Notes - Data Link Layer Design Issues

The following are notes from Computer Networks written by Tanenbaum 5th edition.

• Data link layer uses services of the physical layer to send and receive bits over communication channels
• provide a well defined service interface to the network layer
• deal with transmission errors
• regulate flow of data so that slow receivers are not swamped by fast senders
• data link layer takes packets and encapsulates them into frames for transmission

Services Provided to the Network Layer

• function is to provide services to the network layer
• path data takes shown below

• offers various services
• unack connectionless service
• source machine send independent frames to destination machine
• ack connectionless service
• WiFi (802.11)
• ack connection oriented service
• most sophisticated, source and destination establish reliable data link

Framing

• in order to provide services to network layer, it must use the services provided by the physical layer
• must break up the bit steam using
• Byte Count
• uses a field in the header to specify number of bytes in frame
• can be garbled by transmission error
• rarely used on its own
• Flag Bytes with byte stuffing
• flag bytes used as starting and ending delimiters
• byte stuffing
• sender's data link layer insert a special ESC(escape byte) before each accidental flag byte
• following is an example used in PPP(Point to Point protocols)

• Flag Bits with bit stuffing
• gets around byte stuffing disadvantage that it is tied to 8 bit bytes
• can contain arbitrary bits developed for HDLC(High Level Data Link Control Protocol)

• Physical Layer coding violations
• is a shortcut from the physical layer
• if we do 4B/5B we don't need to map to all possible symbols so the unused signals are violations that signal an error

Error Control

• mark the start and end now we have to see if delivery is in order
• provide sender with some feedback
• introduce timers into data link layer, starts a time so that frame is set to expire
• this way sender knows if frame has been lost
• issue to manage timers

Flow Control

• what to do when a sender wants to transmit frames faster than receiver can accept them
• two approaches are commonly used
• feedback-based flow control receiver sends information to sender to give permission to send
• rate based flow control, built in mechanism to limit rate of sending
• NIC(Network Interface Cards)
• can handle frames as fast as they can arrive so overruns aren't a problem

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