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 Bridges run a distributed spanning tree algorithm
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select which bridges actively forward
developed by Radia Perlman
now IEEE 802.1 specification
Algorithm Overview
 Each bridge has unique id (e.g., B1, B2, B3)
 Select bridge with smallest id as root
 Select bridge on each LAN closest to root as designated
bridge (use id to break ties)
 Each bridge forwards
frames over each LAN for
which it is the designated
bridge
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Algorithm Details
 Bridges exchange configuration messages
 id for bridge sending the message
 id for what the sending bridge believes to be root bridge
 distance (hops) from sending bridge to root bridge
 Each bridge records current best configuration
message for each port
 Initially, each bridge believes it is the root
Algorithm Detail (cont)
 When learn not root, stop generating config messages
 in steady state, only root generates configuration messages
 When learn not designated bridge, stop forwarding
config messages

in steady state, only designated bridges forward config
messages
 Root continues to periodically send config messages
 If any bridge does not receive config message after a
period of time, it starts generating config messages
claiming to be the root
Broadcast and Multicast
 Forward all broadcast/multicast frames
 current practice
 Learn when no group members downstream
 Accomplished by having each member of
group G send a frame to bridge multicast
address with G in source field
Token Ring Overview
 Examples
 16Mbps IEEE 802.5 (based on earlier IBM ring)
 100Mbps Fiber Distributed Data Interface (FDDI)
Token Ring (cont)
 Idea
 Frames flow in one direction: upstream to downstream
 special bit pattern (token) rotates around ring
 must capture token before transmitting
 release token after done transmitting
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•
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immediate release
delayed release
remove your frame when it comes back around
stations get round-robin service
 Frame Format
8
8
48
48
Start of
frame
Control
Dest
addr
Src
addr
32
Body
8
CRC End of
frame
24
Status
Timed Token Algorithm
 Token Holding Time (THT)
 upper limit on how long a station can hold the token
 Token Rotation Time (TRT)
 how long it takes the token to traverse the ring.
 TRT <= ActiveNodes x THT + RingLatency
 Target Token Rotation Time (TTRT)
 agreed-upon upper bound on TRT
Algorithm (cont)
 Each node measures TRT between successive tokens
 if measured-TRT > TTRT: token is late so don’t send
 if measured-TRT < TTRT: token is early so OK to send
 Two classes of traffic
synchronous: can always send
 asynchronous: can send only if token is early
 Worse case: 2xTTRT between seeing token
 Back-to-back 2xTTRT rotations not possible

Token Maintenance
 Lost Token
 no token when initializing ring
 bit error corrupts token pattern
 node holding token crashes
 Generating a Token (and agreeing on TTRT)
 execute when join ring or suspect a failure
 send a claim frame that includes the node’s TTRT bid
 when receive claim frame, update the bid and forward
 if your claim frame makes it all the way around the ring:
• your bid was the lowest
• everyone knows TTRT
• you insert new token
Maintenance (cont)
 Monitoring for a Valid Token
 should periodically see valid transmission (frame or
token)
 maximum gap = ring latency + max frame < =
2.5ms
 set timer at 2.5ms and send claim frame if it fires
Wireless LANs
 IEEE 802.11
 Bandwidth: 1 or 2 Mbps
 Physical Media
 spread spectrum radio (2.4GHz)
 diffused infrared (10m)
Spread Spectrum
 Idea
 spread signal over wider frequency band than required
 originally designed to thwart jamming
 Frequency Hopping
 transmit over random sequence of frequencies
 sender and receiver share…
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pseudorandom number generator
seed
802.11 uses 79 x 1MHz-wide frequency bands
Spread Spectrum (cont)
 Direct Sequence
 for each bit, send XOR of that bit and n random bits
 random sequence known to both sender and receiver
 called n-bit chipping code
 802.11 defines an 11-bit chipping code
1
0
Data stream: 1010
1
0
Random sequence: 0100101101011001
1
0
XOR of the two: 1011101110101001
MACAW
 Sender transmits RequestToSend (RTS) frame
 Receiver replies with ClearToSend (CTS) frame
 Neighbors…
 see CTS: keep quiet
 see RTS but not CTS: ok to transmit
 Receiver sends ACK when it has frame
 neighbors silent until see ACK
 Collisions
 no collisions detection
 known when don’t receive CTS
 exponential backoff
Collisions Avoidance
 Similar to Ethernet
 Problem:
hidden and exposed nodes
Hidden node
Exposed
node
Sending
node
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Glossary of 802.11 Wireless Terms
 Station (STA): A computer or device with a wireless network
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interface.
