![Control Field of I-frame
An I frame contains two 3 bit - flow [N(S)] and error control sequence
[N(R )], either side of P/F bit
N(S) – Number of frame being sent
N(R ) – Number of frame expected in return in a two way exchange
N(R ) – is the acknowledgement field
If the last frame is was error free – N(R ) number will be that of the
next frame in the sequence
If the last frame was not received correctly
N(R ) number will be number of the damaged frame indicating the
need for its retransmission](https://image.slidesharecdn.com/unit-2-250106075556-7775d741/85/MAC-ADDRESS-IP-ADDRESS-AND-OSI-LAYER-ITS-FUNCTIONS-44-320.jpg)
MAC ADDRESS, IP ADDRESS AND OSI LAYER ITS FUNCTIONS
- 1. Unit-2 Data Link Layer • The data-link layer is located between the physical and the network layers. • The data-link layer provides services to the network layer; it receives services from the physical layer • Communication at the data-link layer is node-to-node. • A data unit from one point in the Internet needs to pass through many networks (LANs and WANs) to reach another point. • These LAN and WAN are connected by routers. • Two end hosts and the routers as nodes and the networks in between as links
- 2. Services of Data-link layer • When a packet is travelling in the Internet, the data-link layer of a node (host or router) is responsible for delivering a datagram to the next node in the path. • Data-link layer of the sending node needs to encapsulate the datagram received from the network in a frame. Data-link layer of the receiving node needs to decapsulate the datagram from the frame. • Encapsulation and Decapsulation is needed at each intermediate node. • Reason: Each link may be using a different protocol with a different frame format. Even if one link and the next are using the same protocol, encapsulation and decapsulation are needed because the link-layer addresses are normally different. Framing Link layer addressing Flow Control Error Control Framing : First service provided by the data-link layer is framing
- 3. Link Layer Addressing • In a connectionless internetwork such as the Internet we cannot make a datagram reach its destination using only IP addresses. • The reason is that each datagram in the Internet, from the same source host to the same destination host, may take a different path. • The source and destination IP addresses define the two ends but cannot define which links the datagram should pass through. • We need to remember that the IP addresses in a datagram should not be changed. If the destination IP address in a datagram changes, the packet never reaches its destination; if the source IP address in a datagram changes, the destination host or a router can never communicate with the source if a response needs to be sent back or an error needs to be reported back to the source • We need another addressing mechanism in a connectionless internetwork: the link-layer addresses of the two nodes. • A link-layer address is sometimes called a link address, sometimes a physical address, and sometimes a MAC address. • Since a link is controlled at the data-link layer, the addresses need to belong to the data-link layer.
- 4. When a datagram passes from the network layer to the data-link layer, the datagram will be encapsulated in a frame and two data-link addresses are added to the frame header. These two addresses are changed every time the frame moves from one link to another.
- 5. Consider the figure shown in previous slide: We have three links and two routers. Two hosts: Alice as source and Bob as destination. For each host, we have shown two addresses, the IP addresses (N) and the link-layer addresses (L). A router has as many pairs of addresses as the number of links the router is connected to. In the figure in the previous slide, it is shown three frames, one in each link. Each frame carries the same datagram with the same source and destination addresses (N1 and N8), but the link-layer addresses of the frame change from link to link. In link 1, the link-layer addresses are L1 and L2. In link 2, they are L4 and L5. In link 3, they are L7 and L8. Note that the IP addresses and the link-layer addresses are not in the same order. For IP addresses, the source address comes before the destination address; For link-layer addresses, the destination address comes before the source
- 6. Data Link Control • Flow Control • Error Control
- 9. Sliding Window Sliding window protocols are data link layer protocols for reliable and sequential delivery of data frames. The sliding window is also used in Transmission Control Protocol (TCP). In this protocol, multiple frames can be sent by a sender at a time before receiving an acknowledgment from the receiver. The term sliding window refers to the imaginary boxes to hold frames. Sliding window method is also known as windowing In these protocols, the sender has a buffer called the sending window and the receiver has buffer called the receiving window.
- 10. Sender Sliding Window The size of the sending window determines the sequence number of the outbound frames. If the sequence number of the frames is an n-bit field in the protocol, then the range of sequence numbers that can be assigned is 0 to 2𝑛 −1. Thus, the size of the sending window is 2𝑛 −1. The sequence numbers are numbered as modulo-n. For example, if the sending window size is 8, then the sequence numbers will be 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, and so on. The number of bits in the sequence number is 3 to generate the binary sequence 000, 001, 010, 011,100,100,101,110,111
- 11. Receiver Sliding Window The size of the receiving window is the maximum number of frames that the receiver can accept at a time. It determines the maximum number of frames that the sender can send before receiving acknowledgment.
