1. The Data Link Layer Data Link Types Data Link Protocols Access Methods
Data Link Sub-layers
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2. Data Link Types Point-to-Point Point-to-Multipoint
Links through Switched Networks
Broadcast
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3. Binary Synchronous Communication (BSC)
Developed by IBM, standardised by ISO (Basic Mode)
Connection-oriented
Half Duplex
Point-to-Multipoint or Point-to-Point
Asymmetric (Master-Slave polling)
Synchronous
Character-oriented
Byte Stuffed
VRC/LRC Error Checking (CRC for transparent mode)
Idle RQ (Stop and Wait)
Explicit Negative Acknowledgements (NAKs)
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4. High-level Data Link Control (HDLC)
Developed by ISO (based on IBM’s proprietary SDLC protocol)
Connection-oriented
Full Duplex
Synchronous
Bit-oriented
Bit Stuffed
CRC Error Checking
Two classes (Six modes)
Unbalanced. E.g. Normal Response Mode
Balanced. E.g. Asynchronous Balanced Mode
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5. HDLC Normal Response Mode
Point-to-Multipoint (or Point-to-Point)
Asymmetric (Master-Slave polling)
Continuous RQ (Sliding Window) with Go Back N
No Explicit Negative Acknowledgements
Used for Mainframe-Terminal networks with Point-to-Multipoint or Point-to-Point Private Circuits
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6. HDLC Asynchronous Balanced Mode
Point-to-Point
Symmetric (Equal status for both ends)
Continuous RQ (Go Back N or Selective Reject)
Positive Acknowledgements - Receiver Ready (RR)
Explicit Negative Acknowledgements
Reject (REJ) for Go Back N
Selective Reject (SREJ)
Flow Control - Receiver Not Ready (RNR)
Often used as Layer 2 protocol for IP over Point-to-Point Private Circuits
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7. Link Access Protocol Balanced (LAPB)
Subset of HDLC Asynchronous Balanced Mode
Only supports Go Back N ARQ
Also supports Asynchronous Balance Mode Extended which provides 7 bit sequence numbers (allowing window sizes of 128) for high speed or long delay circuits (E.g. satellite)
Used as the layer 2 protocol by X.25 packet switched networks
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8. HDLC Frame Formats Unnumbered Frames Information Frames
Used for Connection Control with no sequence numbers at all
Information Frames
Used to carry data with sequence numbers, but also contains field for sequence number of acknowledged frames (piggybacking)
Supervisory Frames
Used for Flow and Error Control with no data field but do have sequence numbers of acknowledged frames
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9. Point-to-Point Protocol (PPP)
Based on HDLC
Connection-oriented
Full Duplex
Point-to-Point
Symmetric
Synchronous
Character-oriented
Byte Stuffed
CRC-16 or CRC-32 Error Checking
No flow control or error recovery. Relies on higher layers
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10. PPP Header © Tanenbaum, Prentice Hall International 30/11/10
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11. Point-to-Point Protocol
PPP is used for Internet access over a switched network (E.g. Public Switched Telephone Network)
As well as supporting a frame protocol similar to HDLC, it supports other sub-layer protocols
Link Control Protocol
Network Control Protocol
Two alternative authentication protocols (PAP and CHAP)
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12. Link Control Protocol (LCP)
Encapsulated in a PPP frame
Establishes, maintains, configures, tests and terminates links
Determines quality of link, closing if necessary
Negotiates options
Maximum Receive Unit
Authentication Protocol
Compression
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13. Link Control Protocol Phases
© Tanenbaum, Prentice Hall International
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14. Network Control Protocol (NCP)
Encapsulated in an PPP frame
The NCP is used to configure the Network Layer protocol
A specific Network Control Protocol is used dependent on which Network Layer is supported
For IP, the IP Control Protocol (IPCP) is used. For IPX, it is IPXCP
IPCP will also configure an IP address, if necessary
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15. Password Authentication Protocol (PAP)
PAP is encapsulated in a PPP frame
2 way handshake performed once at link establishment
Sends username password repeatedly in plaintext and hence is not secure
The ISP must close the link, if it is incorrect
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16. Challenge Handshake Authentication Protocol (CHAP)
3 way handshake (challenge, response and accept/reject)
Password is not transmitted across the network and is hence secure
Challenge packet contains a challenge value (usually a few bytes)
Response applies a pre-defined function to the challenge value and the user’s password
Challenger performs the same calculation and checks the response
Challenge can also be repeated at any time
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17. Switched Links
Data Link Protocols, such as HDLC and PPP are often used to carry data over links through switched networks such as:
Public Switched Telephone Network (PSTN)
Integrated Services Digital Network (ISDN)
ADSL Broadband Access Network
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18. Broadcast Links
