Wide Area Network

1. Wide Area Network • WAN protocols • WAN solutions • Interface and cabling • configuration

2. High-Level Data Link Control • HDLC is the most important data link control protocol • Widely used which forms basis of other data link control protocols • Most WAN protocols use HDLC as data link control protocols

3. HDLC Station Types • Primary station (Unbalanced) • Controls operation of link • Frames issued are called commands • Maintains separate logical link to each secondary station • Secondary station • Under control of primary station • Frames issued called responses • Combined station (Balanced) • May issue commands and responses

4. HDLC Link Configurations • Unbalanced • One primary and one or more secondary stations • Supports full duplex and half duplex • Balanced • Two combined stations • Supports full duplex and half duplex

5. HDLC Transfer Modes (1) • Normal Response Mode (NRM) • Unbalanced configuration • Primary initiates transfer to secondary • Secondary may only transmit data in response to command from primary • Used on multi-drop lines • Host computer as primary • Terminals as secondary

6. HDLC Transfer Modes (2) • Asynchronous Balanced Mode (ABM) • Balanced configuration • Either station may initiate transmission without receiving permission • Most widely used • No polling overhead

7. HDLC Transfer Modes (3) • Asynchronous Response Mode (ARM) • Unbalanced configuration • Secondary may initiate transmission without permission form primary • Primary responsible for line • rarely used

8. Frame Structure • Synchronous transmission • All transmissions in frames • Single frame format for all data and control exchanges

9. Frame Structure Diagram

10. Flag Fields • Delimit frame at both ends • 01111110 • May close one frame and open another • Receiver hunts for flag sequence to synchronize • Bit stuffing used to avoid confusion with data containing 01111110 • 0 inserted after every sequence of five 1s • If receiver detects five 1s it checks next bit • If 0, it is deleted • If 1 and seventh bit is 0, accept as flag • If sixth and seventh bits 1, sender is indicating abort

11. Bit Stuffing • Example with possible errors

12. Address Field • Identifies secondary station that sent or will receive frame • Usually 8 bits long • May be extended to multiples of 7 bits • LSB of each octet indicates that it is the last octet (1) or not (0) • All ones (11111111) is broadcast

13. Control Field • Different for different frame type • Information - data to be transmitted to user (next layer up) • Flow and error control piggybacked on information frames • Supervisory - ARQ when piggyback not used • Unnumbered - supplementary link control • First one or two bits of control filed identify frame type • Remaining bits explained later

14. Control Field Diagram

15. Poll/Final Bit • Use depends on context • Command frame • P bit • 1 to solicit (poll) response from peer • Response frame • F bit • 1 indicates response to soliciting command

16. Information Field • Only in information and some unnumbered frames • Must contain integral number of octets • Variable length

17. Frame Check Sequence Field • FCS • Error detection • 16 bit CRC • Optional 32 bit CRC

18. HDLC Operation • Exchange of information, supervisory and unnumbered frames • Three phases • Initialization • Data transfer • Disconnect

19. Supervisory Frames • Receive Ready --- RR • Receive Not Ready --- RNR • Reject --- REJ • Selective Reject --- SREJ Unnumbered Frames • Control operations • Set up and take down communications link • Maintenance

20. The unnumbered format.

21. Operations • Connection Establishment • SABME ----> • <----- UA • Data Transfer • I-frames and • S-frames for flow and error control. • Disconnect • Either entity can issue a DISC.