- 13. Sender
- 14. Receiver
- 15. Difference between Stop and Wait and Sliding Window Protocol Parameter Stop and Wait Protocol Sliding Window Mechanism In Stop and Wait protocol, the sender sends a single frame and waits for an acknowledgment from the receiver. In Sliding window protocol, the sender sends multiple frames at a time and retransmits the damaged frames. Window Size 1 Varies from 1 to n, where n is the number of bits allotted in the protocol to represent the sequence number Sorting Sorting of frames is not needed. Sorting of frames helps to increase the efficiency of the protocol. Efficiency Stop and Wait protocol efficiency is formulated as 1/(1+2a) where a is a ratio of propagation delay to the transmission delay. Sliding Window protocol efficiency is formulated as N/(1+2a) where N is no. of window frames and a is a ratio of propagation delay to the transmission delay. Duplex Stop and Wait protocol is half- duplex in nature. Sliding Window protocol is full-duplex in nature.
- 16. Error Control
- 17. Damaged Frame Stop and Wait Error Control Mechanism
- 18. Lost Frame The McGraw-Hill Companies, Inc., 1998 Stop and Wait Error Control Mechanism
- 19. Lost ACK Stop and Wait Error Control Mechanism
- 20. Go-Back-n Error Control Mechanism Normal Operation
- 21. Go-Back-n Error Control Mechanism Lost Frame
- 22. Go-Back-n - Sender window size
- 23. Damaged Frame Go-Back-n Error Control Mechanism The size of the receiving window is 1.
- 24. Lost ACK Go-Back-n Error Control Mechanism
- 25. Selective Reject Error Control Mechanism The size of the receiving window is < 2n -1. Damaged Frame
- 26. Lost Frame Selective Reject Error Control Mechanism
- 27. Difference between Stop and Wait, Go-Back-n and Selective Reject Parameter Stop and Wait Go-Back-n Selective Reject Sender window size Sender window size is 1. Sender window size is less than 2𝑛 −1. Sender window size is equal to 2𝑛 −1. Receiver Window size Receiver window size is 1 Receiver window size is 1. Receiver window size is n. Acknowledgement Type Individual Cumulative Individual Supported Order no specific order is needed at receiver end In-order delivery only are accepted at receiver end Out-of-order deliveries also can be accepted at receiver end. Re-transmission in case of packet drop, number of re- transmission is 1 in case of packet drop, numbers of re- transmissions are N in case of packet drop, number of re- transmission is 1
- 28. HDLC • High-level Data Link Control (HDLC) • All bit oriented protocols are related to High-level Data Link Control • It is published by ISO. • HDLC supports both half duplex and full duplex modes in point to point and multipoint configurations • is a bit-oriented protocol. It implements ARQ mechanisms
- 29. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC Configuration In this one device is primary and other is secondary If it is point to point only two devices are involved If it is multipoint one primary controlling several secondary's.
- 30. HDLC Configuration Each physical station on a link consists of two logical stations One primary and the other a secondary Separate lines link the Primary of one physical station to secondary aspect of another physical station.
- 31. HDLC Configuration In balanced types both the point to point topology are of combined type. Stations are link by a single line That is controlled by either station
- 32. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC MODES • HDLC supports three modes of communication between station Normal Response Mode (NRM) Asynchronous Response Mode (ARM) Asynchronous Balanced Mode (ABM)
- 33. HDLC MODES Normal Response Mode (NRM) Refers to standard Primary – Secondary relationship In this mode, secondary device must have permission from the primary device before transmitting Once permission has been granted, the secondary may initiate a response transmission of one or more frames containing data. Asynchronous Response Mode (ARM) Refers to standard Primary – Secondary relationship In this mode, secondary device may initiate transmission without permission from the primary whenever the channel is idle. All transmissions from a secondary must be made to the primary for relay to a final destination Asynchronous Balanced Mode (ABM) In this mode, all stations are equal Only combined stations connected in point to point are used. Either combined station may initiate transmission with the other combined station without permission.