Data Link Protocols are often used over broadcast links, but the Point-to-Point and Point-to-Multipoint protocols will not support broadcast links where there are multiple transmitters
The major problem to overcome with broadcast links such as radio and Ethernet LANs is contention (resulting in collisions) accessing the link. Only one device can be permitted to access a broadcast link at any one time
The means of overcoming contention is called the Access Method
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19. Access Methods There are several strategies for handling contention:
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Carrier Sense Multiple Access (CSMA)
Token Passing
Spread Spectrum
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20. Frequency Division Multiple Access (FDMA)
FDMA solves the contention problem by dynamically allocating a different frequency to each station that wants to transmit
Allocations are requested via a common control channel which has its own access method
FDMA was used on 1st Generation (analogue) mobile phone networks
A similar method, called Wavelength Division Multiple Access is used on all optical LANs
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21. Time Division Multiple Access (TDMA)
TDMA solves the contention problem by dynamically allocating different timeslots to each station that wants to transmit
Allocations are requested via a common control channel which has its own access method
TDMA is used on 2nd Generation (GSM) mobile phone networks
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22. Code Division Multiple Access (CDMA)
CDMA is a technique that allows all transmissions to share the same channel by marking each communication with a unique ID (the code)
The receiver is able pick out the one communication it wants to receive by detecting the code
A good analogy is a conversation between two people in a noisy party where lots of other conversations are going on around them, but a listener can focus in on just one speaker
CDMA is used by 3rd Generation mobile phone networks
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23. Carrier Sense Multiple Access
Carrier Sense means listen to make sure that nobody else is transmitting before you start
CSMA/CD (with Collision Detection) - used by Ethernet
Also keep listening after starting to transmit to make sure that nobody else has transmitted while you were
Back off for a random period of time, and retransmit
CSMA/CA (with Collision Avoidance) - used on Wireless LANs
Send a short “request to send” frame and wait for “clear to send” frame before transmitting data
All other stations will see this and will hold off
Any collision of CTS/RTS frames will be dealt with by backing off for a random period
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24. Token Passing A single token is circulated around the network
No station can transmit unless it has the token
Once the station verifies that its transmission has been received it releases the token for another station to use
The network needs to be continually monitored to ensure that the token doesn’t get lost and must be recreated when necessary
Token passing networks unlike CSMA networks can guarantee fair access with a fixed maximum delay
The IBM Token Ring LAN is an example of a network that uses a token passing access method
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25. Spread Spectrum
Spread spectrum techniques, which share a wide frequency band (much wider than the original signal), are used to support multiple access in Wireless LANs. There are many different techniques
Code Division Multiple Access (CDMA)
frequency of transmitted signal is made to vary according to a defined pattern (code)
Frequency Hopping Spread Spectrum (FHSS)
Hops between frequencies according to pseudo-random sequence
Direct Sequence Spread Spectrum (DSSS)
codes one bit as n bits
Orthogonal Frequency Division Multiplexing (OFDM)
used in IEEE a – splits signal across many frequencies which do not interfere with each other
High Rate Direct Sequence Spread Spectrum (HR-DSSS)
used in IEEE b – higher speed version of DSSS
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26. Data Link Sub-layers
The IEEE who have standardised LAN protocols have divided the Data Link Layer into two sub-layers
Logical Link Control (LLC)
Common to all LANs
Independent of physical media, topology and access method
Media Access Control (MAC)
Specific to particular media, topology and access methods
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27. Logical Link Control Functions
Identifies and encapsulates Network Layer Protocols
Flow Control
Error Control
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28. Media Access Control (MAC)
Functions
Access to the physical media
Physical (MAC) Addressing
Ordered Delivery
Managing network topology (including bridging)
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29. MAC Addresses
MAC (or Physical) addresses are burned into Network Interface Cards
They uniquely identify the card
They are 48 bits (6 bytes) long
They are split into two 24 bit parts
Organisationally Unique Identifier (OUI) which identifies the manufacturer of the card and is assigned by the IEEE
A manufacturer assigned serial number
MAC addresses are usually shown as 12 hexadecimal digits, grouped in pairs to represent each byte and separated by colons or hyphens. E.g. 00:06:f5:00:1f:2c
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