Access Point (AP): Device used to bridge the wireless-wired
boundary, or to increase distance as a wireless packet repeater.
Ad Hoc Network: A temporary one made up of stations in
mutual range.
Infrastructure Network: One with one or more Access Points.
Channel: A radio frequency band, or Infrared, used for shared
communication.
Basic Service Set (BSS): A set of stations communicating
wirelessly on the same channel in the same area, Ad Hoc or
Infrastructure.
Extended Service Set (ESS): A set BSSs and wired LANs with
Access Points that appear as a single logical BSS.
Supporting Mobility
ad hoc networking
 Case 2: access points (AP)
 Case 1:
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tethered
each mobile node associates with an AP
Distribution system
AP-1
AP-3
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AP-2
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Overview, 802.11 Architecture
ESS
Existing
Wired LAN
AP
STA
BSS
AP
STA
STA
BSS
STA
Infrastructure
Network
STA
Ad Hoc
Network
STA
BSS
BSS
STA
STA
Ad Hoc
Network
Mobility (cont)
 Scanning (selecting an AP)
 node sends Probe frame
 all AP’s w/in reach reply with ProbeResponse frame
 node selects one AP; sends it AssociateRequest frame
 AP replies with AssociationResponse frame
 new AP informs old AP via tethered network
 When
 active: when join or move
 passive: AP periodically sends Beacon frame
6. 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, ISDN line
 popular point-to-point Data Link Control protocols:
 PPP (point-to-point protocol)
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• Protocol choice for dialup link.
Point-to-Point (serial) links
 Many data link connections are
point-to-point serial links:
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Dial-in or DSL access connects hosts to
access routers
Routers are connected by
high-speed point-to-point links
Access
Router
Modems
Dial-Up Access
 IP hosts and routers are connected
by a serial cable
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Data link layer protocols for pointto-point links are simple:
• Main role is encapsulation of IP
datagrams
• No media access control needed
Router
Router
Router
Point-to-Point Links
Router
Data Link Protocols for Point-to-Point links
 SLIP (Serial Line IP)
• First protocol for sending IP datagrams over dial-up links
(from 1988)
• Encapsulation, not much else
 PPP (Point-to-Point Protocol):
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Successor to SLIP (1992), with added functionality
Used for dial-in and for high-speed routers
 HDLC (High-Level Data Link) :
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Widely used and influential standard (1979)
Default protocol for serial links on Cisco routers
Actually, PPP is based on a variant of HDLC
PPP - IP encapsulation
 The frame format of PPP is similar to HDLC and the 802.2 LLC frame
format:
flag
addr ctrl
7E
FF
03
1
1
1
protocol
data
CRC
flag
7E
2
<= 1500
0021
IP datagram
C021
link control data
8021
network control data
 PPP assumes a duplex circuit
 Note: PPP does not use addresses
 Usual maximum frame size is 1500
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Additional PPP functionality
 In addition to encapsulation, PPP supports:
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multiple network layer protocols (protocol multiplexing)
Link configuration
Link quality testing
Error detection
Option negotiation
Address notification
Authentication
 The above functions are supported by helper protocols:
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LCP
PAP, CHAP
NCP
PPP Support protocols
Link management: The link control protocol (LCP) is
responsible for establishing, configuring, and
negotiating a data-link connection. LCP also monitors
the link quality and is used to terminate the link.
Authentication: Authentication is optional. PPP
supports two authentication protocols: Password
Authentication Protocol (PAP) and Challenge
Handshake Authentication Protocol (CHAP).
Network protocol configuration: PPP has network
control protocols (NCPs) for numerous network layer
protocols. The IP control protocol (IPCP) negotiates
IP address assignments and other parameters when IP
is used as network layer.