4/22/10 1 MS in Telecommunications TCOM 500: Modern Telecommunications Dr. Bernd-Peter Paris George Mason University Spring 2009 MS in Telecommunications Outline • Overview of the functions provided by the data link layer • Framing • Addressing • Flow Control • Error Control • Medium Access Control (MAC) • Data link layer enhances services provided by the physical layer. • Physical layer: transmission of a sequence of bits • Reliable transmission of data-frames between adjacent nodes. Paris 2 TCOM 500: Modern Telecommunications4/22/10 2 MS in Telecommunications INTRODUCTION TO THE DATA LINK LAYER Paris 3 TCOM 500: Modern Telecommunications MS in Telecommunications Link Layer Network Layer Physical Layer Link Layer Network Layer Physical Layer Perspective on Link Layer Paris 4 TCOM 500: Modern Telecommunications Link Layer Network Layer Physical Layer Channel Link Layer Network Layer Physical Layer Transmission of bit sequences Reliable transmission of frames N-PDU N-PDU N-PDU DH N-PDU DH Link Layer Functions: • Framing • Addressing • Flow Control • Error Control • Medium Access Control (MAC)4/22/10 3 MS in Telecommunications Examples: PPP and Ethernet Name Bytes Function Flag 1 Marks start of frame Address 1 Constant all 1’s Control 1 Constant 1100 0000 Protocol 1 or 2 Protocol of Payload (e.g., IP, or LCP) Payload variable N-PDU (bit-stuffed) Padding variable Extends frame to fixed length – no length field FCS 2 (or 4) For error detection. Name Bytes Function Preamble 7 Allow for Synchronization, alternating 1 and 0. SFD 1 Start of Frame 1010 1011 Destination Address 6 HW address of receiver Source Address 6 HW address of sender Ethertype 2 Protocol of payload (e.g., IP, ARP) Payload variable N-PDU CRC 4 Error detection Paris 5 TCOM 500: Modern Telecommunications PPP Frame Structure Ethernet Frame Structure MS in Telecommunications Framing • Framing is used to mark the beginning (and end) of a frame. • This is most pertinent with two directly connected nodes, and when • Fames are transmitted back-to-back. • Facilitates organization of bit sequences into larger units, called data frames. • Framing relies on a fixed bit pattern to mark the boundaries of a frame. • Example: the point-to-point protocol (PPP) uses the 8-bit pattern 0111 1110 at both ends of a frame. • Problem: What happens if the sequence 0111 1110 appears within a frame? • Answer: Bit-stuffing. • Each occurrence of 0111 11 (inside the frame) is replaced with 0111 110. • Conversely, the receiver replaces every occurrence of 0111 110 with 0111 11. • When more than two nodes are connected (e.g., Ethernet), then the frame delimiter is longer and used as a preamble. • For synchronizing the receiver. Paris 6 TCOM 500: Modern Telecommunications4/22/10 4 MS in Telecommunications Addressing • Addressing is most pertinent when the link connects more than two nodes. • With just two nodes, the sender and receiver are obvious. • Example: in PPP, the address field is constant and can be suppressed. • For links connecting, multiple nodes (e.g., Ethernet): • The identity of sender and receiver must be known. • Generally, there is also a broadcast address (all 1’s) to send a message to all nodes on the link. Paris 7 TCOM 500: Modern Telecommunications MS in Telecommunications Flow Control • Flow control aims to prevent that frames arrive at a receiver faster than they can be processed. • E.g., multiple senders send messages to the same host. • Or, receiving node does not have space in its buffer. • For flow control, receiver indicates to sender that it must stop or slow down transmissions. • Implies feedback message from Receiver to sender. • This feedback message is also used for Error Control. • Flow control is not widely implemented in Data Link Protocols. • Instead, end-to-end flow control provided by transport layer is used. Paris 8 TCOM 500: Modern Telecommunications4/22/10 5 MS in Telecommunications Error Control • Error control ensures reliable exchange of frames between adjacent nodes. • Relies on four “ingredients” • Error detection – for finding transmission errors. • Acknowledgements (ACK) – for informing sender that message was received correctly. • Timeouts – to prevent deadlock when ACKs are lost. • Retransmission of packets with errors. • Not widely implemented over reliable links. • Important exception: Wireless links generally use error control. • Instead, use error control provided by transport layer. • Will discuss error control protocols in more detail: • One of the classical problems in networking. Paris 9 TCOM 500: Modern Telecommunications MS in Telecommunications Medium Access Control • When many nodes share a link, then access to that shared “medium” must be coordinated. • This is also called multiplexing. • Fixed assignment multiplexing: • Each node gets a fixed share of the channel. • Time-division multiple access (TDMA) • Frequency-division multiple access (FDMA) • Code-division multiple access (CDMA) • Fixed assignment multiplexing is not well suited for bursty traffic (data): • Random-multiple access (statistical multiplexing). • Another classical problem that we will look at in more detail. Paris 10 TCOM 500: Modern