22. Examples of Operation (1)

23. Examples of Operation (2)

24. 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 DLC protocols: • PPP (point-to-point protocol) • HDLC: High level data link control

25. PPP Design Requirements [RFC 1557] • packet framing: encapsulation of network-layer datagram in data link frame • carry network layer data of any network layer protocol (not just IP) at same time • ability to demultiplex upwards • bit transparency: must carry any bit pattern in the data field • error detection (no correction) • connection liveness: detect, signal link failure to network layer • network layer address negotiation: endpoint can learn/configure each other’s network address

26. PPP non-requirements • no error correction/recovery • no flow control • out of order delivery OK • no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!|

27. PPP Data Frame • Flag: delimiter (framing) • Address: does nothing (only one option) • Control: does nothing; in the future possible multiple control fields • Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

28. PPP Data Frame • info: upper layer data being carried • check: cyclic redundancy check for error detection

29. Byte Stuffing • “data transparency” requirement: data field must be allowed to include flag pattern <01111110> • Q: is received <01111110> data or flag? • Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte • Receiver: • two 01111110 bytes in a row: discard first byte, continue data reception • single 01111110: flag byte

30. Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

31. PPP Data Control Protocol Before exchanging network-layer data, data link peers must • configure PPP link (max. frame length, authentication) • learn/configure network layer information • for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

32. Other DLC Protocols (LAPB,LAPD) • Link Access Procedure, Balanced (LAPB) • Part of X.25 (ITU-T) • Subset of HDLC - ABM • Point to point link between system and packet switching network node • Link Access Procedure, D-Channel • ISDN (ITU-D) • ABM • Always 7-bit sequence numbers (no 3-bit) • 16 bit address field contains two sub-addresses • One for device and one for user (next layer up)

33. Other DLC Protocols (LLC) • Logical Link Control (LLC) • IEEE 802 • Different frame format • Link control split between medium access layer (MAC) and LLC (on top of MAC) • No primary and secondary - all stations are peers • Two addresses needed • Sender and receiver • Error detection at MAC layer • 32 bit CRC • Destination and source access points (DSAP, SSAP)

34. Other DLC Protocols (Frame Relay) (1) • Streamlined capability over high speed packet switched networks • Used in place of X.25 • Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF) • Two protocols • Control - similar to HDLC • Core - subset of control

35. Other DLC Protocols (Frame Relay) (2) • ABM • 7-bit sequence numbers • 16 bit CRC • 2, 3 or 4 octet address field • Data link connection identifier (DLCI) • Identifies logical connection • More on frame relay later

36. Other DLC Protocols (ATM) • Asynchronous Transfer Mode • Streamlined capability across high speed networks • Not HDLC based • Frame format called “cell” • Fixed 53 octet (424 bit) • Details later

37. X.25 • 1976 • Interface between host and packet switched network • Almost universal on packet switched networks and packet switching in ISDN • Defines three layers • Physical • Link • Packet

38. X.25 - Physical • Interface between attached station and link to node • Data terminal equipment DTE (user equipment) • Data circuit terminating equipment DCE (node) • Uses physical layer specification X.21

39. X.25 - Link • Link Access Protocol Balanced (LAPB) • Subset of HDLC • Point to point link between system and packet switching network node

40. X.25 - Packet • External virtual circuits • Logical connections (virtual circuits) between subscribers

41. X.25 Use of Virtual Circuits

42. Virtual Circuit Service • Virtual Call • Dynamically established • Permanent virtual circuit • Fixed network assigned virtual circuit

43. Virtual Call

44. Packet Format

45. Multiplexing • DTE can establish 4095 simultaneous virtual circuits with other DTEs over a single DTC-DCE link • Packets contain 12 bit virtual circuit number

46. Virtual Circuit Numbering

47. Flow and Error Control • HDLC at the link layer • Sliding window at the VC layer

48. Packet Sequences • Complete packet sequences • Allows longer blocks of data across network with smaller packet size without loss of block integrity • A packets • M bit 1, D bit 0 • B packets • The rest • Zero or more A followed by B

49. Reset and Restart • Reset • Reinitialize virtual circuit • Sequence numbers set to zero • Packets in transit lost • Up to higher level protocol to recover lost packets • Triggered by loss of packet, sequence number error, congestion, loss of network internal virtual circuit • Restart • Equivalent to a clear request on all virtual circuits • E.g. temporary loss of network access

50. Asynchronous Transfer Mode (ATM)