- 34. HDLC Frame Types HDLC defines three types of frames: Information frames (I-frames) Supervisory frames (S-frames) Unnumbered frames (U-frames) Each type of frame works as an envelope for the transmission of a different types of message I-frames are used to transport user data and control information relating to user data S-frames are used only to transport control information. U-frames are reserved for system management. Information carried by U-frames is intended for managing the link itself.
- 35. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC Frame Types
- 36. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC Frame Types
- 37. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC Frame Types
- 38. HDLC Frame Fields Each frame in HDLC contains the following six fields Beginning flag field Address field Control field Information field Frame check sequence (FCS) field Ending Flag field
- 39. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC Flag Field Flag It is an 8-bit sequence with the bit pattern 01111110 that identifies the beginning and end of a frame. It serves as a synchronization pattern for the receiver.
- 40. Bit Stuffing and removal HDLC uses a process called bit stuffing Sender wants to transmit a bit sequence having more than five consecutive 1s. It inserts one redundant 0 after the fifth 1 For example 011111111000 becomes 0111110111000 It indicates the receiver that the current sequence is not a flag, its data Once the receiver has seen the stuffed 0, it is dropped from the data and the original bit stream is restored. Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data So that receiver does not mistake the data for a flag.
- 41. HDLC Address Field It contains address of the secondary station. If a primary station created the frame, it contains a to address. If a secondary creates the frame, it contains a from address. An address field can be 1 byte or several bytes, depending on the network. If the address is only one byte, the last bit is always 1. If the address is more than one byte , all bytes but last one will end with 0; and only the last will end with 1.
- 42. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC Control Field
- 43. HDLC Control Field • Control field is a one or two byte segment of the frame • It is used for flow management • It differ depending on frame type • If the first bit is 0 – the frame is an I Frame • If the first bit is 1 and second bit is 0 – the frame is an S- Frame • If the both first and second bits are 1s – the frame is a U frame. • All three types of frames contain a bit called the Poll/Final (P/F) bit
- 44. Control Field of I-frame An I frame contains two 3 bit - flow [N(S)] and error control sequence [N(R )], either side of P/F bit N(S) – Number of frame being sent N(R ) – Number of frame expected in return in a two way exchange N(R ) – is the acknowledgement field If the last frame is was error free – N(R ) number will be that of the next frame in the sequence If the last frame was not received correctly N(R ) number will be number of the damaged frame indicating the need for its retransmission
- 45. Control Field of S-Frame • Control field of S Frame contains an N(R ) field but not N(S) field • S- Frames are used to return N(R ) when the receiver does not have data of its own to send • ACK is contained in the control field of an I – Frame • S- Frame - do not transmit data and so do not require N(S) field • Two bits proceeding the P/F bit - are sued to carry coded flow and error control information • U – Frames have neither N(S) or N(R ) • It is not designed for user data exchange of ACL • It have two code field – one two bits and other three bits either side of P/F bit • These codes are used to identify the type of U frame and its functions Control Field of U-Frame
- 46. Poll / Final • P/F field is a single bit with a dual purpose • When it is set (bit = 1) mean Poll or Final • Poll means when the frame is sent by a primary station to secondary • The address field contains the address of the receiver • Final means when the frame is send by a secondary to a primary • When the address field contains the address of the sender
- 47. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998
- 48. Types of S – Frames Receive ready (RR) : If the value of code is 00, it is an RR S-frame. This kind of frame acknowledges the receipt of a safe frame or group of frames. Receive not ready (RNR): If the value of code is 10, it is an RNR S-frame. It acknowledges the receipt of a frame(s), and announces that the receiver is busy and cannot receive more frames. It acts as a congestion control mechanism. Reject (REJ): If the value of the code subfield is 01, it is a REJ S-frame. This is a NAK frame used in Go-Back-N ARQ to inform the sender, before the sender time expires, that the last frame is lost or damaged. Selective reject (SREJ): If the value of the code subfield is 11, it is an SREJ S-frame. This is a NAK frame used in Selective Repeat ARQ.
- 49. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 HDLC FCS Field It is HDLC’s Error detection field
- 50. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Use of P/F Field
- 51. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Use of P/F Field
- 52. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Use of P/F Field
- 53. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Use of P/F Field
- 54. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 Use of P/F Field
- 55. WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998 U-Frame Control Field Unnumbered frames are used to exchange session management and control information between connected devices. U-frame codes are divided into two sections: a 2-bit and 3-bit before and after the P/F bit, i.e 32 different types of U-frames.