diff --git a/src/backup/sync.js b/src/backup/sync.js
index 8c3d308..848d7a3 100644
--- a/src/backup/sync.js
+++ b/src/backup/sync.js
@@ -5,6 +5,8 @@ const { H_struct } = require("../data_structure.json");
 const { packet_, _nextNode, _list } = require("./intra");
 const TYPE = require('./message_types.json');
 
+const SYNC_WAIT_TIME = 5 * 60 * 1000 //5 mins
+
 const queueSync = {
     list: {},
 
@@ -12,7 +14,7 @@ const queueSync = {
         if (node_i in this.list)
             return console.debug("Another sync is in process for ", node_i);
         console.init(`START: Data sync for ${node_i}`)
-        this.list[node_i] = { ws, ts: Date.now(), q: [], cur: [], hashes: {} };
+        this.list[node_i] = { ws, ts: Date.now(), q: [], cur: new Set(), hashes: {} };
         DB.createTable(node_i)
             .then(result => ws.send(packet_.construct({ type_: TYPE.REQ_HASH, node_i })))
             .catch(error => console.error(error));
@@ -23,11 +25,11 @@ const queueSync = {
             return console.error(`Queue-Sync: Not active for ${node_i}. Cannot request block ${block_n}`);
         let r = this.list[node_i];
         r.hashes[block_n] = hash;
-        if (r.cur.length) //atleast a block is already in syncing process
+        if (r.cur.size) //atleast a block is already in syncing process
             r.q.push(block_n);
         else {
             console.debug(`Queue-Sync: Started block sync for ${node_i}#${block_n}`);
-            r.cur.push(block_n);
+            r.cur.add(block_n);
             r.ws.send(packet_.construct({ type_: TYPE.REQ_BLOCK, node_i, block_n }))
         }
     },
@@ -37,32 +39,40 @@ const queueSync = {
             return console.error(`Queue-Sync: Not active for ${node_i}. Cannot request block ${block_n}`);
 
         let r = this.list[node_i];
-        let p = r.cur.indexOf(block_n);
-        if (p == -1)
+        if (!r.cur.has(block_n))
             return console.warn(`Queue-Sync: Block ${block_n} not currently syncing`);
-        r.cur.splice(p, 1);
+        r.cur.delete(block_n);
         console.debug(`Queue-Sync: Finished block sync for ${node_i}#${block_n}`);
 
         if (r.last_block && block_n === r.last_block)
             r.last_block_done = true;
 
-        if (!r.cur.length) {
-            if (r.q.length) {
-                //request next block
-                let n = r.q.shift();
-                console.debug(`Queue-Sync: Started block sync for ${node_i}#${n}`);
-                r.cur.push(n);
-                r.ws.send(packet_.construct({ type_: TYPE.REQ_BLOCK, node_i, block_n: n }))
-            } else if (r.last_block_done) {
-                //last block is synced and queue is empty. close the sync
-                delete this.list[node_i];
-                //indicate next node for ordering
-                _nextNode.send(packet_.construct({
-                    type_: TYPE_.ORDER_BACKUP,
-                    order: _list.get([node_i])
-                }));
-            }
+        if (!r.cur.size && r.q.length) {
+            //request next block
+            let n = r.q.shift();
+            console.debug(`Queue-Sync: Started block sync for ${node_i}#${n}`);
+            r.cur.add(n);
+            r.ws.send(packet_.construct({ type_: TYPE.REQ_BLOCK, node_i, block_n: n }))
         }
+        //verify hash of block after timeout
+        setTimeout(() => verifyBlockHash(node_i, block_n, r.hashes[block_n]).then(verified => {
+            if (!verified)  //hash mistmatch, resync
+                this.add_block(node_i, block_n, r.hashes[block_n]);
+            else {//hash matched
+                delete r.hashes[block_n];
+                if (!r.cur.size && !r.q.length && r.last_block_done) {
+                    if (Object.keys(r.hashes).length)
+                        console.warn(`Queue-Sync: queue list is empty, but hash list is not empty for ${node_i}`);
+                    //all blocks synced, remove the sync instance
+                    delete this.list[node_i];
+                    //indicate next node for ordering
+                    _nextNode.send(packet_.construct({
+                        type_: TYPE_.ORDER_BACKUP,
+                        order: _list.get([node_i])
+                    }));
+                }
+            }
+        }).catch(error => console.error(error)), SYNC_WAIT_TIME);
     },
 
     last_block(node_i, block_n) {
@@ -123,23 +133,33 @@ function sendBlockHashes(node_i, ws) {
 
 //R: check hash and request data if hash mismatch
 function checkBlockHash(node_i, block_n, hash) {
-    let t_name = _.t_name(node_i);
-    getColumns(t_name).then(columns => {
-        let statement = `SELECT MD5(GROUP_CONCAT(${columns.map(c => `IFNULL(${c}, "NULL")`).join()})) as hash`
-            + `FROM ${t_name} WHERE ${_.block_calc_sql}  = ?`;
-        Database.query(statement, [block_n]).then(result => {
-            if (!result.length || result[0].hash != hash) {
-                //ReSync Block
-                let clear_statement = `DELETE FROM ${t_name} WHERE `
-                    + `CEIL(CAST(${H_struct.VECTOR_CLOCK} AS UNSIGNED) / (${floGlobals.sn_config.hash_n})) = ?`;
-                Database.query(clear_statement, [block_n])
-                    .then(_ => queueSync.add_block(node_i, block_n, hash))
-                    .catch(error => console.error(error))
-            }
-        }).catch(error => console.error(error))
+    verifyBlockHash(node_i, block_n, hash).then(result => {
+        if (!result) //Hash mismatch: ReSync block
+            queueSync.add_block(node_i, block_n, hash)
     }).catch(error => console.error(error))
 }
 
+function verifyBlockHash(node_i, block_n, hash) {
+    return new Promise((resolve, reject) => {
+        let t_name = _.t_name(node_i);
+        getColumns(t_name).then(columns => {
+            let statement = `SELECT MD5(GROUP_CONCAT(${columns.map(c => `IFNULL(${c}, "NULL")`).join()})) as hash`
+                + `FROM ${t_name} WHERE ${_.block_calc_sql}  = ?`;
+            Database.query(statement, [block_n]).then(result => {
+                if (!result.length || result[0].hash != hash) { //Hash mismatch
+                    //ReSync Block
+                    let clear_statement = `DELETE FROM ${t_name} WHERE `
+                        + `CEIL(CAST(${H_struct.VECTOR_CLOCK} AS UNSIGNED) / (${floGlobals.sn_config.hash_n})) = ?`;
+                    Database.query(clear_statement, [block_n])
+                        .then(_ => resolve(false))
+                        .catch(error => reject(error))
+                } else //Hash is verified
+                    resolve(true);
+            }).catch(error => reject(error))
+        }).catch(error => reject(error))
+    })
+}
+
 //R: set last block number
 function setLastBlock(node_i, block_n) {
     queueSync.last_block(node_i, block_n);